Network Design

In module 1, we discussed the importance of defining standard communication models to ensure that different types of devices can communicate (share information). In this module, we will take a more detailed look at those models and create an understanding of how devices are found within our networks and on the World Wide Web.

Each device has a unique Internet Protocol (IP) address, much like a street address, that allows packets of information to be labeled with the sending IP address and destination IP address. Once the packet has been labeled properly, it is sent out across the network through network devices that route the packet to its final destination. IP addresses are numeric in form and work well for computers, but they are hard for humans to manage and remember. The Domain Name System (DNS) was developed to allow us to represent network devices with names instead of numeric IP addresses. The web address www.umuc.edu will be translated by DNS to the appropriate numeric IP address, and that IP address will be sent back to the computer making the request. Once the computer receives the correct IP address, a connection request is initiated, and the two devices start to communicate.

The primer "How the Internet Works" will provide additional detail on how IP addressing and DNS work together. It will walk you through the reference models we introduced in module 1, the OSI model and TCP/IP.

Now that we know how the Internet works, we can begin to take a look at the various components that are used to build telecommunications networks. The next section will start at layer 1 of the OSI model and discuss the connectors and media that are used to start connecting and building our networks.

When we prepare to send information across a network, the data travel down the layers of the OSI model and are converted to a signal at layer 1, the physical layer. Once they are converted to a signal, the data are sent out using network media, conducted or wireless, to their final destination.

Signals

A signal is a mechanism that is used to convey data from one place to another, and it can be in either analog or digital form. In telephony, a signal is the exchange of information between involved points in the network that sets up, controls, and terminates each telephone call. In electronics, a signal is an electric current or electromagnetic field. For example, phones today generally display the signal level by using bars, which indicate the strength of the signal based on your location.

Analog

Analog means continuous; that is, a set of specific points and all the points between them. A good example of an analog device is a watch or clock with an hour hand, a minute hand, and possibly even a second hand. When someone looks at this type of clock or watch, the hour hand does not point exactly to the hour—it points somewhere between the present hour and the next hour. The same is true for the minute hand and minutes, and the second hand and seconds. All three hands are moving continuously. In some situations, an analog phone line may need to be used to transmit data using a device called a modem. The data are converted from the system, which by default only processes and understands in digital format, to analog so they can be transmitted by analog means (modulated). Once they are received at their intended destination, they are converted back to digital (demodulated) so they can be processed and understood by a computer. This is less common today due to the prevalence of native digital networks, but in cases where digital networks have not been established, data transmission using analog means is still possible and feasible.

Digital

Digital means discrete; that is, a set of specific points and no points between them. A good example is a digital clock. It shows the exact hour, minute, second, and possibly tenths or hundredths of a second, but it cannot indicate any time between these discrete states. Figure 2.1(a) shows an analog signal of continuously varying signal strength, where signal strength is shown as amplitude. Figure 2.1(b) shows a digital signal with two discrete values.

Figure 2.1 Analog and Digital Signals

It is possible to use analog signals to transmit either analog or digital information. Likewise, it is possible to use digital signals to transmit either analog or digital information as discussed by using a modem in which the data are converted from digital to analog to be transmitted and from analog to digital once they are received to be processed.

When a signal repeats a pattern over a measurable time frame, it is said to be periodic. The time in seconds (T) that it takes to complete the pattern is called the period, and the completion of one full period is called a cycle. The number of cycles in a second is called the frequency (f). Frequency is measured in cycles per second, or hertz (Hz).

The relationship between period (T) and frequency (f) is

T = 1/f

Example 2.1 shows the relationship between period and frequency.

Example 2.1 Period and Frequency

If the period = 1 ms (millisecond) or .001 seconds, the frequency is 1/.001, or 1,000 cycles/second (Hz). If the frequency is 100Hz, the period is 1/100 = .01 seconds.

If a signal does not repeat itself over time, as with either of the signals in figure 2.1, it is said to be a periodic. Signals that repeat themselves, such as those shown in figure 2.2, are said to be periodic. The signals in figure 2.2 repeat themselves over the three cycles that are shown.

