Information Infrastructure

CH12-BDC7e.pptx

Home >Computer Science homework help >Information Infrastructure

Lecture slides prepared for “Business Data Communications”, 7/e, by William Stallings and Tom Case, Chapter 12 “LAN Architecture and Infrastructure”.

Chapter 12

LAN Architecture and Infrastructure

Local area networks (LANs) are pervasive in businesses of all sizes. Today, they

are standard building blocks for creating enterprise networks and many business

users access the Internet on a daily basis through LAN-connected devices. Being

knowledgeable about LANs and how they work is essential for making informed

decisions about business computing infrastructures.

Recent years have seen rapid changes in the technology, design, and commercial

applications of LANs. A major feature of this evolution is the introduction of a

variety of new schemes for high-speed local networking. These changes have benefitted

businesses in a number of ways, especially in terms of being able to use LANs

to share all types of business data (voice, data, image, and video) among decision

makers. Enhanced data and information sharing has, in turn, contributed to increased

business agility, responsiveness, and innovation.

In this chapter we look at the underlying technology of LANs. Chapters 13

and 14 are devoted to a discussion of specific LAN systems. This chapter begins

with a discussion of various types of LANS and various LAN configuration options.

Next we look at alternatives for wired transmission media, putting off a discussion

of wireless transmission until Chapter 14. This is followed by a discussion of LAN

protocol architecture.

Personal Computer LANs

Uses:

Departmental applications such as collaboration and project management tools and Internet access

“Big data” applications

In-memory computing applications

Real-time analytics supported by high-performance analytic appliance (HANA) boxes

In-house collaboration software applications that support project teams and other business teams

Expensive resources such as copiers, high-speed laser printers, and high-capacity network-attached storage (NAS) systems can be shared

A common LAN configuration is one that supports personal computers. With the

relatively low cost of personal computers, managers within organizations often

independently procure personal computers for departmental applications, such as

collaboration and project management tools, and Internet access. Desktop systems

have traditionally been the most common type of personal computers acquired to

support users in work units, but in recent years, laptop computers, with or without

docking stations, have become increasing common in business LANs. In some organizations,

desktop systems are being phased out in favor of laptops and/or tablets.

In larger businesses, a collection of department-level processors is rarely

capable of meeting all of the organization’s computing needs and centralized

computer processing facilities continue to be important parts of the computing

landscape. This is particularly true in organizations that use enterprise resource

planning (ERP) systems and other enterprise systems to support integrated

business processes across their operating locations. Some applications, such as

econometric forecasting models, may be too big or complex to run efficiently on

a personal computer in someone’s office; in such instances, it makes more sense

to run the applications on powerful servers located in a centralized data center.

As noted in Chapter 3, “big data” applications are most likely to be hosted in

centralized facilities, so are in-memory computing applications and real-time

analytics supported by high-performance analytic appliance (HANA) boxes. In-house

collaboration software applications that support project teams and other

business teams, such as SharePoint or SAP’s StreamWork, are also likely to be

housed in centralized facilities. When employees need to share work and information,

by far the most efficient way to do so is digitally.

Certain expensive resources, such as copiers, high-speed black and white or

laser printers, and high-capacity network-attached storage (NAS) systems, can be

shared by all users of a departmental LAN. In addition, LANs that support individual

work units can tie into larger corporate-wide network facilities. For example,

the corporation may have a building-wide LAN at each of its operating locations

as well as a wide area private network. A communications server can provide each

location with controlled access to enterprise-wide resources.

LANs for the support of personal computers and workstations have become

nearly universal in organizations of all sizes. Even those sites that still depend heavily

on mainframes and centralized data centers have transferred much of the processing

load to personal computer networks. Perhaps the prime example of the way in which

personal computers are being used is to implement client/server business applications.

Backend Networks

Backend networks are used to interconnect large systems such as mainframes, supercomputers,

and mass storage devices that typically need to transfer large volumes of

data between one another. Backend networks are sometimes called computer room

networks because the large devices that they interconnect are often located physically

in centralized climate-controlled computer rooms. This means that backend

networks are physically small in size because the machines that they interconnect are

located close to one another. By putting such large devices on the same network segment,

the data traffic that they interchange is less likely to overwhelm the LAN and

degrade the overall performance of the network. The key requirement for creating

a backend network is for bulk data transfer among a limited number of devices in a

small area. High reliability is generally also a requirement.

Used to interconnect large systems such as mainframes, super-computers, and mass storage devices that typically need to transfer large volumes of data between one another

Sometimes called computer room networks

Physically small in size

Key requirements:

Bulk data transfer among a limited number of devices in a small area

High reliability

Characteristics:

High data rate

Data rates of 1000 Mbps or more are required

High-speed interface

Mainframes and supercomputers may need to exchange high volumes of data over the network

Distributed access

Needed to ensure that all devices get fair, efficient, and reliable access to the network

Limited distance

Typically employed in a computer room or a small number of contiguous rooms

Limited number of devices

Contributes to network efficiency

Typical characteristics

also include the following:

• High data rate: To satisfy the high-volume demand, data rates of 1000 Mbps

or more are required.

• High-speed interface: Data transfer operations between a large host system

and a mass storage device are typically performed through high-speed parallel

I/O interfaces, such as Fibre Channel, rather than slower communications

interfaces. The high-speed interface is needed because the mainframes and

supercomputers may need to exchange high volumes of data over the network.

• Distributed access: Some sort of distributed media access control (MAC)

technique is needed to ensure that all devices get fair, efficient, and reliable

access to the network.

• Limited distance: Typically, a backend network is employed in a computer

room or a small number of contiguous rooms.

• Limited number of devices: The number of mainframes, supercomputers, and

mass storage devices included in a backend network is usually small because

they are expensive. Also, limiting the number of machines contributes to

network efficiency.

Typically, backend networks are found in centralized facilities in large companies

or in research installations that have large data processing budgets. Because of

the scale involved, even a small increase in backend network–enabled productivity

can be worth millions of dollars.

Storage Area Network (SAN)

Separate network of storage devices that are physically removed from, but still connected to, the network

Decouples storage tasks from specific servers and creates a shared storage facility across a high-speed network

Most use Fibre Channel

Business users access storage devices via server systems that are connected to both the LAN and the SAN

Improves client-to-storage access efficiency and direct storage-to-storage communications for backup and replication functions

A concept related to that of the backend network is the storage area network

(SAN). A SAN can be described as a separate network of storage devices that are

physically removed from, but still connected to, the network. SANs evolved from

the concept of taking storage devices and storage traffic off the LAN and creating a

separate backend network specifically designed for data.

