|  | 7.1 |
| 10-Mbps and 100-Mbps Ethernet
|  | 7.1.1 |
| 10-Mbps Ethernet
10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, frame format, transmission process, and a basic design rule. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown in Figure (1 bit time at 10 Mbps = 100 nsec = 0.1 µsec = 1 ten-millionth of a second.) 10BASE5, 10BASE2, and 10BASE-T also have a common frame format. The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. The Layer 2 frame data is converted from hex to binary. As the frame passes from the MAC sublayer to the physical layer, further processes occur prior to the bits being placed from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active: Within 4 to 8 microseconds following a normal transmission to indicate that the outbound frame was successfully transmitted Whenever there is a collision on the medium Whenever there is an improper signal on the medium. Improper signals might include jabber, or reflections that result from a cable short. Whenever a transmission has been interrupted All 10 Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding. Line encoding describes how the bits are actually signaled on the wire. The simplest encodings have undesirable timing and electrical characteristics. So line codes have been designed to have desirable transmission properties. This form of encoding used in 10 Mbps systems is called “Manchester.” Manchester encoding relies on the direction of the edge transition in the middle of the timing window to determine the binary value for that bit period. The top waveform has a falling edge, so it is interpreted as a binary 0. The second waveform shows a rising edge, which is interpreted as a binary 1. In the third waveform, there is an alternating binary sequence. With alternating binary data, there is no need to return to the previous voltage level. As can be seen from the third and fourth wave forms in the graphic, the binary bit values are indicated by the direction of change during any given bit period. The waveform voltage levels at the beginning or end of any bit period are not factors when determining binary values. Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is of the utmost importance when implementing a mixed-media network. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on parameters such as: Cable length and its propagation delay Delay of repeaters Delay of transceivers Interframe gap shrinkage Delays within the station 10-Mbps Ethernet operates within the timing limits offered by a series of not more than five segments separated by no more than four repeaters. This is known as the 5-4-3 rule. No more than four repeaters may be connected in series between any two distant stations. There can also be no more than three populated segments between any two distant stations.
Web Links
Ethernet Encapsulation Cheat Sheet http://www.cisco.com/warp/public/105/ encheat.html