Figure 2.2 Periodic Analog and Digital Signals

When data are represented by a digital signal, there are usually two voltage levels, with one level representing a 1 and the other level representing a 0. In a system where the positive level is a 1 and the negative level is a 0, the seven-bit ASCII code for the letter S is 1010011. Figure 2.3 shows the digital signal representing that bit pattern.

Figure 2.3 Digital Signal for 1010011

Computers use digital signals. Signals in computers might be electrical voltages for copper cable, pulses of light for fiber optic, or infrared/radio waves for wireless networking. In a typical communication, the sending computer uses signals to transform binary data into a form of code that the receiving computer can interpret back (to binary data).

Conducted Media

There are three primary types of cable used to build LANs:

1. coaxial

2. twisted pair

3. fiber optic

Coaxial and twisted pair are copper-based and carry electrical signals, and fiber-optic cables use glass or plastic fibers to carry light signals.

Coaxial Cables

Coaxial cable has two conductors, one inside the other, within a sheath. A central core, made of either a solid copper wire or braided strands of copper, carries the signal. A second conductor made of braided copper mesh surrounds the core, with insulation in between. This second conductor functions as the cable's ground. Again, the entire assembly is encased in an insulating sheath.

Coaxial cable, which provides protection from noise interference, has been used for long-distance telephone transmission, cable television, and cabling in a local area network. There are two types of coaxial cable used in LANs—RG-8 and RG-58—which are similar in construction but vary in thickness and the types of connectors they use.

RG-8 is a thicker cable (0.405 inches in diameter), and the network that uses this medium is sometimes called the thick Ethernet. RG-8 cable usually runs along a floor to create a trunk, and separate AUI (attachment unit interface) cables run from the trunk to the network interface adapters in the computer.

RG-58 is a thinner cable (0.195 inches in diameter), and the network that uses this medium is sometimes called the thin Ethernet.

Thick Ethernet and thin Ethernet are also called 10Base5 and 10Base2, respectively, because they run at 10 Mbps, use baseband transmission, and are limited to maximum cable segment lengths of 500 and 200 (actually 185) meters, respectively.

Figure 2.4(a) Components of a Coaxial Cable

Figure 2.4(b) A Coaxial Cable Terminator Plug, Showing the Center Conductor

Source: User: Colin. [Photo of F connector]. Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Coaxial Cable Connectors

Coaxial cable connectors depend on the type (RG) of the cable. Figure 2.5 shows the type of connectors used with thin Ethernet— BNC (Bayonet Neill-Concelman) connectors—and thick Ethernet (transceivers).

Figure 2.5 Coaxial Cable Connectors

Source: User: Kb. "BNC connectors used to connect video card to monitor." Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Twisted-Pair Cables

Twisted-pair cables comprise pairs of twisted wires. Shielded twisted-pair (STP) cable provides an extra layer of isolation from unwanted electromagnetic interference, while in unshielded twisted-pair (UTP) cable, each conductor is a separate insulated wire. Twisted-pair cables are twisted to prevent the signals on the different wire pairs from interfering with each other (called crosstalk) and to reduce electrical interference from outside sources, such as motors. There are usually 2 to 12 twists per foot, and a higher number of twists provides higher quality. The multiple wire pairs are encased in a single sheath, and the different colors allow users to identify the different wires in the bundle.

The Electronic Industries Alliance (EIA) and the Telecommunications Industry Association (TIA) have developed EIA/TIA rating standards for UTP, ranking them from Category 1 to Category 7, as shown below.

· Category 1 is used for telephone systems. It is good for voice and low-speed data communications at up to 9,600 bps over distances of up to four miles.

· Category 2 is suitable for data transmission at up to 4 Mbps at distances of up to four miles.

· Category 3 can be used both in voice-grade telephone networks and for data transmission. The principal users of this type of cable are 10-Mbps Ethernet and 4-Mbps Token Ring. Two out of four of the wire pairs are used in a typical connection; however, by using all four wire pairs, this same cable can be used for 100-Base-T4 Fast Ethernet and 100-Base-VG-AnyLAN. A single segment of this cable can carry a signal up to 100 meters.

