Another change to the Ethernet CSMA/CD transmit
specification was the addition of frame bursting for gigabit operation. Burst
mode is a feature that allows a MAC to send a short sequence (a burst) of frames
equal to approximately 5.4 maximum-length frames without having to relinquish
control of the medium. The transmitting MAC fills each interframe interval with
extension bits, as shown in Figure 7-8, so that other stations on the network
will see that the network is busy and will not attempt transmission until after
the burst is complete.
Figure 7-8 A Gigabit Frame-Burst Sequence
If the length of the first frame is less than the minimum
frame length, an extension field is added to extend the frame length to the
value indicated in Table 7-1. Subsequent frames
in a frame-burst sequence do not need extension fields, and a frame burst may
continue as long as the burst limit has not been reached. If the burst limit is
reached after a frame transmission has begun, transmission is allowed to
continue until that entire frame has been sent.
Frame extension fields are not defined, and burst mode is
not allowed for 10 Mbps and 100 Mbps transmission rates.
Full-Duplex Transmission�An Optional Approach to Higher
Network Efficiency
Full-duplex operation is an optional MAC capability that
allows simultaneous two-way transmission over point-to-point links. Full duplex
transmission is functionally much simpler than half-duplex transmission because
it involves no media contention, no collisions, no need to schedule
retransmissions, and no need for extension bits on the end of short frames. The
result is not only more time available for transmission, but also an effective
doubling of the link bandwidth because each link can now support full-rate,
simultaneous, two-way transmission.
Transmission can usually begin as soon as frames are ready
to send. The only restriction is that there must be a minimum-length interframe
gap between successive frames, as shown in Figure 7-9, and each frame must
conform to Ethernet frame format standards.
Figure 7-9 Full Duplex Operation Allows Simultaneous
Two-Way Transmission on the Same Link
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