10-Mbps and 100-Mbps Ethernet

10-Mbps Ethernet
7.1.1 This page will discuss 10-Mbps Ethernet technologies.


10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, the frame format, transmission processes, and a basic design rule.

Figure displays the parameters for 10-Mbps Ethernet operation. 10-Mbps Ethernet and slower versions are asynchronous. Each receiving station uses eight octets of timing information to synchronize its receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters. For example, 1 bit time at 10 Mbps = 100 nanoseconds (ns) = 0.1 microseconds = 1 10-millionth of a second. This means that on a 10-Mbps Ethernet network, 1 bit at the MAC sublayer requires 100 ns to transmit.

For all speeds of Ethernet transmission 1000 Mbps or slower, transmission can be no slower than the slot time. Slot time is just longer than the time it theoretically can take to go from one extreme end of the largest legal Ethernet collision domain to the other extreme end, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station to be detected.

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. As the frame passes from the MAC sublayer to the physical layer, other processes occur before the bits move from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. The SQE is a transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. The SQE is also called a heartbeat. The SQE signal is designed to fix the problem in earlier versions of Ethernet where a host does not know if a transceiver is connected. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active in the following instances:

• Within 4 to 8 microseconds after 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, such as 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. Therefore, line codes have been designed with desirable transmission properties. This form of encoding used in 10-Mbps systems is called Manchester encoding.

Manchester encoding uses the transition in the middle of the timing window to determine the binary value for that bit period. In Figure , the top waveform moves to a lower position so it is interpreted as a binary zero. The second waveform moves to a higher position and is interpreted as a binary one. The third waveform has an alternating binary sequence. When binary data alternates, there is no need to return to the previous voltage level before the next bit period. The wave forms in the graphic show that the binary bit values are determined based on the direction of change in a bit period. The voltage levels at the start or end of any bit period are not used to determine 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 most important in mixed-media networks. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on the following types of parameters:

• Cable length and propagation delay
• Delay of repeaters
• Delay of transceivers
• Interframe gap shrinkage
• Delays within the station

10-Mbps Ethernet operates within the timing limits for a series of up to five segments separated by up to four repeaters. This is known as the 5-4-3 rule. No more than four repeaters can be used in series between any two stations. There can also be no more than three populated segments between any two stations.

The next page will describe 10BASE5.