In essence, a SAN is a separate network to handle storage needs. The SAN

decouples storage tasks from specific servers and creates a shared storage facility

across a high-speed network. The collection of networked storage devices can

include hard disks, tape libraries, and CD arrays. Most SANs use Fibre Channel,

which is described in Appendix G.

In traditional client/server LANs, data were stored on devices (typically disk

drives) inside or directly attached to the server. Network-attached storage (NAS)

systems were the next step in the evolution of LAN storage systems. NAS separated

storage devices from the server and connected them directly to the network.

SANs go one step further by allowing storage devices to exist on their own separate

network and communicate directly with each other over very fast interfaces.

Business users access these storage devices via server systems that are connected

to both the LAN and the SAN. The SAN arrangement improves client-to-storage

access efficiency, as well as direct storage-to-storage communications for backup

and replication functions.

Storage Area Network Configuration

Figure 12.1 suggests a typical SAN configuration. Users attached to the

Internet send file requests (store, retrieve) to a bank of servers. These servers do

not maintain the files locally but are connected to a SAN, which supports a number

of mass storage devices. The SAN includes network devices optimized to handle

storage tasks.

Solid-state storage technologies are increasingly being deployed in enterprise

networks as alternatives to SANs or NAS. Unlike traditional storage technologies,

solid-state storage devices do not have moving mechanical components such

as spinning magnetic disks or movable read–write heads. In addition, most use

flash memory and are able to retain data without power. Relative to magnetic disk

storage, solid-state storage technologies have lower latency and data access times;

they also make less noise (almost none) and are more durable (less susceptible to

physical shock). Faster data access contributes the popularity of solid-state storage

solutions among cloud computing service providers.

High-Speed Office Networks

Evolution of office applications has required businesses to move to higher-speed LANs

Bandwidth hungry applications such as video, audio, data conferencing, computer-based training, and e-learning systems have significantly increased network data flow

Other applications demanding bandwidth include fax machines, document scanners, interactive graphic, and collaboration software programs

Traditionally, the office environment has included a variety of devices with low- to

medium-speed data transfer requirements. However, the evolution of office applications

has required businesses to move to higher-speed LANs. Bandwidth hungry

applications such as video, audio, data conferencing, computer-based training, and

e-learning systems have increased network data flow by unprecedented amounts.

Other LAN applications that are bandwidth hogs include fax machines, document

scanners, interactive graphics, and collaboration software programs. Even when

compression techniques are used, such applications can still generate tremendous

data traffic loads. These new demands require LANs with high speed that can

support larger numbers of machines over a greater geographic extent when

compared to backend networks.

Backbone LANs

Interconnect lower-cost, lower-capacity LANs within buildings or departments with a higher-capacity LAN (backbone LAN)

The backbone network provides the infrastructure for the exchange of data and information among the LANs that it interconnects

Typically the backbone’s capacity is greater than that of the networks that connect to it

The increasing use of distributed processing applications and personal computing

devices, including mobile devices, has led to a need for a flexible strategy for local

networking. Support of premises-wide data communications requires a networking

service that is capable of spanning the distances involved and that interconnects

equipment in a single (perhaps large) building or a cluster of buildings. Although

it is possible to develop a single LAN to interconnect all on premises data processing

equipment, this is probably not a practical alternative in most cases. There are

several drawbacks to a single-LAN strategy:

• Reliability: With a single LAN, a service interruption, even of short duration,

could result in a major disruption for users.

• Capacity: A single LAN could be saturated as the number of devices attached

to the network grows over time, especially if the use of bandwidth hungry

applications also grows.

• Cost: A single LAN technology is not typically optimized for the diverse

requirements of interconnection and communication. The presence of large

numbers of low-cost microcomputers dictates that network support for these

devices be provided at low cost. LANs that support very low cost attachments

are not likely to be suitable for meeting the overall communication requirements

of enterprise networks.

A more attractive alternative is to employ lower-cost, lower-capacity

LANs within buildings or departments and to interconnect these networks with

a higher-capacity LAN. This latter network is referred to as a backbone LAN.

If confined to a single building or cluster of buildings, a high-capacity LAN

can perform the backbone function. The backbone network provides the infrastructure

for the exchange of data and information among the LANs that it interconnects.

Typically, the backbone’s capacity is greater than that of the networks that

connect to it.

Factory LANs

Essential to tie together automated equipment and robots to manage the production or manufacturing process

Most dynamic and data-intensive part of a manufacturing organization is the factory floor

Specialized machines are likely to include programmable logic controllers (PLCs)

The more a factory is automated, the greater the need for integrated communications

Require more flexible and reliable LANs than typical business office environments

Factory environments are increasingly dominated by automated equipment:

programmable controllers, automated materials handling devices, machine vision

inspection devices, and various forms of robots. To manage the production or

manufacturing process, it is essential to tie this equipment together.

The most dynamic and data-intensive part of a manufacturing organization

is the factory floor. Microprocessor devices used in manufacturing have the potential

to collect information from the shop floor and accept commands. A variety of

specialized machines from multiple vendors may populate the production line. Each

of these is likely to include programmable logic controllers (PLCs) and when these

are found in each step of the manufacturing process, data processing and information

transmission within the manufacturing environment can be improved.

In general, the more that a factory is automated, the greater is its need

for integrated communications. Only by interconnecting devices and by providing

mechanisms for their cooperation can the automated factory achieve its full

potential.

Factory LANs are a niche market requiring, in general, more flexible and

reliable LANs than are found in typical business office environments.

A Factory LAN Should Be:

In general, a factory LAN should be:

• Of high capacity

• Able to handle a variety of data traffic

• Capable of a having a large geographic footprint

• Highly reliable

• Able to specify and control transmission delays

Of high capacity

Able to handle a variety of data traffic

Capable of having a large geographic footprint

Highly reliable

Able to specify and control transmission delays

Tiered Local Area Networks

Consider the kinds of data processing equipment to be supported in a typical

business organization. In rough terms, we can group this equipment into three

categories:

• Personal computers and workstations: The workhorse in most office environments

is the microcomputer, including personal computers and workstations.

Laptops and mobile tablets have joined the list of business user devices in

many organizations. Most of this equipment is found at the departmental

level, used by individual professionals and secretarial personnel. When used

for network applications, the load generated ranges from modest to heavy

depending on the nature of the applications used by employees to perform

their jobs.

• Server farms: Servers, used within a department or shared by users in a number

of departments, can perform a variety of functions. Generic examples

include supporting expensive peripherals such as mass storage devices, providing

applications that require large amounts of processor resources, and

maintaining databases accessible by many users. Because of this shared use,

these machines may generate substantial traffic.

• Mainframes: For large database and scientific applications, the mainframe is

often the machine of choice. When the machines are networked to exchange

information with one another, bulk data transfers dictate that a high-capacity

network, such as a backend network, be used.

