Further and Faster
Note that 100Base-T Ethernet (and above) is backwardly compatible with 10Base-T Ethernet if both hosts at the ends of a cable implement auto-negotiation. When the hosts are first connected they compare their abilities, which includes full or half duplex and speed of operation. This is done by encoding information in the initial signals that Ether interfaces exchange when they are connected. Having done this, they can agree on their optimal speed and duplex. A host that does not auto-negotiate can safely be assumed to be an old 10Mb Ethernet, though some interfaces are clever enough to analyse the signal they are reading and make an intelligent guess. Thus a switch can plug into 10Mb and 100Mb networks with equal ease knowing that it is using the best protocol for each of the hosts it is talking to.
Now 100Mb (and 10Mb) Ethernet actually only uses two of the four twisted pairs in a UTP cable: one pair for out and one pair for back. You are recommended not to use the other two pairs in a cable since the 100Base-T standard does not take into account possible electrical interference from those pairs. Even so, there are some manufacturers that sell 2-into-1 splitters so you can get two connections along a single cable.
That's not to say you can't use those spare wires for something else. The IEEE 802.3af Power over Ethernet standard describes how to use them to deliver power: useful for supplying a networked device in a place without a handy power source. A good example is an Internet telephone. Powered Ethernet switches are starting to appear: these are being driven by the growth of interest in IP telephony, Voice over IP (VoIP). Final specifications are not yet available, but the specification appears to be about 45 volts DC with a maximum current of at least 250 mA.
Gigabit Ethernet (802.3z) over copper (802.3ab) uses all four pairs with a different IEEE physical encoding of the bits and transmits and receives simultaneously IEEE on all pairs of wires.
Gigabit will run over standard Cat 5 if you are lucky, though you should use Enhanced Cat 5 (or Cat 6 or more) to be sure it will cleanly support the more exacting electrical signals.
The minimum Ethernet frame size of 64 bytes is required in Ethernet for the CSMA/CD method to work. At gigabit speeds, using the original 10Mb Ethernet's minimum packet size would imply a maximum cable length of 10 m: not very practical. A 512 byte limit would be more sensible, allowing a limit of 80 m. But Gigabit Ethernet does keep the 64 byte limit. Short packets are followed by a carrier extension, a special 'hold the wire' signal to fill up to the end of a 512 byte slot. CSMA/CD regards this as part of the packet and so holds off sending until it can be sure the wire is empty.
This means we might have 448 bytes of overhead on a small frame and so Gigabit Ethernet's performance on small packets would not be much better than 100Mb Ethernet. The theoretical limit works out at about 86Mb/s when sending 64 byte frames. As a partial fix for this, Gigabit Ethernet allows packet bursting, where several packets can be sent before relinquishing hold on the wire. If a host has several packets to send, it sends the first one normally (possibly including carrier extension). If that gets through safely, the host still has control and can send more packets (without the need for carrier extension) up to a limit of 8192 bytes in total. It must then release the medium. This decreases the overhead if we want to send many small packets.
All this complication means that for best performance Gigabit Ethernet really ought to be used in full-duplex mode on a switched network: then there is no need to worry about collisions and we can dispense with carrier extension and packet bursting. Notice, also, that a full-duplex connection will have no inherent distance limit (apart from electrical considerations).
Now 10Gb Ethernet is standard Ethernet with just three differences:
- It runs over optical fibre connected with a XAUI socket.
- It is full duplex only and does not have half duplex and CSMA/CD.
- It operates at 10Gb/s.
There is a proposal for 10Gb over twisted pair copper (10Gbase-T, using Cat 6A cable), but this may take some time to work through. Work on a 40Gb Ethernet standard has begun. Just think on that: 10Gb Ethernet is 1000 times as fast as the original Ethernet over a timescale of less than 20 years!
The designers and engineers of Ethernet are confident that it will get ever faster and more commonly used, driving out all other physical/link layer technologies. One of the more ambitious claims is that eventually even computer buses will be Ethernets connecting machine components (like video cards, sound cards, the processor, and memory) each of which has its own globally unique (IPv6) address. For example, the Internet SCSI (iSCSI) standard has been developed to adapt the popular SCSI bus protocol to run over TCP/IP 3. It operates at 10Gb/s.
