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Lecture slides prepared for “Business Data Communications”, 7/e, by William Stallings and Tom Case, Chapter 13 “Ethernet, Switches, and Virtual LANs”.

Chapter 13

Ethernet, Switches, and Virtual LANs

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

of local area networks (LANs). A major feature of this evolution is new schemes

for high-speed local area networking. To keep pace with the changing local networking

needs of business, several approaches to high-speed LAN design have become pervasive

in enterprise networks. The most important of these include the following:

• Fast Ethernet and Gigabit Ethernet: The extension of 10-Mbps CSMA/CD

(carrier sense multiple access with collision detection) to higher speeds is a

logical strategy, because it tends to preserve the investment in existing systems.

• High-speed wireless LANs: Wireless LAN technology and standards have at

last come of age, and high-speed standards and products have been introduced.

• Fibre Channel: As noted in Chapter 12, this standard provides a low-cost, easily

scalable approach to achieving very high data rates and is used in storage area

networks (SANs) and for other types of storage networking.

Table 13.1 lists characteristics of these approaches. The remainder of this

chapter is devoted to Ethernet. Chapter 14 covers wireless LANs. Appendix G

covers Fibre Channel.

Table 13.1 Characteristics of Some High-Speed LANs

	Fast Ethernet	Gigabit Ethernet	Fibre Channel	Wireless LAN
Data Rate	100 Mbps	1 Gbps, 10 Gbps, 100 Gbps	100 Mbps to 3.2 Gbps	1 Mbps to 54 Mbps
Transmission Media	UTP, STP, optical fiber	UTP, shielded cable, optical fiber	Optical fiber, coaxial cable, STP	2.4-GHz, 5-GHz microwave
Access Method	CSMA/CD	Switched	Switched	CSMA/Polling
Supporting Standard	IEEE 802.3	IEEE 802.3	Fibre Channel Association	IEEE 802.11

Table 13.1

Characteristics of some high-speed LANs

Traditional Ethernet

Most widely used high-speed LANs in today’s enterprise networks

Developed by the IEEE 802.3 standards committee

Much of the content of the Ethernet standard is focused on the medium access control sublayer and physical layer, which are found on layers 2 and 1 of the OSI reference model

Standards for 40 Gbps and 100 Gbps versions of Ethernet are being formulated by IEEE 802.3 committees

Power over Ethernet (PoE) is another important development in the evolution of 802.3 networks

The most widely used high-speed LANs in today’s enterprise networks are

referred to as Ethernet and were developed by the IEEE 802.3 standards committee.

Collectively, Ethernet and Ethernet-like LANs are the dominant force in the wired

LAN market. As with other LAN standards, much of the content of the Ethernet

standard is focused on the medium access control sublayer and physical layer, which

are found, respectively, on layers 2 and 1 of the Open Systems Interconnection

(OSI) reference model.

Early Ethernet networks that complied with the original IEEE 802.3 standard

operated at 10 Mbps. Subsequently, standards were developed for Ethernet networks

operating at 100 Mbps, 1 Gbps, and 10 Gbps. Today, standards for 40-Gbps

and 100-Gbps versions of Ethernet are being formulated by IEEE 802.3 committees.

Power over Ethernet (PoE) is another important development in the evolution of

802.3 networks.

Before looking at PoE and high-speed Ethernet LANs, we provide a brief

overview of the original 10-Mbps Ethernet because it is a good vehicle for understanding

Ethernet’s traditional media access control (MAC) protocol. We also

introduce the concept of switched LANs to enable you to better understand why

fully-switched Ethernet networks dominate today’s business networks.

Classical Ethernet operates at 10 Mbps over a bus topology LAN using the

CSMA/CD (carrier sense multiple access with collision detection) medium access

control protocol. In this section, we introduce the concepts of bus LANs and CSMA/

CD operation, and then briefly discuss transmission medium options.

Bus Topology LAN

Early Ethernet LANs had a bus topology and the original IEEE 802.3 standard

assumed that a bus topology was in place. In a bus topology LAN, all stations

attach, through appropriate hardware interfacing known as a tap, directly to a

linear transmission medium, or bus. Full-duplex operation between the station

and the tap allows data to be transmitted onto the bus and received from the bus.

A transmission from any station’s tap propagates the length of the medium in

both directions and can be received by all other stations. At each end of the bus is

a terminator, which absorbs any signal, removing it from the bus, and keeping the

signal from echoing back across the medium.

Two communication challenges are present in bus LANs. First, because a transmission

from any one station can be received by all other stations, there needs to be

some way of indicating for whom the transmission is intended. Second, a mechanism

is needed to regulate station transmissions. To see the reason for this, consider that

if two stations on the bus attempt to transmit at the same time, their signals overlap

and become garbled. Or consider that one station decides to transmit continuously for

a long period of time, blocking other users from accessing the transmission medium.

To address these challenges, stations are required to transmit data in small

blocks, known as frames . Each frame consists of a portion of the data that a station

wishes to transmit, plus a frame header that contains control information. Each

station on the bus is assigned a unique address, or identifier, and the destination

address for the intended recipient of a frame is included in its header.

All stations attach, through hardware interfacing known as a tap, directly to a linear transmission medium, or bus

Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus

Two communication challenges:

There needs to be some way of indicating for whom the transmission is intended

A mechanism is needed to regulate station transmissions

To address these challenges stations are required to transmit data in small blocks, known as frames

Each station on the bus is assigned a unique address, or identifier, and the destination address for the intended recipient of a frame is included in its header

Frame Transmission on a Bus LAN

Figure 13.1 illustrates the scheme. In this example, station C wishes to transmit

a frame of data to A. The frame header includes A’s address. As the frame

propagates along the bus, it passes B. B observes that A is the intended recipient

and ignores the frame. A, on the other hand, sees that the frame is addressed to

itself and therefore copies the data from the frame as it goes by.

So the frame structure solves the first challenge mentioned previously: It provides

a mechanism for indicating the intended recipient of the data. It also provides

the basic tool for solving the second challenge, the regulation of access. In particular,

the stations take turns sending frames in some cooperative fashion, as explained in

the next subsection.

Medium Access Control

For CSMA/CD, a station wishing to transmit first listens to the medium (the bus) to determine if another transmission is in progress (carrier sense)

If two or more stations attempt to transmit at the same time there will be a collision

The signals from both transmissions will be garbled and not received successfully by their intended recipients

For CSMA/CD, a station wishing to transmit first listens to the medium (the bus)

to determine if another transmission is in progress (carrier sense). If the medium is

idle, the station may transmit. It may happen that two or more stations attempt to

transmit at about the same time. If this happens, there will be a collision ; the signals

from both transmissions will be garbled and not received successfully by their

intended recipients. The following procedure is the essence of CSMA/CD; it specifies

what a station with data to transmit should do if the medium is found busy and

what it should do if a collision occurs:

1. If the medium is idle, transmit; otherwise, go to step 2.

2. If the medium is busy, continue to listen until it is idle, then transmit immediately.

3. If a collision is detected during transmission, transmit a brief jamming signal

to assure that all stations know that there has been a collision and then cease

transmission.