The requirements indicated by this spectrum suggest that combining all of

these technologies in a single LAN is not, in many cases, the most cost-effective

solution. A single network would have to have very high speed to support the

aggregate demand. However, the cost of attachment to a LAN tends to increase

as a function of the network data rate. For example, a 10-Gbps Ethernet adapter

card can cost several hundred dollars, while a 100-/1000-Mbps Ethernet adapter

may cost $ 15 or less. Accordingly, attaching low-cost personal computers to a very

high speed LAN can be very expensive.

An alternative approach, which is becoming increasingly common, is to

employ two or three tiers of LANs (Figure 12.2). Within a department, a low-cost,

moderate-speed LAN supports a cluster of personal computers and workstations.

These departmental LANs are lashed together with a backbone LAN of higher

capacity. In addition, shared systems are also supported off of this backbone. If

mainframes are also part of the office equipment suite, then a separate high-speed,

backend network that supports these devices may be linked, as a whole, to the

backbone LAN to support the traffic between the mainframes and the departmental

LANs. We shall see that LAN standards and products address the need for all

three types of LANs.

Tiered LAN Strategies

One final aspect of the tiered architecture should be mentioned: the way in which

such a networking implementation comes about in an organization. This varies

widely from one business organization to the next, but two general scenarios can be

defined. It is useful to be aware of both scenarios because of their implications for

the selection and management of LANs.

In the first scenario, the LAN decisions are made from the bottom up, with

each department making decisions more or less in isolation. In this scenario, the

particular application requirements of a department are typically well known. For

example, an engineering department has very high data rate requirements to support

its CAD environment, whereas the sales department has much more modest

data rate requirements for its order entry and order inquiry needs. Because the

applications are well known, decisions about the infrastructure needed for each

department LAN can be made quickly. Departmental budgets are often sufficient

to cover all or most of the costs of these networks, so approval by upper level

management may not be required. When a bottom-up scenario is followed, the

potential exists for each department to develop its own cluster network (tier 3). In

the meantime, if this is a large organization, the information services department

may acquire a high-speed (tier 1) LAN or backend network to interconnect its

mainframes.

Over time, departments with their own cluster tier realize the need to connect

to other networks in the enterprise in order to access other computing resources.

For example, the marketing department may have to access cost information from

the finance department as well as last month’s order volumes from sales. When

cluster-to-cluster communication requirements become important, the company

make a conscious decision to provide interconnect capability. This interconnection

may be realized through the LAN backbone (tier 2).

The advantage of this scenario is that, since the department manager is

closest to the department’s needs, local interconnect strategies can be responsive

to the specific applications used by workers in the department, and acquisition can

be timely. There are several disadvantages to this approach. First, there is the problem

of suboptimization. If procurement is not centralized within the organization,

department-by-department purchase requests may wind up costing the company

more, especially when similar types of equipment are being purchases. In addition,

larger-volume purchases may result in more favorable terms from quantity

discounts, site licenses for software, etc. Second, the company is eventually faced

with the need to interconnect all departmental LANs. If there are a wide variety of

such LANs equipped with hardware from many different vendors, the interconnection

problem becomes more challenging.

For these reasons, an alternative scenario is becoming increasingly

common: a top-down design of a LAN strategy. In this case, the company decides

to map out a total local networking strategy. The decision is centralized because

it impacts the entire operation or company. The advantage of this approach is

built-in compatibility to interconnect the users. The difficulty with this approach

is, of course, the need to be responsive and timely in meeting needs at the departmental

level.

Bottom-up strategy

Individual departments create LANs independently

Decisions about the infrastructure needed for each department LAN can be can be made quickly

Potential exists for each department to develop its own cluster network

Top-down strategy

The company decides to map out a total local networking strategy

Decision is centralized because it impacts the entire operation or company

Advantage is built-in compatibility to interconnect the users

Transmission Medium

Physical path between transmitter and receiver

Guided Media

Waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable, or optical fiber

The medium itself is more important in determining the limitations of transmission

Unguided Media

Transmits electromagnetic signals but does not guide them

Referred to as wireless transmission

Examples are the atmosphere and outer space

The bandwidth of the signal produced is more important than the medium in determining transmission characteristics

In a data transmission system, the transmission medium is the physical path

between transmitter and receiver. Transmission media can be classified as guided

or unguided. In both cases, communication is in the form of electromagnetic waves.

With guided media , the waves are guided along a solid medium, such as copper

twisted pair, copper coaxial cable, or optical fiber. The atmosphere and outer space

are examples of unguided media , which provide a means of transmitting electromagnetic

signals but do not guide them; this form of transmission is usually referred

to as wireless transmission .

The characteristics and quality of a data transmission system are determined

both by the characteristics of the medium and the characteristics of the signal. In

the case of guided media, the medium itself is more important in determining the

limitations of transmission. For unguided media, the bandwidth of the signal produced

by the transmitting antenna is more important than the medium in determining

transmission characteristics. One key property of signals transmitted by antenna

is directionality. In general, signals at lower frequencies are omnidirectional; that is,

the signal propagates in all directions from the antenna. At higher frequencies, it is

possible to focus the signal into a directional beam.

Transmission Medium and Signal Design Factors

In considering the design of data transmission systems, key concerns are data

rate and distance: The greater the data rate and distance capability, the greater

the importance of having a well-designed network. A number of design factors

relating to the transmission medium and the signal determine the data rate and

distance:

• Bandwidth: All other factors remaining constant, the wider the bandwidth of

a signal, the higher the data rate that can be achieved.

• Transmission impairments: Impairments, such as attenuation, limit effective

distance. For guided media, twisted pair generally suffers more impairment

than does coaxial cable, which in turn suffers more than optical fiber.

• Interference: Interference from competing signals in overlapping frequency

bands can distort or wipe out a signal. Interference is of particular concern for

unguided media but is also a problem with guided media. For guided media,

interference can be caused by emanations from nearby cables. For example,

twisted pairs are often bundled together and conduits often carry multiple

cables. Interference can also be experienced from unguided transmissions.

Proper shielding of a guided medium can minimize this problem.

• Number of receivers: A guided medium can be used to construct a point-to-point

link or a shared link with multiple attachments. In the latter case, each

attachment introduces some attenuation and distortion on the line, limiting

distance and/or data rate.

Bandwidth

The wider the bandwidth of a signal, the higher the data rate that can be achieved

Transmission impairments

Impairments such as attenuation limit effective distance

Interference

Interference from competing signals in overlapping frequency bands can distort or wipe out a signal

Number of receivers

A guided medium can be used to construct a point-to-point link or a shared link with multiple attachments

Figure 12.3 depicts the electromagnetic spectrum and indicates the frequencies

at which various guided media and unguided transmission techniques operate.

