NET 121b: Essentials of Networking

Chapter 5: Network Cabling and Devices


This chapter discusses cables used on various kinds of networks. The topics of this chapter are:

  1. Bandwidth usage and dialog methods
  2. Cable types and connectors
  3. Specific devices on Ethernets and token rings

This chapter begins with a concept that belongs to the Physical layer of the OSI model: bandwidth use. The bandwidth topic is split into two methods for using the bandwidth of a medium.

  • Baseband - this method uses the entire bandwidth of the medium to send one signal at a time. This means one transmitter at a time, so users have to take turns or fight for access.
  • Broadband - this method uses separate channels in a medium, such as frequencies, to send multiple signals simultaneously. User have little need to wait for access in this kind of system.

The next topic, Dialog Control, belongs on the Session layer of the OSI model, but it relates to how cables are used on a network. It concerns three ways a dialog can be conducted:

  • Simplex - this is communication in one direction, more like a monologue than a dialog. It is compared to a public speech or a television transmission.
  • Half-Duplex - this is a dialog that can flow both directions, but only one direction at a time. After one side transmits, the channel has to be "reversed" for the other side to transmit. It is like CB or Ham radio, using only one frequency at a time and taking turns.
  • Full-Duplex - this is a dialog in which both sides can transmit and receive at the same time. It is like a telephone conversation, in which both sides have a live earphone and microphone.

Computers use electric currents and various forms of electromagnetic waves to communicate. We can class networks as being cable or wireless, for obvious reasons. We will discuss five attributes for each type of medium in this chapter:

  • Cost - which media cost more or less than others
  • Ease of installation - how easy is it to set up
  • Capacity - also called bandwidth, this means how much data can be on the net at once
  • Attenuation - When a signal passes along a medium, it tends to fade, or attenuate, over distance. We compare media to see which ones have better (longer) attenuation rates.
  • Immunity from EMI and RFI - Electromagnetic Interference happens when your medium picks up static, or bad data you don't want. Radio Frequency Interference means that you are getting the interference from an actual radio signal source, not just from stray static electricity. Media that are susceptible to EMI and RFI are also susceptible to eavesdropping (called "signal capture" in this book).

The text describes several types of cable media (follow the link to a Microsoft TechNet article about media. It is not perfect, but it is pretty good.):

  • twisted pair - come in two types:
    • unshielded - UTP does not have an EMI resistant sheath
    • shielded - STP has an EMI resistant sheath
  • coaxial - Coax similar to that used for cable TV, but NOT identical
  • fiber optic - glass or plastic channels that conduct light, often red laser light

(For the purists among you, I will note that the speed of light through these media is about two thirds the speed of light in a vacuum.)

The graphic on page 5-5 shows several twisted pairs of wires. Each wire is covered with an insulator, and the two wires in the pair complete a circuit. These wires suffer from crosstalk, leakage of signal. The twists help cancel out such leaks. The graphic shows a UTP cable with eight wires in it, making four pairs.

The wires in each pair are twisted around each other. This type of cable comes in several varieties: two pair, three pair and four pair are common. Also, each variety may be available in grades, such as CAT 1 (Category 1) and CAT 5 (Category 5). There are several such categories, and a major difference between them is the number of twists per foot in each pair. CAT 1 will have less than 5 twists per foot, CAT 5 will have 25 or more twists per foot (so it is better, and costs more). Note that the better the class of cable, the more bits per second can be passed across it.

Connecting a system with twisted pair wiring is easy. It is illustrated on page 5-6. A possible problem is that the wiring closets in any building are often in need of being "cleaned up". The "closet" on each floor of a building contains punch-down blocks, patch panels, and hubs (or switches). Many are disorganized and messy. People who try to clean them up, however, must be careful not to disconnect circuits that are needed.

The factors for UTP:

  • Cost - inexpensive
  • Installation - cheap and easy
  • Capacity - 1 to 100 Megabits per second (Mbps), but 10 Mbps is common (100 Mbps, if fast Ethernet)
  • Attenuation - nothing is perfect, so this is high (poor)
  • Immunity from EMI - also poor. Recommendation: run UTP lines perpendicular to fluorescent lights.

UTP cables are usually connected to devices with RJ-45 connectors. Your text does not show an RJ-45 connector (or any other) very well. In the enlarged picture on the right, note the eight gold-colored connections for the eight wires usually found in UTP cables. The wires are used in pairs to form circuits. See the Networking Technologies notes for more information on this sort of connector.

