802.3 AT 10Mbps

802.3 is essentially OSI compliant and defines a physical as well as a MAC layer. At 10Mbps the 802.3 physical layer is composed of an upper physical layer signalling (PLS) sublayer and a lower physical medium attachment (PMA) sublayer. The PLS sublayer is medium independent, responsible for generating and detecting the Manchester code used by all 10Mbps variants, which ensures that clocking information is transmitted along with the data. The PMA sublayer is implemented by a functional unit called an MAU (Medium Access Unit) which attaches directly to the medium, transmits and receives medium signals and identifies collisions. The interface between the PMA and PLS sublayers is known as the Attachment Unit Interface (AUI). In 10Base5, the MAU, known as a transceiver, is separate from the station itself and is attached directly to the Ethernet coaxial cable. The AUI in 10base5 is a drop-cable up to 50m long which carries five twisted pairs connecting the station's NIC (which implements the MAC and PLS) and transceiver . In 10Base2 and 10BaseT, however the MAU and AUI are themselves integrated into the NIC which then connects directly to the medium.

Like the PLS sublayer, the MAC sublayer is common to all variants of 10Mbps 802.3 and its PDUs or frames have a simple structure, shown in Figure 1.

Figure 1           IEEE 802.3frame structure

In Ethernet II, on the other hand, this field is used to indicate the type of payload carried by the frame. For example 080016 signifies an IP payload. In fact the smallest legal value of this field is 060016, and since the greatest value of the 802.3 Length is 05DC16 it is always possible to tell Ethernet and 802.3 frames apart and they can therefore coexist on the same network. 802.3 was intended to be used with 802.2 LLC as its standard payload, the latter using a 7-bit subaddress to specify protocol type. However, this is not compatible with the 16-bit EtherType of Ethernet II, so the SNAP (Subnetwork Access Protocol) extension was developed. With a SNAP-extended header, an LLC PDU can carry a 16-bit EtherType.

To send a frame, a station on an 802.3 network first listens to the Ether (carrier sense function). If the Ether is busy, the station defers, but, after the current activity stops, it uses a 1-persistent strategy and will wait only for a short, fixed delay, the inter-frame gap, before beginning to transmit. If there is no collision, the transmission will complete successfully. If, however, a collision is detected, the frame transmission stops and the station begins to send a jamming signal to make sure that all other stations realise what has happened. The station then backs off for a random time interval before trying again. The back-off interval is computed using an algorithm called truncated binary exponential backoff, which works as follows.

The station always waits for some multiple of a 51.2ms time interval, known as a slot. The station chooses a number randomly from the set {0,1} and waits for that number of slots. If there is another collision it waits again, but this time for a number chosen from {0,1,2,3}. After k collisions on the same transmission it chooses its number randomly from {0, …, 2k-1}, until k = 10, when the set is frozen. After k = 16, the so-called attempt limit, the MAC unit gives up and reports a failure to the layer above.

The collision does, however, typically result in a short malformed packet called a runt. The runt is just the part of a packet that the first sender managed to transmit before the collision occurred. If the Ethernet construction rules are adhered to, the runt will be easily identified because it will be shorter than the Ethernet minimum (64 bytes). Its CRC will also be incorrect. Runts are common and cause little difficulty on individual Ethernet segments, but it is important that they should be identified and filtered by a switch to prevent them being forwarded unnecessarily onto other segments.