In this section, we look at the guided media alternatives for LANs; a discussion of

wireless media alternatives for LANs is deferred to Chapter 14.

Guided Transmission Media

A twisted pair consists of two insulated copper wires arranged in a regular spiral

pattern (Figure 12.4a). A wire pair acts as a single communication link. Typically, a

number of these pairs are bundled together into a cable by wrapping them in a tough

protective sheath. Over longer distances, cables may contain hundreds of pairs.

Twisted pair is much less expensive than the other commonly used guided transmission

media (coaxial cable, optical fiber) and is easier to work with. Compared to

other transmission media, twisted pair is limited in distance, bandwidth, and data rate.

The medium is quite susceptible to interference and noise because of its potential for

coupling with electromagnetic fields. For example, a wire run parallel to an AC power

line will pick up 60-Hz energy. Impulse noise also easily intrudes into twisted pair.

Several measures are taken to reduce impairments. Shielding the wire with

metallic braid or sheathing reduces interference. The twisting of the wire reduces

low-frequency interference, and the use of different twist lengths in adjacent pairs

reduces crosstalk.

Types of Twisted Pair

Unshielded Twisted Pair (UTP)

Ordinary telephone wire is an example

Least expensive

Easy to work with and install

Most widely deployed communications medium in enterprise networks

Shielded Twisted Pair (STP)

Twisted pair is shielded with a metallic braid or sheathing that reduces interference

Provides better performance at lower data rates

More expensive and difficult to work with

Preferred over UTP in “noisy” work environments

Twisted pair comes in two varieties:

unshielded and shielded. One of the best known examples of unshielded twisted

pair (UTP) is ordinary telephone wire. Office buildings, by universal practice, are

prewired with excess UTP, more than is needed for simple telephone support.

Business buildings are also wired with one or more of the several categories of

UTP that have been developed for data communications. Because UTP is the least

expensive of all the transmission media commonly used for LANs and is easy to

work with and install, it is the most widely deployed communications medium in

enterprise networks.

UTP is subject to external electromagnetic interference, including interference

from nearby twisted pair and from noise generated in the environment. A way

to improve the characteristics of this medium is to shield the twisted pair with a

metallic braid or sheathing that reduces interference. This shielded twisted pair

(STP) provides better performance at lower data rates. However, because it is more

expensive and more difficult to work with than UTP, it is less widely used in business

networks than UTP. However, in “noisy” work environments, STP or optical fiber is

preferred over UTP.

Categories of UTP Cabling

Category 3 UTP

Cables and associated connecting hardware whose transmission characteristics are specified up to 16 MHz

Category 4 UTP

Cables and associated connecting hardware whose transmission characteristics are specified up to 20 MHz

Category 5 UTP

Cables and associated connecting hardware whose transmission characteristics are specified up to 100 MHz

Most office buildings are prewired with a type

of twisted-pair cable commonly referred to as voice grade. Because voice-grade

twisted pair is already installed, it may appear to be an attractive alternative for use

as a medium in LAN implementations. Unfortunately, the data rates and distances

achievable with voice-grade twisted pair are limited.

In 1991, the Electronic Industries Association published standard EIA-568,

Commercial Building Telecommunications Cabling Standard , which specifies the

use of voice-grade UTP as well as STP for in-building data applications. At that

time, the specification was felt to be adequate for the range of frequencies and data

rates found in office environments. Up to that time, the principal interest for LAN

designs was in the range of data rates from 1 Mbps to 16 Mbps. Subsequently, as

users migrated to higher-performance workstations and applications, there was

increasing interest in providing LANs that could operate up to 100 Mbps over

inexpensive cable. In response to this need, TIA/EIA-568-A was issued in 1995;

TIA/EIA 568-B was published in 2001. Both of these have been superseded by

TIA/EIA 568-C, which was published in 2009. The new standard reflects advances

in cable and connector design and test methods. It covers STP and UTP.

TIA/EIA-568-A and B recognized three categories of UTP cabling:

• Category 3: UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 16 MHz

• Category 4: UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 20 MHz

• Category 5: UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 100 MHz

Of these, Category 3 and Category 5 cables have historically received the most

attention for LAN applications. However, in recent years, Category 5e (enhanced)

and Category 6 cables have largely replaced Category 5 for new LAN implementations.

These are addressed in TIA/EIA 568-C.

Over limited distances, and with proper design, data rates of up to 100 Mbps are

achievable with Category 5. Category 5 and Category 5e cabling is capable of supporting

faster Ethernet LANs such as 100BASE-TX and 1000BASE-T. Category 6 UTP

provides performance up to 250 MHz and is capable of supporting 100BASE-TX

(Fast Ethernet), 1000BASE-T/1000BASE-TX (Gigabit Ethernet), or 10BASE-TX

(10-Gigabit Ethernet). The “T” or “TX” in each of these Ethernet varieties stands

for “twisted pair.” As you might guess, the ability to support fast LANs has made

Category 5e and Category 6 cables increasingly common for pre-installation in new

office buildings.

A key difference between the categories of twisted-pair cable is the number of

twists in the cable per unit distance. For example, Category 5 is much more tightly

twisted, with a typical twist length of 0.6–0.85 cm (3–4 twists per inch), compared to

7.5–10 cm (or 3–4 twists per foot) for Category 3. Category 5e has more twists per

inch than Category 5. The tighter twisting of Category 5e and Category 5 is one of the

factors that make these more expensive than Category 3, but this is also one of the

reasons why they provide much better performance than does Category 3.

Table 12.1 Comparison of Shielded and Unshielded Twisted Pair

Table 12.1 summarizes the performance of Category 5, Category 5e, and

Category 6 UTP, as well as the STP specified in EIA-568. The first parameter used

for comparison, attenuation, is fairly straightforward. The strength of a signal falls

off with distance over any transmission medium. For guided media, attenuation is

generally exponential and therefore is typically expressed as a constant number of

decibels per unit distance (see Appendix 12A).

Attenuation introduces three considerations for the network designer. First, a

received signal must have sufficient magnitude so that the electronic circuitry in the

receiver can detect and interpret the signal. Second, the signal must maintain a level

sufficiently higher than noise to be received without error. Third, attenuation is an

increasing function of frequency.

Near-end crosstalk (NEXT) as it applies to twisted-pair wiring systems is the

coupling of the signal from one pair of conductors to another pair. These conductors

may be the metal pins in a connector or wire pairs in a cable. The near end

refers to coupling that takes place when the transmit signal entering the link couples

back to the receive conductor pair at that same end of the link (i.e., the near transmitted

signal is picked up by the near receive pair).