An STP (Shielded Twisted Pair) cable is illustrated on page 5-7. This cable is more expensive than unshielded cable, and is less flexible due to the stiff shielding. The shield, however, makes it more EMI resistant than UTP.

The factors for STP:

  • Cost - Moderately expensive
  • Installation - harder than UTP, needs special connectors (note the IBM-style Token Ring connector shown here)
  • Capacity - 1 to 500 Megabits per second (Mbps) is possible, but 16 Mbps is common (for Token Ring)
  • Attenuation - high (poor)
  • Immunity from EMI - also poor, but not as bad as UTP.

IBM data connectorIBM Data connectors are typically used with Shielded Twisted Pair cable on a Token Ring network.
Coaxial cable is called that because it has two conductors, one wire in center and a conductive sheath around it, that share a "common axis". Most people have seen this style of cable used with cable television.

The wiring standards used for network coax are different from those used for cable TV. You may want to know this list:

  • 50 ohm cable, available as RG-8 and RG-11. Used in Thick Ethernet, also called "Ether Hose".
  • 50 ohm cable, available as RG-58. Used in Thin Ethernet
  • 75 ohm cable, available as RG-59. This one is for TV, not networks. However, since cable TV providers now commonly supply connectivity to the Internet, you can consider RG-59 to be a network medium as well. Be aware that a business network that does not involve cable TV would not be constructed with RG-59.
  • 93 ohm cable, available as RG-62. Used in ARCnet. (Actually, you can use almost anything for ARCnet.)

The number associated with each RG specification tells you the relative size of the center conductor. Smaller numbers mean thicker wires. The coaxial line is essentially a single bus, going from one station to the next. At each end of the line, the cable has to have a terminator on it. At one end, it also has to be grounded. If using thin Ethernet, T-connectors are used. If using thick Ethernet, vampire taps are used. They are called vampire taps because little teeth bite into the cable (to contact the shield), and a big tooth bites deeper to contact the central conductor when you screw the clamp down. Note that the vampire tap provides a place to tap into the cable, and a transceiver to translate between the PC and the network. The workstation also needs a patch cable to connect to the tap.

The factors for Coax:

  • Cost - Relatively low to Moderately expensive (depending on thickness of the cable)
  • Installation - simple to install, hard to modify
  • Capacity - high rates are possible, but 10 Mbps is common
  • Attenuation - high, but less than twisted pair
  • Immunity from EMI - moderate

The example on the right shows a typical T-connector with BNC fittings. The fitting on the bottom of the image might attach to a port on a NIC that looks like the barrel on either end of the top of the T. Attachment is achieved by pushing the connector onto the barrel of the port, then twisting the collar of the connector to lock onto the pin that is part of the port. In other words, it mounts like a bayonet.

The next (enlarged) picture shows a BNC connector attached to a thin Ethernet cable. Such a connector would be used to attach to one of the T-connector barrels in the photo above. The other end of the cable would run to the next node on the network.

Fiber optic can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor. Two configurations exist. Loose configuration has a liquid filler between the outer sheath and the conductor. Tight configuration has wire or stiff fibers around the conductor to add strength to the cable.

Fiber optic comes in two modes: single mode conducts a single signal, while multi-mode conducts many signals simultaneously. You may want to know that the most common type used is 62.5 micron core with 125 micron cladding, multimode.

Fiber optic is much harder to install and splice than electrical conductors. As illustrated on page 5-19, this type of connection requires two connectors for each station, a line in and a line out.

The factors for fiber optic:

  • Cost - Expensive, mostly for installation
  • Installation - difficult
  • Capacity - 100 Mbps at up to 20 kilometers per segment
  • Attenuation - very low
  • Immunity from EMI - immune. This is light, not electricity.

Ethernet hubs are discussed on page 5-22. The term concentrator is often used for a hub, since the hub is used to collect connections at one point. Four types of hubs are listed:

  • passive hub- a passive hub connects devices, but does not regenerate signals
  • active hub - an active hub connects devices, and does regenerate signals. Regeneration includes error correction, when possible.
  • switching hub, or switch - only forwards the signal through the port that will allow the transmission to be delivered to the device to which it is addressed. This feature improve the effective throughput of the network.
  • intelligent hub - This is more of a marketing term. It has no standard definition, so it can mean whatever a manufacturer chooses to mean. It should be a more advanced hub than the others, but it may not be.

