The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

7.5. Implications of Full Duplex Operation

The use of full duplex mode has a number of important implications:

• Full duplex operation eliminates the link length restrictions of CSMA/CD.

• Full duplex operation increases the aggregate channel capacity.

• Full duplex operation increases the potential load on a switch.

7.5.1. Eliminating the Link Length Restriction of Half Duplex Ethernet

The use of CSMA/CD as an access control mechanism implies an intimate relationship between the minimum length of a frame and the maximum round-trip propagation delay of the network. We need to make sure that, if a collision occurs on any transmission, all transmitting stations know of the collision so that they can take proper action. This implies that the minimum length of a frame must be longer than the maximum round-trip propagation time of the network, plus an allowance for the jam time, synchronization delays, and so on, so that the station will still be transmitting the frame when it is informed of the collision.

Because full duplex operation does not use CSMA/CD, this distance restriction no longer applies. Regardless of the data rate of the LAN, the length of a full duplex Ethernet link is limited only by the physical transmission characteristics of the medium. While twisted pair links may be used at distances on the order of 100 m, optical fiber may be used at distances up to 2 to 3 km (multimode fiber) and 20 to 50 km or more (single mode fiber). With appropriate line drivers and signal regeneration, ...