Part III: Faster Ethernet Technologies
In the early 1990′s the IEEE’s 802.3 committee (the committee put in charge of Ethernet standards since 10base2) met again to develop a faster Ethernet. Discussion was carried out as to how this new Ethernet should be structured. Should everything be redone from scratch? Should core attributes stay the same? It was eventually decided do just that and allow the new, faster Ethernet backwards compatible with current Ethernet setups. Thus the only major change from 10Mbit Ethernet to 100Mbit Ethernet, or “fast Ethernet”, was to decrease the time it takes transfer one bit from 100 nanoseconds to 10 nanoseconds (1 bit takes 100 nanoseconds to transfer on 10Mbit Ethernet — verify this simple calculation if you have doubts). This could easily have been done by utilizing a wiring scheme such as the old 10base5 or 10base2 and decreasing the distance by a factor of ten (recall the discussion in the previous section as to why this would have needed to be done). However, 10baseT Ethernet with its twisted copper pair topology and inclusion of hubs provided enough advantages at the end and won out. Thus, 100baseT Ethernet would not be supported on the tap/transceiver or BNC connections of the older 10base5 and 10base2 technologies. However, the real question after deciding to go with 10baseT became what kind of wiring should be supported? Category 3 UTP (unshielded twisted pair) became the obvious choice as it was deployed widely in offices and as phone cables at the time.
The downside was that CAT 3 UTP used a signal speed of only 25MHz (10base5 and 10base2 already used 20MHz!). How is it possible then to achieve the necessary transfer rate of 100Mbit/s? The solution was to use all four twisted pairs of CAT 3 UTP cabling. Doing so would allow for the ability to transfer 4 bits in parallel in each of the 25 million cycles per second (MHz), equaling a transfer rate of 100Mbit/s (4bits x 25MHz). Furthermore, it should also be noted that Manchester encoding is no longer used in fast Ethernet, as technology had improved and distances were short enough, allowing sender and receiver to stay synchronized with one another without it.
Besides category 3 cable, category 5 UTP cable could also be used for fast Ethernet. The advantage of CAT 5 UTP cabling is that the signal rate is increased by factor 5 to up to 125MHz. Consequently, only two twisted pairs would need to be used to achieve the same kind of transfer rates that required four twisted pairs with CAT 3 UTP cable.
Category 5 Ethernet Cable
Manchester encoding is not used here either, but instead a technique called 4B/5B. This basically takes a group of 5 clock periods (e.g. 10101) which can have a total of 32 combinations (2*2*2*2*2). 16 of these combinations are used for transmitting data, while the other 16 are used for control purposes (e.g. determining the frame cutoff etc..). Another advantage of this setup is that it supports full duplex transfers if a switch utilized, i.e. 100Mbit/s receive and 100Mbit/s send at the same time. Why is a switch required for full duplex data transfers? More on this in the next section.
Not soon after the fast Ethernet standard was developed (also known as 802.3u) the IEEE 802 committee got together again to develop an even faster Ethernet. This Ethernet was to be 10 times faster than “fast Ethernet” and still be backwards compatible. Just as 100baseT Ethernet, the new Ethernet would not support the sharing of a transmission medium for multiple devices. Instead if would be point-to-point, i.e. machine to machine or machine to hub/switch. The new Ethernet was born and given the 802.3.z standard. Its transfer rate was 1000Mbit/s and so was quickly dubbed “gigabit” (the metric prefix for 10^3 is Giga). Gigabit, like its predecessor, uses category 5 UTP cabling, however this time all four twisted pairs are used in data transfer. Each pair of copper cables represents two bits, and since there are 4 pairs to choose from there’s the ability to send 4×2 bits = 8 bits in parallel. Recall that the signaling speed of CAT 5 cabling is 125MHz, and after carrying out some simple math we will see that the transfer rate of 1000Mbit/s is achieved (8bits x 125MHz).
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