Token ring concentrators, are called Multistation Access Units (MSAUs, or MAUs). The nodes on the ring are actually wired to several MSAUs in a star-wired manner. The MSAUs are wired to each other through their ring in and ring out ports. This brings up a phrase your author has not used yet. This sort of network is wired as a star. That is its physical topology. It works like a ring. That is its logical topology. All networks can be described in terms of each of these two properties. A network's physical topology is how it is shaped or wired. Its logical topology is how it is used: typically like a bus or a ring.

This text does not discuss the classic wireless network media types in this chapter. Since they have been mentioned in other chapters already, I will provide you with some notes here.

It may be redundant to remind you that wireless media means that there is no cable of any sort between certain parts of the network. (There are still wires inside lots of components).

Radio is the label used for frequencies from 10 KHz to 1 GHz. Several bands are used. Frequencies that are used for networks can be divided into regulated and unregulated frequencies. Only a few frequencies are unregulated in the United States. It is not possible to guarantee error free transmission in the unregulated frequencies. This is because anyone else can broadcast in those frequencies, causing errors in your transmissions. For this reason, broadcasts are usually limited to low power in unregulated bands, to minimize interference.

Three types of radio usage:

  • Low power, single frequency
  • High power, single frequency
  • Spread spectrum (multi-frequency)

The comparison factors for wireless media are different from those for wired media. The factors for low power, single frequency:

  • Frequency range - any frequency available, usually in the upper GHz
  • Cost - moderate
  • Installation - easy if using a preconfigured antenna
  • Capacity - 1 Mbps, sometimes up to 10 Mbps
  • Attenuation - relatively high
  • Immunity from EMI - low (poor) immunity.

The factors for high power, single frequency:

  • Frequency range - any frequency available, usually in the upper GHz
  • Cost - moderate, towers and repeaters increase the cost
  • Installation - complex
  • Capacity - 1 Mbps, sometimes up to 10 Mbps
  • Attenuation - relatively low
  • Immunity from EMI - low (poor) immunity.

Spread spectrum radio usage puts the incoming data stream on several frequencies at once. This discourages eavesdropping. Using direct sequence modulation, the signal is put on several frequencies, some of which may contain false signals. Using frequency hopping, the frequency being used is changed on a preset pattern, which the sender and receiver know. The factors for spread spectrum:

  • Frequency range - any frequency available, usually in the upper GHz
  • Cost - moderate
  • Installation - simple to moderately complex
  • Capacity - 2 Mbps to 6 Mbps
  • Attenuation - relatively high
  • Immunity from EMI - low (poor) immunity, but better immunity from eavesdropping

Microwave signals are used in two formats: terrestrial (earth-based) and satellite systems. Terrestrial systems are used in line of sight connections where it is not possible to put a wire, such as across several city blocks. The factors for terrestrial microwave:

  • Frequency range - 4 to 6 or 21 to 23 GHz
  • Cost - moderate to high
  • Installation - difficult
  • Capacity -1 Mbps to 10 Mbps
  • Attenuation - relatively high, varies with weather
  • Immunity from EMI - low

Satellite systems are used to connect sites that are widely separated. Usually, signals are sent to geosynchronous satellites, orbiting 22,300 miles above the earth. This orbit puts the satellite in the same part of the sky relative to a ground based observer at all times. The factors for satellite microwave:

  • Frequency range - 11 to 14 GHz
  • Cost - high
  • Installation - very difficult (Yes, someone has to be a rocket scientist.)
  • Capacity -1 Mbps to 10 Mbps
  • Attenuation - relatively high, varies with weather
  • Immunity from EMI - low

Infrared systems come in two types: point-to-point and broadcast. Point-to-point systems are like the remote controls we use for televisions. Some systems also use lasers. The factors for point-to-point infrared:

  • Frequency range - 100 GHz to 1000 THz
  • Cost - low to moderate
  • Installation - moderate to difficult
  • Capacity - 1 to 16 Mbps
  • Attenuation - varies with weather and light purity
  • Immunity from EMI - moderate

Broadcast infrared systems are used in single room settings, as these waves will bounce off walls, but not penetrate them. The advantage is that you can put a system in each room where required, and the users may move their machines around as they like. The factors for broadcast infrared:

  • Frequency range - 100 GHz to 1000 THz
  • Cost - low
  • Installation - simple
  • Capacity - up to 1 Mbps
  • Attenuation - high
  • Immunity from EMI - low