4. After transmitting the jamming signal, wait a random amount of time, referred

to as the backoff interval, then attempt to transmit again (repeat from step 1).

CSMA/CD Operation

Figure 13.2 illustrates the technique. The upper part of the figure shows a bus

LAN layout. The remainder of the figure depicts activity on the bus at four successive

instants in time. At time t0 , station A begins transmitting a packet addressed to D.

At t1 , both B and C are ready to transmit. B senses a transmission and so defers. C,

however, is still unaware of A’s transmission and begins its own transmission. When

A’s transmission reaches C, at t2 , C detects the collision and ceases transmission. The

effect of the collision propagates back to A, where it is detected some time later, t3 , at

which time A ceases transmission.

The advantage of CSMA/CD is its simplicity. It is easy to implement the logic

required for this protocol. Furthermore, there is little to go wrong in the execution

of the protocol. For example, if for some reason a station fails to detect a collision,

the worst that can happen is that it continues to transmit its frame, wasting some

time on the medium. Once the transmission is over, the algorithm continues to

function as before.

IEEE 802.3 MAC Frame Format

Figure 13.3 depicts the frame format for the 802.3 protocol. It consists of the

following fields:

• Preamble: A seven-octet pattern of alternating 0s and 1s used by the receiver

to establish bit synchronization.

• Start Frame Delimiter (SFD): The sequence 10101011, which indicates the

actual start of the frame and enables the receiver to locate the first bit of the

rest of the frame.

• Destination Address (DA): Specifies the station(s) for which the frame is

intended. It may be a unique physical address, a group address, or a broadcast

address.

• Source Address (SA): Specifies the station that sent the frame.

• Length/Type: Takes on one of two meanings, depending on its numeric value.

If the value of this field is less than or equal to 1500 decimal, then the Length/

Type field indicates the number of MAC Client Data octets contained in the

subsequent MAC Client Data field of the basic frame (length interpretation).

If the value of this field is greater than or equal to 1536 decimal, then the

Length/Type field indicates the nature of the MAC client protocol (Type

interpretation). The Length and Type interpretations of this field are mutually

exclusive.

• MAC Client Data: Data unit supplied by LLC. The maximum size of this field

is 1500 octets for a basic frame, 1504 octets for a Q-tagged frame, and 1982

octets for an envelope frame.

• Pad: Octets added to ensure that the frame is long enough for proper CD

operation.

• Frame Check Sequence (FCS): A 32-bit cyclic redundancy check (CRC),

based on all fields except preamble, SFD, and FCS.

• Extension: This field is added, if required for 1-Gbps half-duplex operation.

The extension field is necessary to enforce the minimum carrier event duration

on the medium in half-duplex mode at an operating speed of 1 Gbps.

A station’s source address is also called its MAC address. If the station is a personal

computer (PC), the MAC address is hardwired into its network adapter card, when

the card is manufactured. A card’s backoff interval may also be randomly assigned

at the time of manufacture. If the card fails and has to been replaced, the machine

will have a new MAC address and may have a different backoff interval.

IEEE 802.3 Medium Notation

Notation format: <data rate in Mbps> <signaling method><maximum segment length in hundreds of meters>

e.g 10Base5 provides 10Mbps baseband, up to 500 meters

T and F are used in place of segment length for twisted pair and fiber

The IEEE 802.3 committee has traditionally defined a number of alternative physical

configurations. This is both good and bad. On the good side, the standard has

been responsive to evolving technology, including fiber optic cable. On the bad side,

LAN customers may be faced with a bewildering array of options. However, the

committee has been at pains to ensure that the various options can be easily integrated

into a configuration that satisfies a variety of business needs. Thus, enterprise

network designers with complex sets of requirements may find the flexibility and

variety of the 802.3 standards to be an asset.

To distinguish the various implementations that are available, the original

802.3 committee adopted a concise notation:

<data rate in Mbps> <signaling method><maximum segment length in

hundreds of meters>

Table 13.2 IEEE 802.3 10Mbps Physical Layer Medium Alternatives

	10BASE5	10BASE2	10BASE-T	10BASE-F
Transmission Medium	Coaxial cable	Coaxial cable	Unshielded twisted pair	850-nm optical fiber pair
Topology	Bus	Bus	Star	Star
Maximum Segment Length (m)	500	185	100	500
Nodes per Segment	100	30	-	33
Cable Diameter	10 mm	5 mm	0.4 – 0.6 mm	62.5/125 μm

Table 13.2 summarizes several of the options that were available for early

10-Mbps Ethernet LANs. Note that 10BASE-T and 10BASE-F do not quite follow

the notation: “T” stands for twisted pair and “F” stands for optical fiber. As will

become apparent in our discussion of high-speed Ethernet LANs, the inclusion of

a media identifier has become the norm and today’s concise notation is as follows:

Bridges

Provide a means of interconnecting similar LANs

Take place at the data link layer of the OSI reference model

Use MAC address to direct frames

Designed for use between LANs or LAN segments that use identical protocols for the physical and link layers

In virtually all cases, there is a need to expand beyond the confines of a single LAN,

to provide interconnection to other LANs and to wide area networks (WANs). Two

general approaches are used for this purpose: bridges and routers. The bridge is the

simpler of the two devices and provides a means of interconnecting similar LANs.

The router is a more general-purpose device, capable of interconnecting a variety

of LANs and WANs.

Both bridging and routing are forms of data control, but work through different

methods. Bridging takes place at the data link layer of the OSI reference model

while routing takes place at the network layer. This difference means that a bridge

uses MAC address to direct frames while a router bases its forwarding decisions on

network layer (e.g., IP) addresses.

Basic bridges are designed for use between LANs or LAN segments that use

identical protocols for the physical and link layers (e.g., all conforming to IEEE 802.3).

Because the devices all use the same protocols, the amount of processing required at

the bridge is minimal. More sophisticated bridges are capable of mapping from one

MAC format to another (e.g., to interconnect an Ethernet and a Fibre Channel LAN).

Because the bridge is used in a situation in which all the LANs have the same

characteristics, the reader may ask, why not simply have one large LAN? Depending

on circumstance, there are several reasons for the use of multiple LANs or LAN

segments connected by bridges in place of one large LAN:

• Reliability: The danger in connecting all data processing devices in an

organization to one network is that a fault on the network may disable communication

for all devices. By using bridges, the network can be partitioned

into self-contained units. Each self-contained unit is called a collision domain

because it is a group of computers amongst which data collisions can occur.

Segmenting a larger network into collision domains can help to prevent a fault

in one network segment from affecting the entire network.

• Performance: In general, performance on a LAN declines with an increase in

the number of devices or the length of the wire. Adding devices to an Ethernet

LAN increasing the probability of collisions and an increase in collisions can

degrade network performance. Dividing a large LAN into smaller segments

can give improved performance, especially when devices can be clustered so

that intranetwork traffic significantly exceeds internetwork traffic.

• Security: The establishment of multiple LANs may improve security of communications.