Table 12.2 Twisted Pair Categories and Classes

UTP = Unshielded twisted pair

S/UTP = Screened unshielded twisted pair

S/STP = Screened shielded twisted pair

Since the publication of TIA/EIA-568-C, there has been ongoing work on the

development of standards for premises cabling. These are being driven by two issues.

First, the Gigabit Ethernet specification requires the definition of parameters that

are not specified completely in any published cabling standard. Second, there is a

desire to specify cabling performance to higher levels, namely Enhanced Category 5

(Cat 5e), Category 6, Category 6e, Category 6a (augmented), and Category 7.

Table 12.2 compares these schemes to the existing standards.

Coaxial Cable (Coax)

Consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor

Inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material

Outer conductor is covered with a jacket or shield

Much less susceptible to interference and crosstalk than twisted pair

More expensive than STP but provides greater capacity

Coaxial cable, like twisted pair, consists of two conductors but is constructed

differently to permit it to operate over a wider range of frequencies. It consists

of a hollow outer cylindrical conductor that surrounds a single inner wire conductor

(Figure 12.4b). The inner conductor is held in place by either regularly spaced

insulating rings or a solid dielectric material. The outer conductor is covered with

a jacket or shield. A single coaxial cable has a diameter of 1–2.5 cm. Because of its

shielded, concentric construction, coaxial cable is much less susceptible to interference

and crosstalk than is twisted pair. Coaxial cable can be used over longer

distances and support more stations on a shared line than twisted pair.

Coaxial cable, like STP, provides good immunity from electromagnetic interference.

Coaxial cable is more expensive than STP but provides greater capacity.

Traditionally, coaxial cable was an important transmission medium for

LANs, beginning with the early popularity of Ethernet. However, in recent years,

the emphasis has been on low-cost limited distance LANs using twisted pair, and

high-performance LANs using optical fiber. The effect is the gradual but steady

decline in the use of coaxial cable for LAN implementation, to the point that it is

rarely used today except in legacy LANs.

Optical Fiber

Thin (2 to 125 µm), flexible medium capable of conducting an optical ray

Has a cylindrical shape and consists of three concentric sections

The two innermost are two types of glass with different indexes of refraction

Center one is called the core

The next layer is the cladding

These are covered by a protective, light-absorbing jacket

Optical fibers are grouped together into optical cables

An optical fiber is a thin (2–125 μm), flexible medium capable of conducting an

optical ray. Various glasses and plastics can be used to make optical fibers. The

lowest losses have been obtained using fibers of ultrapure fused silica. Ultrapure

fiber is difficult to manufacture; higher-loss multicomponent glass fibers are more

economical and still provide good performance. Plastic fiber is even less costly and

can be used for short-haul links, for which moderately high losses are acceptable.

An optical fiber has a cylindrical shape and consists of three concentric sections

(Figure 12.4c). The two innermost are two types of glass with different indexes of

refraction. The center one is called the core, and the next layer the cladding. These

two sections of glass are covered by a protective, light-absorbing jacket. Optical

fibers are grouped together into optical cables.

The following characteristics distinguish optical fiber from twisted pair or coaxial cable:

One of the most significant technological breakthroughs in information transmission

has been the development of practical fiber optic communications systems.

Optical fiber already enjoys considerable use in long-distance telecommunications,

and its use in military applications is growing. The continuing improvements in

performance and decline in prices, together with the inherent advantages of optical

fiber, have made it increasingly attractive for local area networking. The following

characteristics distinguish optical fiber from twisted pair or coaxial cable:

• Greater capacity: The potential bandwidth, and hence data rate, of optical

fiber is immense; data rates of hundreds of Gbps over tens of kilometers have

been demonstrated. Compare this to the practical maximum of hundreds of

Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up

to 100 Mbps to 10 Gbps over a few tens of meters for twisted pair.

• Smaller size and lighter weight: Optical fibers are considerably thinner than

coaxial cable or bundled twisted-pair cable—at least an order of magnitude

thinner for comparable information transmission capacity. For cramped conduits

in buildings and underground along public rights-of-way, the advantage

of small size is considerable. The corresponding reduction in weight reduces

structural support requirements.

• Lower attenuation: Attenuation is significantly lower for optical fiber than for

coaxial cable or twisted pair and is constant over a wide frequency range.

• Electromagnetic isolation: Optical fiber systems are not affected by external

electromagnetic fields. Thus the system is not vulnerable to interference,

impulse noise, or crosstalk. By the same token, fibers do not radiate energy,

causing little interference with other equipment and providing a high degree

of security from eavesdropping. In addition, fiber is inherently difficult to tap.

Greater capacity

The potential bandwidth, and hence data rate, of optical fiber is immense

Smaller size and lighter weight

Optical fibers are considerably thinner

Reduction in weight reduces structural support requirements

Lower attenuation

Attenuation is significantly lower for optical fiber and is constant over a wide frequency range

Electromagnetic isolation

Optical fiber systems are not affected by external electromagnetic fields

Fibers do not radiate energy, causing little interference with other equipment and providing a high degree of security from eavesdropping

Fiber is inherently difficult to tap

Fiber Optic Types

Multimode step-index fiber

The reflective walls of the fiber move the light pulses to the receiver

Multimode graded-index fiber

Acts to refract the light toward the center of the fiber by variations in the density

Single mode fiber

The light is guided down the center of an extremely narrow core

Two different types of light source are used in fiber optic systems:

Light-emitting diode (LED)

Less costly, operates over a greater temperature range, has a longer operational life

Injection laser diode (ILD)

Operates on the laser principle, is more efficient, can sustain higher data rates

Optical fiber systems operate in the range of about 1014 –1015 Hz; this covers

portions of the infrared and visible spectrums. The principle of optical fiber transmission

is as follows. Light from a source enters the cylindrical glass or plastic

core. Rays at shallow angles are reflected and propagated along the fiber; other

rays are absorbed by the surrounding material. This form of propagation is called

step-index multimode , referring to the variety of angles of reflection. With multimode

transmission, multiple propagation paths exist, each with a different path

length and hence time to traverse the fiber. This causes signal elements (light pulses)

to spread out in time, which limits the rate at which data can be accurately received.

Put another way, the need to leave spacing between the pulses limits data rate. This

type of fiber is best suited for transmission over very short distances. When the

fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the

core to the order of a wavelength, only a single angle or mode can pass: the axial

ray. This single-mode propagation provides superior performance for the following

reason. Because there is a single transmission path with single-mode transmission,

the distortion found in multimode cannot occur. Single mode is typically used

for long-distance applications, including telephone and cable television. Finally, by

varying the index of refraction of the core, a third type of transmission, known as

graded-index multimode, is possible. This type is intermediate between the other

two in characteristics. The higher refractive index at the center makes the light rays

moving down the axis advance more slowly than those near the cladding. Rather

than zigzagging off the cladding, light in the core curves helically because of the

graded index, reducing its travel distance. The shortened path and higher speed

allow light at the periphery to arrive at a receiver at about the same time as the

straight rays in the core axis. Graded-index fibers are often used in LANs.