· Category 4 can be used for data transmission at up to 20 Mbps at distances of up to 100 meters. It is used mostly in 16-Mbps Token Ring networks.

· Category 5 can be used in 100Base-TX Fast Ethernet, Synchronous Optical Network (SONET), and Optical Carrier (OC3) Asynchronous Transfer Mode (ATM) for data transmission at up to 100 Mbps at distances of up to 100 meters.

· Category 5e is an extended version of Category 5 with tighter specifications, and is suitable for Gigabit Ethernet in 1000Base-T.

· Category 6 is under development and will probably support speeds of up to 200 Mbps at distances of up to 100 meters.

· Category 7 is also under development and may support speeds of 600 Mbps at distances of up to 100 meters.

Shielded twisted-pair wire is available in Categories 1 through 5. The main difference between UTP and STP is that STP has only two wire pairs and has foil and mesh shielding around each pair that provides additional protection from interference. STP also provides a higher level of security than UTP because it reduces electrical emissions that can be detected outside the cable. See figure 2.6 for an illustration of twisted-pair cable.

Figure 2.6 Twisted-Pair Cable

Twisted-Pair Connectors

Both STP and UTP use RJ-45 telephone connectors. These are similar to RJ-11 telephone connectors. Although RJ-11 and RJ-45 connectors look alike at first glance, they are very different. The RJ-45 is slightly larger and will not fit into an RJ-11 telephone jack. The RJ-45 also houses eight cable connections, while the RJ-11 houses only four. Figure 2.7(a) and figure 2.7(b) show the difference between the RJ-45 and RJ-11 connectors.

Figure 2.7(a) RJ-45 Plug and Jack

Source: User: Sylvain Leroux. "8P8C with T568B wiring (RJ45) plug and jack." Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Figure 2.7(b) RJ-11 Plug and Jack

Source: User: Sylvain Leroux. "6P2C (RJ11) plug and jack." Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Fiber-Optic Cable

Fiber-optic cable has a core made of glass or plastic that carries light signals. A plastic cladding that reflects the light surrounds the core, and both the core and cladding are wrapped in a protective buffer and an outer jacket. A fiber-optic cable carries light in only one direction, and a pair of cables is required for two-way transmission.

Fiber-optic cable is completely resistant to electromagnetic interference and is less subject to attenuation—the tendency of a signal to weaken as it travels over a cable or other medium. It can span a distance of up to 120 kilometers without excessive signal degradation. Fiber-optic cable is used in high-security applications because it does not emit signals that could be detected outside the cable.

There are two primary types of fiber-optic cable, single-mode and multimode, with the thickness of the core and the cladding being the main difference between them.

Single-mode transmission uses a thin fiber-optic cable that is only 8.3 microns wide, with a cladding of 125 microns. This is called 8.3/125 single-mode fiber. The narrow fiber core requires a highly focused light source, such as a single-wavelength laser, but it can transmit at high rates over longer distances and is more commonly found in outdoor installations that span long distances, such as telephone and cable television networks.

Most multimode transmission uses a fiber-optic cable width of 62.5 microns with a cladding of 125 microns, and thus is called 62.5/125 fiber. This mode can use a light-emitting diode (LED) rather than a laser as a light source, and it carries multiple wavelengths, but it is limited to short-distance use.

Fiber-optic cables use one of two connectors: the straight-tip (ST) connector or the subscriber connector (SC). Fiber-optic cable is more expensive than twisted-pair or coaxial cable. Fiber-optic cable is illustrated in figure 2.8(a) and figure 2.8(b).

Figure 2.8(a) Components of Fiber-Optic Cable

Figure 2.8(b) Fiber-Optic Cable

Source: User: Hustvedt. [Photo of a TOSLINK fiber-optic cable]. Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Fiber-Optic Connectors

Straight-tip (ST) connectors appear as illustrated in figure 2.9(a), and they connect by inserting and twisting. Subscriber connectors, or SC connectors, appear as illustrated in figure 2.9(b), and they attach by inserting until a click is heard or felt.