It is desirable to keep different types of traffic (e.g., accounting,

personnel, strategic planning) that have different security needs on physically

separate media. At the same time, the different types of users with different

levels of security need to communicate through controlled and monitored

mechanisms.

• Geography: Clearly, two separate LANs are needed to support devices clustered

in two geographically distant locations. Even in the case of two buildings

separated by a highway, it may be far easier to use a microwave bridge link

than to attempt to string cable between the two buildings.

Reasons for use

Reliability

Performance

Security

Geography

Bridge Operation

Figure 13.4 shows the action of a bridge connecting two LANs, A and B, using

the same MAC protocol. In this example, a single bridge attaches to both LANs;

frequently, the bridge function is performed by two “half-bridges,” one on each

LAN. The functions of the bridge are few and simple:

• Read all frames transmitted on A and accept those addressed to any station

on B.

• Using the medium access control protocol for B, retransmit each frame on B.

• Do the same for B-to-A traffic.

Key Aspects of Bridge Function

Makes no modification to the content or format of the frames it receives

Each frame to be transferred is simply copied from one LAN and repeated with exactly the same bit pattern on the other LAN

Should contain enough buffer space to meet peak demands

Must contain addressing and routing intelligence

May connect more than two LANs

Several design aspects of a bridge are worth highlighting:

• The bridge makes no modification to the content or format of the frames it

receives, nor does it encapsulate them with an additional header. Each frame

to be transferred is simply copied from one LAN and repeated with exactly

the same bit pattern on the other LAN. Because the two LANs use the same

LAN protocols, it is permissible to do this.

• The bridge should contain enough buffer space to meet peak demands. Over a

short period of time, frames may arrive faster than they can be retransmitted.

• The bridge must contain addressing and routing intelligence. At a minimum,

the bridge must know which addresses are on each network to know which

frames to pass. Further, there may be more than two LANs interconnected

by a number of bridges. In that case, a frame may have to be routed through

several bridges in its journey from source to destination.

• A bridge may connect more than two LANs.

In summary, the bridge provides an extension to the LAN that requires no modification

to the communications software in the stations attached to the LANs. It

appears to all stations on the two (or more) LANs that there is a single LAN on

which each station has a unique address. The station uses that unique address and

need not explicitly discriminate between stations on the same LAN and stations on

other LANs; the bridge takes care of that.

Hubs

Alternative to bus topology

Each station is connected to the hub by two lines (transmit and receive)

When a single station transmits, the hub repeats the signal on the outgoing line to each station

Network devices that operate at the physical layer of the OSI reference model

The line between a station and a hub consists of two unshielded twisted pairs

Because of the high data rate and transmission qualities of UTP the length of a line is limited to about 100 m

As an alternative, an optical fiber link may be used with the maximum length being about 500 m

In recent years, there has been a proliferation of types of devices for interconnecting

LANs that go beyond bridges and the routers. These devices can conveniently be

grouped into the categories of layer 2 switches and layer 3 switches. We begin with a

discussion of hubs and then explore these two concepts.

A hub is an alternative to the bus topology. Each station is connected to the

hub by two lines (transmit and receive). The hub acts as a repeater: When a single

station transmits, the hub repeats the signal on the outgoing line to every station.

Like other repeaters, hubs are relatively simple network devices that operate at the

physical layer (layer 1) of the OSI reference model. Hubs do not manage the traffic

that comes through them. Any frame entering one of its ports broadcasts out or is

“repeated” on every other port, except for the port of entry. Since every packet is

repeated on every other port, collisions can occur and these can affect the performance

of the entire network.

Ordinarily, the line between a station and a hub consists of two unshielded

twisted pairs. Because of the high data rate and transmission qualities of UTP, the

length of a line is limited to about 100 m. As an alternative, an optical fiber link may

be used. In this case, the maximum length is about 500 m.

Two-Level Star Topology

Multiple levels of hubs can be cascaded in a hierarchical configuration.

Figure 13.5 depicts a two-level configuration. There is one header hub (HHUB) and

one or more intermediate hubs (IHUBs). Each hub may have a mixture of stations

and other hubs attached to it from below. This layout fits well with building wiring

practices. Typically, there is a wiring closet on each floor of an office building, and

a hub can be placed in each one. Each hub could service the stations on its floor.

LAN Hubs and Switches

A hub uses a star wiring arrangement to attach stations to the hub. Note

that although this topology is physically a star, it is logically a bus. In this

arrangement, a transmission from any one station is received by the hub and

retransmitted on all of the outgoing lines to every station. If two stations transmit

at the same time, there will be a collision. Therefore, to avoid collisions, only

one station should transmit at a time. All attached devices share the LANs total

transmission (e.g., 10 Mbps) just as they do in a shared medium bus LAN such as

that depicted in Figure 13.6a.

The hub has several advantages over the simple bus arrangement. It exploits

standard building wiring practices in the layout of cable. In addition, the hub can

be configured to recognize a malfunctioning station that is jamming the network

and to cut that station out of the network. Figure 13.6b illustrates the operation of a

hub. Here again, station B is transmitting. This transmission goes from B, across the

transmit line from B to the hub, and from the hub along the receive lines of each of

the other attached stations.

In recent years, the layer 2 switch has replaced the hub in popularity, particularly

for high-speed LANs. The layer 2 switch is also sometimes referred to as a switching

hub or a multiport network bridge that processes and routes data at the data link

layer (layer 2) of the OSI model.

To clarify the distinction between hubs and switches, Figure 13.6a shows a

typical bus layout of a traditional 10-Mbps LAN. A bus is installed that is laid out so

that all the devices to be attached are in reasonable proximity to a point on the bus.

In the figure, station B is transmitting. This transmission goes from B, across the

link from B to the bus, along the bus in both directions, and along the access lines of

each of the other attached stations. In this configuration, all the stations must share

the total capacity of the bus, which is 10 Mbps.

We can achieve greater performance with a layer 2 switch. In this case, the

central hub acts as a switch, similar to a packet switch. An incoming frame from a

particular station is switched to the appropriate output line to be delivered to the

intended destination. At the same time, other unused lines can also be used for

switching other traffic. Figure 13.6c shows an example in which B is transmitting a

frame to A and at the same time C is transmitting a frame to D. So, in this example,

the current throughput on the LAN is 20 Mbps, although each individual device is

limited to 10 Mbps.

Advantages of Switched Hubs

The layer 2 switch has several attractive features:

1. No change is required to the software or hardware of the attached devices

to convert a bus LAN or a hub LAN to a switched LAN. In the case of an

Ethernet LAN, each attached device continues to use the Ethernet medium

access control protocol (CSMA/CD) to access the LAN. From the point of

view of the attached devices, nothing has changed in the access logic.

2. Each attached device has a dedicated capacity equal to that of the entire original

LAN, assuming that the layer 2 switch has sufficient capacity to keep up with

all attached devices. For example, in Figure 13.6c, if the layer 2 switch can sustain

a throughput of 20 Mbps, each attached device appears to have a dedicated

capacity for either input or output of 10 Mbps.

