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Transmission Media
Transmission Media includes the Layer 1 technologies that transfer bits: network cabling and wireless technologies. We will discuss cabling now and wireless later in this chapter. The simplest part of the OSI model is the part you can touch: network cables, at Layer 1. It is important to understand the types of cabling that are commonly used, and the benefits and drawbacks of each.
Fundamental network cabling terms to understand include EMI, noise, crosstalk, and attenuation. Electromagnetic Interference (EMI) is interference caused by magnetism created by electricity. Any unwanted signal (such as EMI) on a network cable is called noise. Crosstalk occurs when a signal crosses from one cable to another. Attenuation is the weakening of signal as it travels further from the source.
Twisted Pair Cabling
Unshielded Twisted Pair (UTP) network cabling, shown in Fig. 5.12, uses pairs of wires twisted together. All electricity creates magnetism; taking two wires that send electricity in opposite directions (such as sending and receiving) and twisting them together dampens the magnetism. This makes Twisted Pair cabling less susceptible to EMI.
UTP cable. Source: https://upload.wikimedia.org/wikipedia/commons/c/cb/UTP_cable.jpg. Image by Baran Ivo. Image under permission of Creative Commons
Twisted pair cables are classified by categories according to rated speed. Tighter twisting results in more dampening: a Category 6 UTP cable designed for gigabit networking has far tighter twisting than a Category 3 fast Ethernet cable. Shielded Twisted Pair (STP) contains additional metallic shielding around each pair of wires. This makes STP cables less susceptible to EMI, but more rigid and more expensive. Table 5.6 summarizes the types and speeds of Category cabling.
Table 5.6 Category Cabling Speed.
| Category | Speed |
|---|---|
| Cat 3 | 10 Mbps |
| Cat 4 | 16 Mbps |
| Cat 5 | 100 Mbps |
| Cat 5e | 1 Gbps |
| Cat 6 | 1+ Gbps |
| Cat 6a | 10 Gbps |
| Cat 7 | 10–100 Gbps |
Note that Cat 3, Cat 4, and Cat 5 are legacy standards (Cat 1 and Cat 2 were never formally standardized). Both Cat 5e and Cat 6 operate at 1000 Mbps (aka 1 Gbps) at lengths up to 100 meters, but Cat 6 can operate at 10 Gbps at lengths up to 55 meters. Cat 6a can operate at 10 Gbps at lengths up to 100 meters. Cat 7 operates at 10 Gbps at lengths up to 100 meters, but can operate at 100 Gbps at lengths up to 15 meters.