Two different types of light source are used in fiber optic systems: the light emitting

diode (LED) and the injection laser diode (ILD). Both are semiconductor

devices that emit a beam of light when a voltage is applied. The LED is less costly,

operates over a greater temperature range, and has a longer operational life. The ILD,

which operates on the laser principle, is more efficient and can sustain higher data rates.

There is a relationship among the wavelength employed, the type of transmission,

and the achievable data rate. Both single mode and multimode can support several different

wavelengths of light and can employ laser or LED light source. In optical fiber,

light propagates best in three distinct wavelength “windows,” centered on 850, 1300,

and 1550 nm. These are all in the infrared portion of the frequency spectrum, below

the visible-light portion, which is 400–700 nm. The loss is lower at higher wavelengths,

allowing greater data rates over longer distances. Most local applications today use

850-nm LED light sources. Although this combination is relatively inexpensive, it

is generally limited to data rates under 100 Mbps and distances of a few kilometers.

To achieve higher data rates and longer distances, a 1300-nm LED or laser source is

needed. The highest data rates and longest distances require 1500-nm laser sources.

Structured Cabling

Need a cabling plan that deals with the selection of cable and the layout of the cable in a building

Plan should be easy to implement and accommodate future growth

Structured cabling system is a generic wiring scheme with the following characteristics:

Refers to the telecommunications infrastructure wiring within a building or campus

Includes cabling to support all types of information transfer, including voice, LANs, video and image transmission, and other forms of data transmission

Independent of vendor and end-user equipment

Designed to encompass distribution to all work or living areas within the building

As a practical matter, network managers need a cabling plan that deals with the

selection of cable and the layout of the cable in a building. The cabling plan should

be easy to implement and accommodate future growth. In campus environments,

the cabling plan also includes cable connections among buildings within a campus

area network (CAN).

To aid in the development of cabling plans, standards have been issued that

specify the cabling types and layout for office, data center, and apartment buildings.

These standards are referred to as structured cabling systems . A structured cabling

system is a generic wiring scheme with the following characteristics:

• The scheme refers to the telecommunications infrastructure wiring within a

building or campus.

• The scope of the system includes cabling to support all types of information

transfer, including voice, LANs, video and image transmission, and other

forms of data transmission.

• The cabling layout and cable selection are independent of vendor and end-user

equipment.

• The cable layout is designed to encompass distribution to all work or living

areas within the building, so that relocation of equipment does not require

rewiring but simply requires plugging the equipment into a preexisting outlet

in the new location.

One advantage of such standards is that they provide guidance for pre-installation

of cable in new buildings so that future voice and data networking needs

can be met without the need to rewire the building. The standards also simplify

cable layout design for network managers. Two standards for structured cabling

systems have been issued: TIA/EIA-568, issued jointly by the Electronic Industries

Association and the Telecommunications Industry Association, and ISO 11801,

issued by the International Organization for Standardization. The two standards are

quite similar; the details in this section are from the TIA/EIA-568 document.

Elements of a Structured Cabling Layout

A structured cabling strategy is based on the use of a hierarchical, star-wired

cable layout. Figure 12.5 illustrates the key elements for a typical commercial

building. External cables, from the local telephone company and from wide area

networks (WANs), terminate in an equipment room that is generally on the ground

floor or a basement level. Patch panel and cross-connect equipment in the equipment

room connect the external cables to internal distribution cable. Typically, the first

level of distribution consists of backbone cables. In the simplest implementation, a

single backbone cable or set of cables run from the equipment room to telecommunications

closets (called wiring closets ) on each floor. A telecommunications closet

differs from the equipment room only in that it is less complex; the telecommunications

closet generally contains cross-connect equipment for interconnecting cable on

a single floor to the backbone. The cables distributed on a single floor are referred to

as horizontal cabling . This cabling connects the backbone to wall outlets that service

individual telephone and data equipment.

Cable Distances

The use of a structured cabling plan enables an enterprise to use the transmission

media appropriate for its requirements in a systematic and standardized

fashion. Figure 12.6 indicates the recommended media for each portion of the

structured cabling hierarchy. For horizontal cabling, a maximum distance of 90 m

is recommended independent of media type. This distance is adequate to provide

coverage for an entire floor for many commercial buildings. For buildings with very

large floor space, backbone cable may be required to interconnect multiple telecommunications

closets on a single floor. For backbone cabling, distances range

from 90 to 3000 m, depending on cable type and position in the hierarchy.

LAN Protocol Architecture

LAN architecture primarily focuses on the protocols used by LAN devices to share transmission media

Is best described in terms of a layering of protocols that organize the basic functions of a LAN

Physical media access control (MAC)

Logical link control (LLC)

LAN infrastructure focus primarily on the hardware and media that provide the

network platform needed to transmit data and information among attached devices.

As we have seen in the preceding sections, cabling is a key aspect of LAN infrastructure,

especially in environments where tiered LANs make sense. LAN architecture

is less closely related to connections among physical equipment. Instead, it primarily

focuses on the protocols used by LAN devices to share transmission media.

The architecture of a LAN is best described in terms of a layering of protocols

that organize the basic functions of a LAN. This section opens with a description

of the standardized protocol architecture for LANs, which encompasses physical,

media access control (MAC), and logical link control (LLC) layers. This section

then provides an overview of the MAC and LLC layers.

IEEE 802 Protocol Layers Compared to OSI Model

Protocols defined specifically for LAN and MAN transmission address issues

relating to the transmission of blocks of data over the network. In Open Systems

Interconnection (OSI) terms, higher-layer protocols (layer 3 or 4 and above) are independent

of network architecture and are applicable to LANs, MANs, and WANs.

Thus, a discussion of LAN protocols is concerned principally with lower layers of

the OSI model, especially layers 1 and 2.

Figure 12.7 relates the LAN protocols to the OSI architecture (Figure L.1).

This architecture was developed by the IEEE 802 committee and has been adopted

by all organizations working on the specification of LAN standards. It is generally

referred to as the IEEE 802 reference model.

Working from the bottom up, the lowest layer of the IEEE 802 reference

model corresponds to the physical layer of the OSI model and includes such functions

as encoding/decoding of signals and bit transmission/reception. In addition,

the physical layer includes a specification of the transmission medium. Generally,

the transmission medium is considered “below” the lowest layer of the OSI model.