3. The layer 2 switch scales easily. Additional switches can be attached to the

layer 2 switch by increasing the capacity of the layer 2 switch correspondingly.

No modifications needed to workstations when replacing shared-medium hub

Each device has a dedicated capacity equivalent to entire LAN

Easy to attach additional devices to the network

Types of Switched Hubs

Two major types of layer 2 switches are available for use in business networks:

• Store-and-forward switch: The layer 2 switch accepts a frame on an input line,

buffers it briefly, and then routes it to the appropriate output line.

• Cut-through switch: The layer 2 switch takes advantage of the fact that the

destination address appears at the beginning of the MAC frame. The layer 2

switch begins repeating the incoming frame onto the appropriate output line

as soon as the layer 2 switch recognizes the destination address.

The cut-through switch yields the highest possible throughput but at some

risk of propagating bad frames, because the switch is not able to perform the CRC

(described in Chapter 5) prior to retransmission. The store-and-forward switch

involves a delay between sender and receiver while the CRC is performed, but

boosts the overall integrity of the network.

Store-and-forward switch

Accepts a frame on input line

Buffers it briefly

Routes it to appropriate output line

Involves a delay between sender and receiver while the CRC is performed but boosts the overall integrity of the network

Cut-through switch

Takes advantage of the fact that the destination address appears at the beginning of the MAC frame

Begins repeating the frame as soon as it recognizes the destination address

Higher throughput, increased chance of error

Differences Between Switched Hubs and Bridges

A layer 2 switch can be viewed as a full-duplex version of the hub. It can also

incorporate logic that allows it to function as a multiport bridge. The following are

differences between layer 2 switches and bridges:

• Bridge frame handling is done in software. A layer 2 switch performs the

address recognition and frame forwarding functions in hardware.

• A bridge can typically only analyze and forward one frame at a time, whereas a

layer 2 switch has multiple parallel data paths and can handle multiple frames

at a time.

• A bridge uses store-and-forward operation. With a layer 2 switch, it is possible

to have cut-through instead of store-and-forward operation.

Because a layer 2 switch has higher performance and can incorporate the functions

of a bridge, bridges are much less common in today’s LANs than they were in

the past. New installations typically include layer 2 switches with bridge functionality

rather than bridges.

Bridges typically only analyze and forward one frame at a time

A layer 2 switch can handle multiple frames at a time

Much less common in today’s LANs

New installations typically include layer 2 switches with bridge functionality rather than bridges

Bridges use store-and-forward operation

Layer 2 switches use cut-through instead of store-and-forward operation

Bridge frame handling is done in software

A layer 2 switch performs the address recognition and frame forwarding functions in hardware

Layer 3 Switches

Can be described as a hardware-based router

Packet-by-packet layer 3 switch

Operates in the identical fashion as a traditional router

Can achieve an order of magnitude increase in performance compared to the software-based router

Flow-based layer 3 switch

Enhances performance by identifying flows of IP packets that have the same source and destination

Once identified, a predefined route can be established through the network to speed up the forwarding process

To accommodate the higher traffic loads

associated with today’s high-speed LANs, a number of vendors have developed

layer 3 switches, which implement the packet-forwarding logic of the router in hardware.

Hence, a layer 3 switch can be described as a hardware-based router.

There are various layer 3 schemes on the market, but fundamentally they fall

into two categories: packet by packet and flow based. The packet-by-packet layer 3

switch operates in the identical fashion as a traditional router. However, because the

forwarding logic is in hardware, the packet-by-packet switch can achieve an order of

magnitude increase in performance compared to the software-based router.

A flow-based layer 3 switch tries to enhance performance by identifying flows

of IP packets that have the same source and destination. This can be done by observing

ongoing traffic or by using a special flow label in the packet header (allowed in IPv6

but not in IPv4; see Figure 8.7). Once a flow is identified, a predefined route can be

established through the network to speed up the forwarding process. Again, huge

performance increases over a pure software-based router are achieved.

Typical Premises Network Configuration

Figure 13.7 is a typical example of the approach taken to local networking in

an organization with a large number of PCs and workstations (thousands to tens

of thousands). Desktop systems have links of 100 Mbps to 1000 Mbps (1 Gbps)

into a LAN controlled by a layer 2 switch. Wireless LAN connectivity is also likely

to be available for mobile users. Layer 3 switches are at the local network’s core,

forming a local backbone. Typically, these switches are interconnected at 1 Gbps

or 10 Gbps and connect to layer 2 switches at 1 Gbps to 10 Gbps. Servers connect

directly to layer 2 or layer 3 switches at 1 Gbps or possibly 10 Gbps. A lower cost

software-based router provides WAN connection. The circles in the figure

identify separate LAN subnetworks; a MAC broadcast frame is limited to its own

subnetwork.

Fast Ethernet

Refers to a set of specifications developed by the IEEE 802.3 committee to provide a low-cost, Ethernet-compatible LAN operating at 100 Mbps

Blanket designation for these standards is 100BASE-T

Committee defined a number of alternatives to be used with different transmission media

If one were to design a high-speed (100 Mbps or more) LAN from scratch, one

would not choose CSMA/CD as the basis for the design. CSMA/CD is simple to

implement and robust in the face of faults. However, it does not scale well. As

the load on a bus increases, the number of collisions increases, degrading performance.

Furthermore, as the data rate for a given system increases, performance also

decreases. The reason for this is that at a higher data rate, a station can transmit

more bits before it recognizes a collision, and therefore more wasted bits are transmitted

when collisions occur.

These problems can be overcome. To accommodate higher loads, a system can

be designed to have a number of different segments, interconnected with switching

hubs. As mentioned previously, the switches can act as barriers, separating the LAN

into collision domains, so that a collision in one domain does not spread to other

domains. The use of switched Ethernet hubs in effect eliminates collisions, further

increasing efficiency.

Despite some drawbacks to the use of CSMA/CD as a MAC protocol,

Ethernet-style LANs have been developed that operate at 100 Mbps, 1 Gbps, and

10 Gbps. The reasons for this are instructive. From the vendor’s point of view,

the CSMA/CD protocol is well understood and vendors have experience building

the hardware, firmware, and software for such systems. Scaling the system up to

100 Mbps or more may be easier than implementing an alternative protocol and

topology. From the customer’s point of view, it is relatively easy to integrate

older Ethernet systems running at 10 Mbps with newer systems running at higher

speeds if all the systems use the same frame format and the same access protocol.

In other words, the continued use of Ethernet-style LANs is attractive because

Ethernet is already there. This same situation is encountered in other areas of

data communications. Vendors and customers do not always, or even in the majority

of cases, choose the technically superior solution. Cost, ease of management,

and other factors relating to the already-existing base of equipment are often

more important factors in the selection of new LAN equipment than technically

superior alternatives. This is the reason that Ethernet-style systems continue to

dominate the LAN market and show every sign of continuing to do so in the

foreseeable future.

Fast Ethernet refers to a set of specifications developed by the IEEE

802.3 committee to provide a low-cost, Ethernet-compatible LAN operating

at 100 Mbps. The blanket designation for these standards is 100BASE-T.

