Data Acquisition

Figure 9.6Summary of 100Base-T standardsThis approach is possible because the original 802.3 specification defines the MAC layerindependently of the various physical PMD layers it supports. As you will recall, theMAC layer defines the format of the Ethernet frame and defines the operation of theCSMA/CD access mechanism. The time dependent parameters are defined in the 802.3specification in terms of bit-time intervals and so are speed-independent. The 10 MbpsEthernet inter-frame gap is actually defined as an absolute time interval of 9.60microseconds, equivalent to 96 bit times; while the 100 Mbps system reduces this by tentimes to 960 nanoseconds.One of the limitations of the 100Base-T system is the size of the collision domain,which is 250 m. This is the maximum sized network in which collisions can be detected;being one tenth of the size of the maximum 10 Mbps network. This limits the distancebetween our workstation and hub to 100 m, the same as for 10Base-T, but usually onlyone hub is allowed in a collision domain. This means that networks larger than 200 mmust be logically connected together by store and forward type devices such as bridges,routers or switches. However, this is not a bad thing since it segregates the traffic withineach collision domain, and hence reducing the number of collisions on the network. Theuse of bridges and routers for traffic segregation, in this manner, is often done onindustrial CSMA/CD networks.The dominant 100Base-T system is 100Base-TX, which accounts for about 95% of allfast Ethernet shipments. The 100Base-T4 systems were developed to use four pairs ofcategory 3 cable; however few users had the spare pairs available and T4 systems are notcapable of full-duplex operation, so this system has not been widely used. The 100Base-T2 system has not been marketed at this stage, however its underlying technology usingdigital signal processing (DSP) techniques is used for the 1000Base-T systems on twocategory 5 pairs. With category 3 cable diminishing in importance, it is not expected thatthe 100Base-T2 systems will become significant.Ethernet signals are encoded using the Manchester encoding scheme. This methodallows a clock to be extracted at the receiver end to synchronize the transmission/reception process. The encoding is performed by performing an ‘exclusive-or’ between a20 MHz clock signal and the data stream. In the resulting signal, a 0 is represented by ahigh to low change at the center of the bit cell, whilst a 1 is represented by a low to highchange at the center of the bit cell. There may or may not be transitions at the beginning