Special care should be taken with the tips of any type of fiber-optic connector. Ensure that you never touch the ends or let them come in contact with any item other than a connector. Always keep the end caps on your fiber ends; this will prevent many connectivity problems. As a safety precaution, never look through the end of a fiber connector; the laser light that is being emitted can cause damage to your eyes.

Figure 2.9(a) Fiber-Optic ST Connector

Source: User: Ytrottier. "ST optical fiber connector." Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Figure 2.9(b) Fiber-Optic SC Connector

Source: User: Adamantios. "SC optical fiber connector." Used under Creative Commons Attribution-Share Alike 3.0 Unported license.

Although it is generally considered more secure than some other media, fiber-optic cable is still susceptible to tapping, so it is really important to ensure that physical protection mechanisms are in place to protect the cable.

Wireless Media

Wireless technologies, such as terrestrial and satellite microwave, have been used for years to provide connectivity over large geographic areas; however, many of the solutions have been very expensive. Over the past decade, wireless technologies have improved in reliability, cost, and security, making it the technology of choice when designing new networks.

As we discussed in module 1, the Federal Communications Commission is responsible for managing the radio frequency spectrum. Each different type of wireless medium uses the same basic technology, but with a different set of assigned frequencies. In this section, we will discuss some of the more common types of wireless media.

Microwave

A microwave transmission uses a microwave link that transmits tightly focused beams of radio signals with a wavelength ranging from 300 mm to 10 mm (1 GHz to 30 GHz) to send and receive microwave signals. The microwave links can be made up of a series of microwave radio antennas located on top of buildings or mountains. The microwave signals consist of tightly focused beams of radio signals. Each microwave link must be in sight of other links in order to transmit the signal. This is known as line-of-sight transmission and is required for many wireless technologies. Microwave technology is capable of sending a signal 20 to 30 miles.

An example of an application for microwave technology would be a set of office buildings that are within the same general geographic area, but the distance between each building is too far to run a dedicated line. As long as there is a clear line of sight between the office buildings, a microwave antenna could be placed at each end, and signals could be shared.

Figure 2.10 Microwave Transmission

Source: User: GeographBot. (2008). [Photo of Goosemoor transmitter tower]. Used under Creative Commons Attribution-Share Alike 2.0 Generic license.

Satellite

Satellite communications allow communication to take place around the world. Satellite technology is very similar to microwave. The major difference is that instead of the signal traveling from one land-based link to the next, the transmission is sent to a satellite in space (this is called uplinking) and sent back down to the receiving ground station on Earth. Satellite transmissions also have a line-of-sight requirement, but they have the advantage of being able to send signals a much greater distance around the world. A key use of satellite technology is to reach areas that cannot feasibly be reached with some type of conducted medium because of the difficulty of digging ditches and tunnels to run cable. Imagine trying to transmit data to villages in the mountains of Tibet. It would take years and quite a lot of money to provide cable capability over the mountains, but satellite communications would only require the equipment to uplink and the receiving ground station. In the case of television, satellite transmission is a means of sending a signal to areas that cable TV cannot reach practicably. See figure 2.10 for an illustration of a satellite ground station.

Figure 2.11 Satellite Transmission

Infrared

When you think of infrared technology, the first example that might enter your mind is a TV remote control. Infrared is also a line-of-sight transmission technology that sends a focused ray of light (infrared) a very short distance. Most applications of infrared technology for computers are used to connect wireless peripherals (e.g., a keyboard or a mouse).

Bluetooth

Bluetooth does not need to be in line of sight. It uses low-power radio communication to make its wireless link. Bluetooth was designed for providing simple wireless connectivity for computer peripherals and personal devices, including cell phones. Bluetooth has the advantage over infrared of being able to transmit over a longer range, up to 30 feet, and it has the ability to connect to more than one device.

Wireless Networking