The committee defined a number of alternatives to be used with different

transmission media.

Table 13.3 IEEE 802.3 100 Mbps Physical Layer Medium Alternatives

	100BASE-TX		100BASE-FX	100BASE-T4
Transmission Medium	2 pair, STP	2 pair, Category 5 UTP	2 optical fibers	4 pair, Category 3, 4, or 5 UTP
Maximum Segment Length	100 m	100 m	100 m	100 m
Network Span	200 m	200 m	400 m	200 m

Table 13.3 summarizes key characteristics of the 100BASE-T options.

All of the 100BASE-T options use the IEEE 802.3 MAC protocol and frame

format. 100BASE-X refers to a set of options that use the physical medium

specifications. All of the 100BASE-X schemes use two physical links between

nodes: one for transmission and one for reception. 100BASE-TX makes use of

shielded twisted pair (STP) or high-quality (Category 5e or higher) UTP. (See

Chapter 12 for a discussion of Category 3 and Category 5 cables.) 100BASE-FX

uses optical fiber.

For all of the 100BASE-T options, the topology is similar to that of 10BASE-T,

namely a star-wire topology.

A traditional Ethernet 10 Mbps bus LAN is half duplex: A station can either

transmit or receive a frame, but it cannot do both simultaneously. With full-duplex

operation, a station can transmit and receive simultaneously. In a 100-Mbps Ethernet

running in full-duplex mode, the theoretical transfer rate becomes 200 Mbps. To

operate in full-duplex mode, the attached stations must have full-duplex adapter

cards; these are now standard equipment in switched Ethernet environments.

The central points in a fully-switched Ethernet network are the switches.

Each station attaches to a switch and the link between the switch and the station

is essentially a separate collision domain. In full-duplex Ethernet LANs, traffic can

be simultaneously flowing in both directions between the station and the switch,

which essentially means that there are no collisions and the CSMA/CD algorithm

is no longer needed. However, the same 802.3 MAC frame format is used and the

attached stations can continue to execute the CSMA/CD algorithm, even though

there are no collisions to be detected.

Gigabit Ethernet

The strategy for Gigabit Ethernet is the same as that for Fast Ethernet. While

defining a new medium and transmission specification, Gigabit Ethernet retains

the CSMA/CD protocol and frame format of its 10-Mbps and 100-Mbps predecessors.

It is compatible with both 100BASE-T and 10BASE-T, preserving a smooth

migration path. Most business organizations have moved to 100BASE-T and many

have jumped to Gigabit Ethernet for at least some of their LANs. These LANs

are putting huge traffic loads on backbone networks, which further increases

demand for Gigabit Ethernet and 10 Gigabit Ethernet.

Strategy is the same as that for Fast Ethernet

Retains the CSMA/CD protocol and frame format of its 10-Mbps and 100-Mbps predecessors

Compatible with both 100BASE-T and 10BASE-T, preserving a smooth migration path

Huge traffic loads on backbone networks increase the demand for Gigabit Ethernet and 10 Gigabit Ethernet

Example Gigabit Ethernet Configuration

Figure 13.8 shows a typical application of Gigabit Ethernet. A 1-/10-Gbps

LAN switch provides backbone connectivity for central servers and high-speed

workgroup switches. Each workgroup LAN switch supports both 1-Gbps links, to

connect to the backbone LAN switch and to support high-performance workgroup

servers, and 100-Mbps links, to support high-performance workstations, servers,

and 100-/1000-Mbps LAN switches.

Gigabit Ethernet Medium Options

The current 1-Gbps specification for IEEE 802.3 includes the following

physical layer alternatives (Figure 13.9):

• 1000Base-Lx: This long-wavelength option supports duplex links of up to

550 m of 62.5-μm or 50-μm multimode fiber or up to 5 km of 10-μm single mode

fiber. Wavelengths are in the range of 1270–1355 nm.

• 1000Base-Sx: This short-wavelength option supports duplex links of up to

275 m using 62.5-μm multimode or up to 550 m using 50-μm multimode fiber.

Wavelengths are in the range of 770–860 nm.

• 1000Base-Cx: This option supports 1-Gbps links among devices located

within a single room or equipment rack, using copper jumpers (specialized STP

cable that spans no more than 25 m). Each link is composed of a separate STP

running in each direction.

• 1000Base-T: This option makes use of four pairs of Category 5 UTP to support

devices over a range of up to 100 m.

10-Gbps Ethernet

Principal driving requirement is the increase in Internet and intranet traffic

This technology:

Enables Internet service providers (ISPs) and network service providers (NSPs) to create very high-speed links at a low cost, between co-located, carrier-class switches and routers

Allows the construction of metropolitan area networks (MANs) and WANs that connect geographically dispersed LANs between campuses or points of presence (PoPs)

In recent years, 10-Gbps Ethernet switches have made considerable inroads in

the LAN market. The principal driving requirement for 10 Gigabit Ethernet is

the increase in Internet and intranet traffic. A number of factors contribute to the

explosive growth in both Internet and intranet traffic:

• An increase in the number of network connections

• An increase in the connection speed of each end-station (e.g., 10-Mbps users

moving to 100 Mbps, analog 56-kbps users moving to DSL and cable modems)

• An increase in the deployment of bandwidth-intensive applications such as

high-quality video

• An increase in Web hosting and application hosting traffic

Initially, network managers are using 10-Gbps Ethernet to provide high-speed,

local backbone interconnection between large-capacity switches. As the demand for

bandwidth increases, 10-Gbps Ethernet will be deployed throughout the entire network

and will include server farm, backbone, and campus-wide connectivity. This

technology enables Internet service providers (ISPs) and network service providers

(NSPs) to create very high-speed links at a low cost, between co-located, carrier-class

switches and routers.

The technology also allows the construction of metropolitan area networks

(MANs) and WANs that connect geographically dispersed LANs between campuses

or points of presence (PoPs). Thus, Ethernet begins to compete with ATM

and other wide area transmission/networking technologies.

10-Gbps Ethernet Versus ATM

Carrier Ethernet, Metro

Ethernet, and Wide Area Ethernet services are increasingly common components

of enterprise networks. In most cases where the primary form of business communications

traffic is data and TCP/IP is the preferred mode of transport, 10-Gbps

Ethernet provides substantial value over ATM transport for both network end

users and service providers:

• No expensive, bandwidth-consuming conversion between Ethernet packets

and ATM cells is required; the network is Ethernet, end to end.

• The combination of IP and Ethernet offers quality of service and traffic policing

capabilities that approach those provided by ATM, so that advanced traffic

engineering technologies are available to users and providers.

• A wide variety of standard optical interfaces (wavelengths and link distances)

have been specified for 10-Gbps Ethernet, optimizing its operation and cost

for LAN, MAN, or WAN applications.

No expensive, bandwidth consuming conversion between Ethernet packets and ATM cells is required

The combination of IP and Ethernet offers quality of service and traffic policing capabilities that approach those provided by ATM

A wide variety of standard optical interfaces have been specified for 10-Gbps Ethernet, optimizing its operation and cost for LAN, MAN, or WAN applications

10-Gbps Ethernet Distance Options

Maximum link distances for 10-Gbps Ethernet ranges from 300 m to 40 km.