However, the choice of transmission medium is critical in LAN design, and so a

specification of the medium is included.

Above the physical layer are the functions associated with providing service to

LAN users. These include the following:

• On transmission, assemble data into a frame with address and error-detection

fields.

• On reception, disassemble frame, and perform address recognition and error

detection.

• Govern access to the LAN transmission medium.

• Provide an interface to higher layers and perform flow and error control.

These are functions typically associated with OSI layer 2. The functions in the

last bullet item are grouped into a logical link control (LLC) layer. The functions

in the first three bullet items are treated as a separate layer, called media access

control (MAC). The separation is done for the following reasons:

• The logic required to manage access to a shared-access medium is not found in

traditional layer 2 data link control.

• For the same LLC, several MAC options may be provided.

LAN Protocols in Context

Figure 12.8 illustrates the relationship between the layers of the architecture

(compare Figure 8.5). Higher-level data, such as an IP datagram, are passed down

to LLC, which appends control information as a header, creating an LLC protocol

data unit (PDU) . This control information is used in the operation of the LLC protocol.

The entire LLC PDU is then passed down to the MAC layer, which appends

control information at the front and back of the packet, forming a MAC frame .

Again, control information in the frame is needed for the operation of the MAC

protocol. For context, the figure also shows the use of TCP/IP and an application

layer above the LAN protocols.

Logical Link Control (LLC)

Logical link control (LLC) is a common link protocol for all LANs. LLC specifies

the mechanisms for addressing stations across the medium and for controlling the

exchange of data between two users. It can be thought of as residing between the

network layer and the media access control sublayer of the data link layer. LLC

enables LANs with different MAC protocols (such as Ethernet and Token Ring)

to interface with a common network layer protocol, such as IP.

Specifies the mechanisms for addressing stations across the medium and for controlling the exchange of data between two users

Can be thought of as residing between the network layer and the media access control sublayer of the data link layer

Enables LANs with different MAC protocols to interface with a common network layer protocol, such as IP

Three services are provided as alternatives for attached devices using LLC:

Three services are

provided as alternatives for attached devices using LLC:

• Unacknowledged connectionless service: This service is a datagram-style

service. It is a very simple service that does not involve any of the flow control

and error control mechanisms. Thus, the delivery of data is not guaranteed.

However, in most devices there is some higher layer of software that deals

with reliability issues.

• Connection-mode service: This service is similar to that offered by typical data

link control protocols, such as HDLC (see Chapter 6). A logical connection is

set up between two users exchanging data, and flow control and error control

are provided.

• Acknowledged connectionless service: This is a cross between the previous

two services. It provides that datagrams are to be acknowledged, but no prior

logical connection is set up.

The unacknowledged connectionless service requires minimum logic and is

useful in two contexts. First, it is often the case that higher layers of software provide

the necessary reliability and flow-control mechanism, and it is efficient to avoid

duplicating them. For example, TCP provides the mechanisms needed to ensure

that data are delivered reliably. Second, there are instances in which the overhead

of connection establishment and maintenance is unjustified or even counterproductive.

One example is data collection activity that involves the periodic sampling of data

sources, such as sensors and automatic self-test reports from security equipment or

network components. In a monitoring application, the loss of an occasional data

unit would not cause distress, as the next report should arrive shortly. Thus, in most

cases, the unacknowledged connectionless service is the preferred option.

The connection-mode service could be used in very simple devices, such as

terminal controllers, that have little software operating above this level. In these

cases, it would provide the flow control and reliability mechanisms normally implemented

at higher layers of the communications software.

The acknowledged connectionless service is useful in several contexts. With

connection-mode service, the logical link control software must maintain some sort

of table for each active connection, to keep track of the status of that connection. If

the user needs guaranteed delivery, but there are a large number of destinations for

data, connection-mode service may be impractical because of the large number of

tables required. An example is a process control or automated factory environment

where a central site may need to communicate with a large number of processors

and programmable controllers. Another use of this is the handling of important and

time-critical alarm or emergency control signals in a factory. Because of their importance,

an acknowledgment is needed so that the sender can be assured that the signal

got through. Because of the urgency of the signal, the user might not want to take

the time to first establish a logical connection and then send the data.

The LLC PDU includes destination and source service access point (DSAP,

SSAP) addresses. These refer to the next higher-layer protocol that uses LLC

(typically IP). The LLC PDU also includes a control field that provides a sequencing

and flow control mechanism. Such a control field is typical for data link control

protocols and is described in Chapter 6.

Unacknowledged connectionless service

Datagram-style service

Very simple and does not involve any of the flow control and error control mechanisms

Connection-mode service

Similar to that offered by typical data link control protocols such as HDLC

A logical connection is set up between two users exchanging data

Acknowledged connectionless service

Cross between the previous two services

Provides that datagrams are to be acknowledged, but no prior logical connection is set up

Flow control and error control are provided

Media Access Control (MAC)

Function is to provide a means of controlling access to the transmission medium in order to provide an orderly and efficient use of that capacity

The relationship between LLC and the MAC protocols can be seen by considering the transmission formats involved:

User data are passed down to the LLC layer, which prepares a link-level frame, known as an LLC protocol data unit (PDU)

This PDU is then passed down to the MAC layer, where it is enclosed in a MAC frame

All LANs and MANs (metropolitan area networks) consist of collections of devices

that must share the network’s transmission capacity. Some means of controlling

access to the transmission medium is needed to provide an orderly and efficient use

of that capacity. This is the function of a media access control (MAC) protocol.

The relationship between LLC and the MAC protocols can be seen by considering

the transmission formats involved. User data are passed down to the LLC

layer, which prepares a link-level frame, known as an LLC protocol data unit

(PDU). This PDU is then passed down to the MAC layer, where it is enclosed in a

MAC frame.

LLC PDU in a MAC Frame Format

The exact format of the MAC frame differs somewhat for the various MAC

protocols in use. In general, all of the MAC frames have a format similar to that of

Figure 12.9. The fields of this frame are as follows:

• MAC: This field contains any protocol control information needed for the

functioning of the MAC protocol. For example, a priority level could be indicated

here.

• Destination MAC address: The destination physical attachment point on the

LAN for this frame. This is the physical (MAC) address of the device within

the LAN that is the intended recipient of the frame.

• Source MAC address: The source physical attachment point on the LAN for

this frame. This is the physical (MAC) address of the sender of the frame.

• LLC PDU: The LLC data from the next higher layer. This includes the user

data plus the source and destination service access point (SSAP and DSAP),

which indicate the user of LLC.