The links operate in full-duplex mode only, using a variety of optical fiber physical

media. Four physical layer options are defined for 10-Gbps Ethernet (Figure 13.10):

• 10GBASE-S (short): Designed for 850-nm transmission on multimode fiber.

This medium can achieve distances up to 300 m.

• 10GBASE-L (long): Designed for 1310-nm transmission on single-mode fiber.

This medium can achieve distances up to 10 km.

• 10GBASE-E (extended): Designed for 1550-nm transmission on single-mode

fiber. This medium can achieve distances up to 40 km.

• 10Gbase-Lx4: Designed for 1310-nm transmission on single-mode or multimode

fiber. This medium can achieve distances up to 10 km. This medium uses

wavelength-division multiplexing (WDM) to multiplex the bit stream across

four light waves.

Market Drivers for 100-Gbps Ethernet

Ethernet is widely deployed and is the preferred technology for wired local

area networking. Ethernet dominates enterprise LANs, broadband access,

and data center networking, and has also become popular for communication

across MANs and even WANs. Further, it is now the preferred carrier wire line

vehicle for bridging wireless technologies, such as Wi-Fi and WiMAX, into local

Ethernet networks.

This popularity of Ethernet technology is due to the availability of cost effective,

reliable, and interoperable networking products from a variety of vendors.

Over the years, a number of industry consortiums have participated in the development

of ever-faster versions of Ethernet, including the Fast Ethernet Alliance

(100 Mbps), the Gigabit Ethernet Alliance, the 10 Gigabit Ethernet Alliance, the

Ethernet Alliance, and the Road to 100G Alliance. As a testament to the continuing

evolution of Ethernet, the first three of the alliances just mentioned no longer

exist. The Ethernet Alliance is devoted to promoting the development of Ethernet,

whatever the speed. The Road to 100G Alliance is focused on the development of

standards and technologies for 100-Gbps Ethernet.

As this alliance evolution reflects, the development of converged and unified

communications, the evolution of massive server farms, and the continuing

expansion of VoIP, TVoIP, and Web 2.0 applications have driven the need for

ever-faster Ethernet switches. [HUFF06] lists the following market drivers for

100-Gbps Ethernet:

• Data center/Internet media providers: To support the growth of Internet

multimedia content and Web applications, content providers have been

expanding data centers, pushing 10-Gbps Ethernet to its limits. Likely to be

high-volume early adopters of 100-Gbps Ethernet.

• Metro-video/service providers: Video on demand has been driving a new

generation of 10-Gbps Ethernet metropolitan/core network buildouts. Likely

to be high-volume adopters in the medium term.

• Enterprise LANs: Continuing growth in convergence of voice/video/data and

in unified communications is driving up network switch demands. However,

most enterprises still rely on 1-Gbps or a mix of 1-Gbps and 10-Gbps Ethernet,

and adoption of 100-Gbps Ethernet is likely to be slow.

• Internet exchanges/ISP core routing: With the massive amount of traffic

flowing through these nodes, these installations are likely to be early adopters

of 100-Gbps Ethernet.

Data center/Internet media providers

To support the growth of Internet multimedia content and Web applications, content providers have been expanding data centers, pushing 10-Gbps Ethernet to its limits

Metro-video/service providers

Video on demand has been driving a new generation of 10-Gbps Ethernet metropolitan/core network buildouts

Enterprise LANs

Continuing growth in convergence of voice/video/data and in unified communications is driving up network demands

Internet exchanges/ISP core routing

With the massive amount of traffic flowing through these nodes, these installations are likely to be early adopters of 100-Gbps Ethernet

Table 13.4 Media Options for 40-Gbps and 100-Gbps Ethernet

	40 Gbps	100 Gbps
1-m backplane	40GBASE-KR4
10-m copper	40GBASE-CR4	1000GBASE-CR10
100-m multimode fiber	40GBASE-SR4	1000GBASE-SR10
10-km single-mode fiber	40GBASE-LR4	1000GBASE-LR4
40-km single-mode fiber		1000GBASE-ER4

Naming nomenclature:

Copper: K = backplane; C = cable assembly

Optical: S = short reach (100m); L = long reach (10 km); E = extended long reach (40 km)

Coding scheme: R = 64B/66B block coding

Final number: number of lanes (copper wires or fiber wavelengths)

In 2007, the IEEE 802.3 working group authorized the IEEE P802.3ba 40Gb/s

and 100Gb/s Ethernet Task Force . Table 13.4 indicates the physical layer objectives

for this task force. As can be seen, these high-speed switches will be standardized to

operate at distances from 1 m to 40 km over a variety of physical media.

Example 100-Mbps Ethernet Configuration for Massive Blade Server Site

An example of the application of 100-Gbps Ethernet is shown in Figure 13.11,

taken from [NOWE07]. The trend at large data centers, with substantial banks of

blade servers, is the deployment of 10-Gbps ports on individual servers to handle

the massive multimedia traffic provided by these servers. Such arrangements

are stressing the on-site switches needed to interconnect large numbers of servers.

A 100-Gigabit Ethernet rate was proposed to provide the bandwidth required to

handle the increased traffic load. It is expected that 100 Gigabit Ethernet will be

deployed in switch uplinks inside the data center as well as providing interbuilding,

intercampus, MAN, and WAN connections for enterprise networks.

The success of Fast Ethernet, Gigabit Ethernet, and 10-Gbps Ethernet highlights

the importance of network management concerns in choosing a network

technology. Both ATM and Fiber Channel, explored subsequently, may be technically

superior choices for a high-speed backbone, because of their flexibility and

scalability. However, the Ethernet alternatives offer compatibility with existing

installed LANs, network management software, and applications. This compatibility

has accounted for the survival of 30-year-old technology in today’s fast evolving

network environment.

LAN Configuration

Figure 13.12 shows a relatively common type of hierarchical LAN configuration. In

this example, the devices on the LAN are organized into four groups, each served

by a LAN switch. The three lower groups might correspond to different departments,

which are physically separated, and the upper group could correspond to a

centralized server farm that is used by all the departments.

Let us consider the transmission of a single MAC frame from workstation

X. Suppose the destination MAC address in the frame (see Figure 12.9) is

workstation Y. This frame is transmitted from X to the local switch, which then

directs the frame along the link to Y. If X transmits a frame addressed to Z or W,

then its local switch routes the MAC frame through the appropriate switches to

the intended destination. All these are examples of unicast addressing , in which

the destination address in the MAC frame designates a unique destination.

A MAC frame may also contain a broadcast address , in which case the destination

MAC address indicates that all devices on the LAN should receive a copy of the

frame. Thus, if X transmits a frame with a broadcast destination address, all of the

devices on all of the switches in Figure 13.12 receive a copy of the frame. The total

collection of devices that receive broadcast frames from each other is referred to

as a broadcast domain .

In many situations, a broadcast frame is used for a purpose, such as network

management or the transmission of some type of alert, that has a relatively local

significance. Thus, in Figure 13.12, if a broadcast frame has information that is only

useful to a particular department, then transmission capacity is wasted on the other

portions of the LAN and on the other switches.

A Partitioned LAN

One simple approach to improving efficiency is to physically partition the LAN

into separate broadcast domains, as shown in Figure 13.13. We now have four separate

LANs connected by a router. In this case, an IP packet from X intended for Z is

handled as follows. The IP layer at X determines that the next hop to the destination

is via router V. This information is handed down to X’s MAC layer which prepares

a MAC frame with a destination MAC address of router V. When V receives the

frame, it strips off the MAC header, determines the destination, and encapsulates

the IP packet in a MAC frame with a destination MAC address of Z. This frame is

then sent to the appropriate Ethernet switch for delivery.

The drawback to this approach is that the traffic pattern may not correspond to

the physical distribution of devices. For example, some departmental workstations

may generate a lot of traffic with one of the central servers. Further, as the networks

expand, more routers are needed to separate users into broadcast domains and provide

connectivity among broadcast domains. Routers introduce more latency than

switches because the router must process more of the packet to determine destinations

and route the data to the appropriate end node.

A VLAN Configuration

A more effective alternative is the creation of virtual LANs (VLANs). In essence,

a VLAN is a logical subgroup within a LAN that is created by software rather

than by physically moving and separating devices. It combines user stations and

network devices into a single broadcast domain regardless of the physical LAN

segment they are attached to and allows traffic to flow more efficiently within populations

of mutual interest. The VLAN logic is implemented in LAN switches and

functions at the MAC layer. Because the objective is to isolate traffic within the

VLAN, in order to link from one VLAN to another, a router is required. Routers

can be implemented as separate devices, so that traffic from one VLAN to another

is directed to a router, or the router logic can be implemented as part of the LAN

switch, as shown in Figure 13.14.

VLANs provide the ability for any organization to be physically dispersed

throughout the company while maintaining its group identity. For example, accounting

personnel can be located on the shop floor, in the research and development

center, in the cash disbursement office, and in the corporate offices, while at

the same time all members reside on the same virtual network, sharing traffic only

with each other.

In Figure 13.14, four VLANs are defined. A transmission from workstation

X to server Z is within the same VLAN, so it is efficiently switched at the MAC

level. A broadcast MAC frame from X is transmitted to all devices in all portions of

the same VLAN. But a transmission from X to printer Y goes from one VLAN to

another. Accordingly, router logic at the IP level is required to move the IP packet

from X to Y. In Figure 13.14, that logic is integrated into the switch, so that the

switch determines whether or not the incoming MAC frame is destined for another

device on the same VLAN. If not, the switch routes the enclosed IP packet at the

IP level.

The figure also includes one “legacy” switch that does not implement VLAN

software. In this case, all of the end systems of the legacy device must belong to the

same VLAN, because the legacy switch is unable to recognize traffic differentiated

by VLAN.

Communicating VLAN Membership

Switches must have a way of understanding which stations belong to which VLAN

Frame tagging

A header is typically inserted into each frame on interswitch trunks to uniquely identify to which VLAN a particular MAC-layer frame belongs

IEEE 802.1Q is the standard the IEEE 802 committee has developed for frame tagging

Switches must have a way of understanding VLAN membership (i.e., which

stations belong to which VLAN) when network traffic arrives from other

switches; otherwise, VLANs would be limited to a single switch. One possibility

is to configure the information manually or with some type of network management

signaling protocol, so that switches can associate incoming frames with the

appropriate VLAN.

A more common approach is frame tagging, in which a header is typically

inserted into each frame on interswitch trunks to uniquely identify to which

VLAN a particular MAC-layer frame belongs. The IEEE 802 committee has

developed a standard for frame tagging, IEEE 802.1Q, which we examine in the

next section.

IEEE 802.1Q VLAN Standard

Defines the operation of VLAN bridges and switches that permits the definition, operation, and administration of VLAN topologies within a bridged/switched LAN infrastructure

Traffic between switches must indicate LAN membership

A tag is inserted with a VLAN identifier (VID) with a value in the range of 1 to 4094

Each VLAN in a LAN configuration is assigned a globally unique VID

The tag also contains a priority level

The IEEE 802.1Q standard, last updated in 2005, defines the operation of VLAN

bridges and switches that permits the definition, operation, and administration of

VLAN topologies within a bridged/switched LAN infrastructure. In this section, we

will concentrate on the application of this standard to 802.3 LANs.

Recall that a VLAN is an administratively configured broadcast domain,

consisting of a subset of end stations attached to a LAN. A VLAN is not limited

to one switch but can span multiple interconnected switches. In that case, traffic

between switches must indicate VLAN membership. This is accomplished in

802.1Q by inserting a tag with a VLAN identifier (VID) with a value in the range

from 1 to 4094. Each VLAN in a LAN configuration is assigned a globally unique

VID. By assigning the same VID to end systems on many switches, one or more

VLAN broadcast domains can be extended across a large network. The tag also

contains a priority level.

Figure 13.14 illustrates a LAN configuration that includes three switches that

implement 802.1Q and one “legacy” switch or bridge that does not. The MAC

frames that traverse trunks between VLAN-aware switches include the 802.1Q tag.

This tag is stripped off before a frame is routed to a legacy switch. For end systems

connected to a VLAN-aware switch, the MAC frame may or may not include the

tag, depending on the implementation. The important point is that the tag is used

between VLAN-aware switches so that appropriate routing and frame handling can

be performed.

Power Over Ethernet (PoE)

Makes it possible to distribute both power and data using Ethernet cabling

The IEEE standard requires Category 3 cable for low power levels and Category 5 cable or higher for high power levels

Uses two or more twisted pairs in the Ethernet cable to distribute power, the other pairs are used to transmit data

PoE switches and DC battery arrays are the primary power supplies in PoE networks

Power over Ethernet (PoE) is another form of Ethernet that is increasingly found

in enterprise networks. As the name suggests, PoE makes it possible to distribute

both power and data using Ethernet cabling. The IEEE standard for PoE requires

Category 3 cable for low power levels and Category 5 cable or higher for high power

levels. PoE uses two or more twisted pairs in the Ethernet cable to distribute power;

the other pairs are used to transmit data. PoE switches and DC battery arrays are

the primary power supplies in PoE networks.

Power Over Ethernet (PoE)

PoE provides network designers with more flexibility for deploying LAN

devices. In many instances, PoE eliminates the need to locate equipment close to

AC power sources. As is illustrated in Figure 13.15, PoE can be used to power VoIP

phones, Wi-Fi access points, and LCD monitors. PoE can also be used to distribute

power to IP surveillance cameras, Web cams, industrial devices (such as sensors,

controllers, and meters), lighting controllers, remote network switches, access control

devices (such as keyless entry and intercom systems), and remote POS (point

of sale) kiosks. Industry pundits predict that PoE will increasingly be used to power

the motherboards of network-attached workstations.