• CRC: The cyclic redundancy check field (also known as the Frame Check

Sequence, FCS, field). This is an error-detecting code, such as is used in other

data link control protocols (Chapter 6). CRC’s role in the error control process

is described in Chapter 5. The CRC is calculated based on the bits in the

entire frame. The sender calculates the CRC and adds it to the frame. The

receiver performs the same calculation on the incoming frame and compares

that calculation to the CRC field in that incoming frame. If the two values

don’t match, then one or more bits have been accidentally altered in transit;

this typically triggers retransmission of the frame by the sender.

In most data link control protocols, the data link protocol entity is responsible

not only for detecting errors using the CRC, but for recovering from those errors by

retransmitting damaged frames. In the LAN protocol architecture, these two functions

are split between the MAC and LLC layers. The MAC layer is responsible

for detecting errors and discarding any frames that contain errors. The LLC layer

optionally keeps track of which frames have been successfully received and retransmits

unsuccessful frames.

Summary

Personal computer LANs

Backend networks and storage area networks

High-speed office networks

Backbone LANs

Factory LANs

LAN configuration

Tiered LANs

Chapter 12: LAN Architecture and Infrastructure

Guided transmission media

Twisted pair

Coaxial cable

Optical fiber

Structured cabling

LAN protocol architecture

IEEE 802 reference model

Logical link control

Media access control

Chapter 12 summary.

Figure 12.1 Storage Area Network Configuration

Storage Devices

Internet SAN

Web servers, application servers or database servers

Users

Figure 12.2 Tiered Local Area Networks

Tier 1: Mainframe and Supercomputer LAN

Tier 2: LAN Backbone

Servers

Fibre Channel switch

Ethernet switch (1 Gbps to 10 Gbps)

Ethernet switch (100 to 1000 Mbps)

Tier 3: Workstation cluster LANs

102 Frequency

(Hertz) 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015

Power and telephone Rotating generators Musical instruments Voice microphones

Microwave Radar Microwave antennas Magnetrons

Infrared Lasers Guided missiles Rangefinders

Radio Radios and televisions Electronic tubes Integrated circuits Cellular Telephony

ELF VF

ELF = Extremely low frequency

VF = Voice frequency

VLF = Very low frequency

LF = Low frequency

MF = Medium frequency

HF = High frequency

VHF = Very high frequency

UHF = Ultrahigh frequency

SHF = Superhigh frequency

EHF = Extremely high frequency

VLF LF MF HF VHF UHF SHF EHF

Twisted Pair

Coaxial Cable

Visible light

Optical Fiber

FM Radio and TV

AM Radio Terrestrial and Satellite Transmission

Wavelength in space (meters)

106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6

Figure 12.3 Electromagnetic Spectrum for Telecommunications

(a) Twisted pair

(b) Coaxial cable

—Outer conductor is braided shield

—Inner conductor is solid metal

—Separated by insulating material

—Covered by padding

Light at less than

critical angle is

absorbed in jacket

Angle of

incidence

Angle of

reflection

Outer sheath Outer conductor

Insulation

Inner

conductor

—Glass or plastic core

—Laser or light emitting diode

—Specially designed jacket

—Small size and weight

Figure 12.4 Guided Transmission Media

Core

Jacket

Cladding

twist

length —Separately insulated

—Twisted together

—Often "bundled" into cables

—Usually installed in building

during construction

Attenuation (dB per 100m) Near-end Crosstalk (dB) Frequency Cat

5 Cat 5e

Cat 6

STP Cat 5

Cat 5e

Cat 6

STP

1 2.0 2.0 2.0 1.1 62 65.3 74.3 58 4 4.1 4.1 3.8 2.2 53 56.3 65.3 58 16 8.2 8.2 7.6 4.4 44 47.2 56.2 50.4 25 10.4 10.4 9.5 6.2 41 44.3 53.3 47.7 100 22.0 22.0 19.8 12.3 32.2 35.3 44.3 38.5 250 -- -- 32.8 21.4 -- -- 38.3 31.3

Category 5 Class D

Category 5e Category 6 Class E

Category 6e Category 6a Category 7 Class F

Bandwidth 100 MHz 100 MHz 250 MHz 500 MHz 500 MHz 600 MHz

Cable Type UTP STP

UTP STP

UTP S/UTP

S/UTP S/STP

S/STP

Link Cost (Cat 5 =1)

1 1.2 1.5 1.6 3.0 10.0

Differences from Preceding Standard

Replaced Cat 3 using only two pairs

More twists per inch than 5; four pairs required in each cable

Thicker wire gauge than and more twists per inch than 5e

More twists per inch than Cat 6. Grounded foil shielding

New 6a connectors that are 3dB better than 6e connectors

Stricter requirements for crosstalk and noise than Class E

Speed 100Mbps per 100m

350Mbps per 100m 1Gbps/50m

1Gbps/100m 10Gbps/50m

10Gbps per 100m

10Gbps per 100m 40Gbps/50m

External

Cable

Equipment

room

Telecommunications

closet

Backbone

cable

Horizontal

cable

Figure 12.5 Elements of a Structured Cabling Layout

Telecommuni- cations outlet

D Horizontal cross-connect

Main cross-connect

Telecommuni- cations outlet

D Horizontal cross-connect

B Intermediate cross-connect

Media Type A B C D

UTP (voice transmission) 800 m 500 m 300 m 90 m

Category 5 UTP up to 100 Mbps 90 m 90 m 90 m 90 m

Category 6 UTP up to 1000 Mbps 90 m 90 m 90 m 90 m Category 7 STP up to 10 Gbps 90 m 90 m 90 m 90 m

62.5 µm optical fiber 2000 m 500 m 1500 m 90 m

Single-mode optical fiber 3000 m 500 m 2500 m 90 m

Figure 12.6 Cable Distances Specified in TIA/EIA-568

Physical

Data Link

Medium

Network

Transport

Session

Presentation

Application

OSI Reference

Model

Physical

Medium Access

Control

Medium

Logical Link Control

( ) ( ) ( )

Upper

Layer

Protocols LLC Service

Access Point

(LSAP)

Scope

IEEE 802

Standards

Figure 12.7 IEEE 802 Protocol Layers Compared to OSI Model

IEEE 802

Reference

Model

TCP segment

IP datagram

LLC protocol data unit

MAC frame

Application data

TCP header

IP header

LLC header

MAC header

MAC trailer

Figure 12.8 LAN Protocols in Context

Application Layer

TCP Layer

IP Layer

LLC Layer

MAC Layer

Figure 12.9 LLC PDU in a Generic MAC Frame Format

MAC Frame

LLC Address FieldsI/G

I/G = Individual/Group C/R = Command/Response

DSAP value C/R SSAP value

MAC Control

Destination MAC Address

Source MAC Address LLC PDU CRC

LLC PDU DSAP

1 octet 1 1 or 2 variable

SSAP LLC Control Information