Advantages of PoE

There are numerous advantages of PoE. For example, PoE can be used in

situations where AC power would be expensive, infeasible, or inconvenient to use

to power network devices. While USB could also be an option in such situations,

PoE is often a superior choice to either USB or AC wire.

• It can be deployed over Category 5 UTP, which is less expensive than using

USB repeaters or AC wire.

• Gigabit connections are possible; this is faster than USB 2.0 and AC power

line networking capabilities. Furthermore, a 10-Gbps PoE standard is being

developed.

• Companies with multiple international locations can deploy PoE everywhere

without regard to local differences in AC power levels, plugs, outlets, or

reliability. It can be deployed in buildings without having to worry about AC

cabling building codes.

Like other versions of Ethernet, PoE standards continue to evolve. The original

IEEE 802.3af-2003 PoE standard provides up to 15.4 W of direct current (DC) power

(or a minimum of 44 V DC and 350 mA) to each device. This standard only assures

12.95 W to each powered device because some power is dissipated in the cable.

Since the release of the original PoE standard, work has continued on both

lower and higher power PoE systems. The IEEE 802.3af working group is working

on an Energy Efficient Ethernet standard that is expected to consume 60% less

electricity than the original PoE standard. Another committee is working on standards

for PoE networks that deliver more power to devices. The IEEE 802.3at-2009

PoE standard provides up to 25.5 W of power; this standard is also known as PoE+.

A 60 W standard of PoE is also in the works.

Higher power levels are likely to result in PoE being used to power workstation

motherboards. However, with more power being distributed through Ethernet

cables, appropriate accommodations in structured cabling schemes must be made.

Using PoE with cable bundles is sometimes thought to be a fire hazard because of

the heat generated as power is distributed from PoE switches to network attached

devices. While this cannot be ruled out, there is not a great deal of supporting evidence.

However, with high-power PoE, it may be wise to separate cables within

cables trays and conduit to facilitate heat dissipation. It is also wise to use Category

6, 6a, and 7 cables because these are superior than Category 5e in dissipating heat.

Adding temperature-sensing thermocouples to PoE cable bundles may also be a

wise infrastructure investment.

Can be used in situations where AC power would be expensive, infeasible, or inconvenient to use to power network devices

It can be deployed over Category 5 UTP which is less expensive than using USB repeaters or AC wire

Gigabit connections are possible

Companies with multiple international locations can deploy PoE everywhere without regard to local differences in AC power levels, plugs, outlets, or reliability

It can be deployed in buildings without having to worry about AC cabling building codes

Summary

Traditional Ethernet

Bus topology LAN

Medium access control

MAC frame

IEEE 802.3 medium options at 10 Mbps

Bridges

Hubs

Layer 2 switches

Layer 3 switches

Chapter 13: Ethernet, Switches, and Virtual LANs

High-speed Ethernet

Fast Ethernet

Gigabit Ethernet

10-Gbps Ethernet

100-Gbps Ethernet

Virtual LANs

The use of virtual LANs

Communicating VLAN membership

IEEE 802.1Q VLAN standard

PoE

Chapter 13 summary.

C transmits frame addressed to A

Frame is not addressed to B; B ignores it

A copies frame as it goes by

Figure 13.1 Frame Transmission on a Bus LAN

B C

A B C

Bus Frame

Figure 13.2 CSMA/CD Operation

A DC

A's transmission

C's transmission

Signal on bus

TIME t 0

A's transmission

C's transmission

Signal on bus

TIME t 1

A's transmission

C's transmission

Signal on bus

TIME t 2

A's transmission

C's transmission

Signal on bus

TIME t 3

Preamble

Start of Frame Delimiter

Destination Address

Source Addresss

Length/Type

MAC Client Data

octets transmitted

top to bottom

bits transmitted left to right

fr am

pa ck

et Pad

Figure 13.3 IEEE 802.3 MAC Frame Format

Frame Check Sequence

Extension

7 octets

1 octets

6 octets

2 octets

46 to 1500 or 1504

or 1982 octets

4 octets

LSB MSB

LAN A

LAN B

Bridge

Figure 13.4 Bridge Operation

Frames with

addresses 11 through

20 are accepted and

repeated on LAN B

Frames with

addresses 1 through

10 are accepted and

repeated on LAN A

Station 1 Station 2 Station 10

Station 11 Station 12 Station 20

Station Station Station Station

Station

HHUB

Figure 13.5 Two-Level Star Topology

IHUBIHUB

Two cables

(twisted pair or

optical fiber)

Transmit

Receive

Shared Bus - 10 Mbps

1 0 M

b p

1 0 M

b p

1 0 M

b p

1 0 M

b p

A B C D

(a) Shared medium bus

A B C D

(b) Shared medium hub

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacity

up to 10 Mbps

A B C D

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacity

N 10 Mbps

Figure 13.6 LAN Hubs and Switches

WAN

Router

Layer 3 switch

Layer 2 switch

100 Mbps

Figure 13.7 Typical Premises Network Configuration

54 Mbps

1 Gbps

10 Gbps

Laptop with wireless connection

Layer 2 switch

1 Gbps switch

100/1000-Mbps Switches

Central Servers

Workgroup Workgroup

Figure 13.8 Example Gigabit Ethernet Configuration

100 Mbps link

1 Gbps link

50 m25 m 500 m 5000 m250 m

Maximum distance

Figure 13.9 Gigabit Ethernet Medium Options (log scale)

2500 m

10-µm single-mode fiber

50-µm multimode fiber

62.5-µm multimode fiber

50-µm multimode fiber

62.5-µm multimode fiber

Category5 UTP

Shielded cable

1000BASE-LX

1000BASE-SX

1000BASE-T

1000BASE-CX

100 m 300 m10 m 1 km 10 km 40 km 100 km

Maximum distance

50-µm multimode fiber

62.5-µm multimode fiber

10GBASE-S

(850 nm)

Figure 13.10 10-Gbps Ethernet Distance Options (log scale)

10GBASE-L

(1310 nm)

Single-mode fiber

10GBASE-E

(1550 nm)

Single-mode fiber

50-µm multimode fiber10GBASE-LX4

(1310 nm) 62.5-µm multimode fiber

N 100GbE

100GbE

10GbE &

40GbE

Figure 13.11 Example 100-Gbps Ethernet Configuration for Massive Blade Server Site

Figure 13.12 A LAN Configuration

Inaho for takeout. Love to All Tricia

X Y

Internet

Server

Router Ethernet

switch

Printer

Workstation

Figure 13.13 A Partitioned LAN

Internet

Server

Router Ethernet

switch

Printer

Workstation

X Y

Figure 13.14 A VLAN Configuration

Internet

VLAN A

VLAN C

VLAN A

VLAN B

VLAN D

Server

Ethernet switch with VLAN and IP routing capability

Legacy switch/bridge

Printer

X Y

Workstation

Laptop Computers

VoIP Phones

WiFi Access Points

Desktop Computers/Thin Clients

Figure 13.15 Power over Ethernet (PoE)

Power over Ethernet Switch

Category 5e, 6, 6a, or 7 UTP Distributes Power and Conveys Data