CSMA/CD can be described as a listen-before-acting access method. Thus, the first function associated with transmit media access management is to find out whether any data is already being transmitted on the network and, if so, to defer transmission. During the listening process, each station attempts to sense the carrier signal of another station, hence the prefix carrier sense (CS) for this access method. Although broadband networks use RF modems that generate a carrier signal, a baseband network has no carrier signal in the conventional sense of a carrier as a periodic waveform altered to convey information. Thus, a logical question you may have is how the MAC sublayer on a baseband network can sense a carrier signal if there is no carrier. The answer to this question lies in the use of a digital signaling method, known as Manchester encodingon 10-Mbps Ethernet LANs, that a station can monitor to note whether another station is transmitting. Although NRZI encoding is used on broadband networks, the actual data is modulated after it is encoded.
Thus, the presence or absence of a carrier is directly indicated by the presence or absence of a carrier signal on a broadband network.
Collision Detection
As discussed in Chapter 3, under Manchester encoding, a transition occurs at the middle of each bit period. This transition serves as both a clocking mechanism, enabling a receiver to clock itself to incoming data, and as a mechanism to represent data. Under Manchester coding, a binary 1 is represented by a high-to-low transition, while a binary 0 is represented by a low-to-high voltage transition. Thus, an examination of the voltage on the medium of a baseband network enables a station to determine whether a carrier signal is present.
If a carrier signal is found, the station with data to transmit will continue to monitor the channel. When the current transmission ends, the station will then transmit its data, while checking the channel for collisions. Because Ethernet and IEEE 802.3 Manchester-encoded signals have a 1-volt average DC voltage level, a collision results at an average DC level of 2 volts. Thus, a transceiver or network interface card can detect collisions by monitoring the voltage level of the Manchester line signal.
Jam Pattern
If a collision is detected during transmission, the transmitting station will cease transmission of data and initiate transmission of a jam pattern. The jam
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pattern consists of 32 to 48 bits. These bits can have any value other than the CRC value that corresponds to the partial frame transmitted before the jam. The transmission of the jam pattern ensures that the collision lasts long enough to be detected by all stations on the network.
When a repeater is used to connect multiple segments, it must recognize a collision occurring on one port and place a jam signal on all other ports. Doing so results in the occurrence of a collision with signals from stations that may have been in the process of beginning to transmit on one segment when the collision occurred on the other segment. In addition, the jam signal serves as a mechanism to cause nontransmitting stations to wait until the jam signal ends before attempting to transmit, alleviating additional potential collisions from occurring.
Wait Time
Once a collision is detected, the transmitting station waits a random number of slot times before attempting to retransmit. The termslotrepresents 512 bits on a 10-Mbps network, or a minimum frame length of 64 bytes. The actual number of slot times the station waits is selected by a randomization process, formally known as atruncated binary exponential backoff. Under this randomization process, a randomly selected integer r defines the number of slot times the station waits before listening to determine whether the channel is clear. If it is, the station begins to retransmit the frame, while listening for another collision.
If the station transmits the complete frame successfully and has additional data to transmit, it will again listen to the channel as it prepares another frame for transmission. If a collision occurs on a retransmission attempt, a slightly different procedure is followed. After a jam signal is transmitted, the station simply doubles the previously generated random number and then waits the prescribed number of slot intervals before attempting a retransmission. Up to 16 retransmission attempts can occur before the station aborts the transmission and declares the occurrence of a multiple collision error condition.
Figure 4.6 illustrates the collision detection process by which a station can determine that a frame was not successfully transmitted. At time t0 both stations A and B are listening and fail to detect the occurrence of a collision, and at time t1station A commences the transmission of a frame. As station A’s frame begins to propagate down the bus in both directions, station B begins the transmission of a frame, since at time t2 it appears to station B that there is no activity on the network.
Shortly after time t2 the frames transmitted by stations A and B collide, resulting in a doubling of the Manchester encoded signal level for a very short
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A B
A B
A B
A B
A B
t0
t1
t2
t3
t4
Stations A & B listening
Station A begins transmission
Station B begins transmission
Station B detects collision and transmits pattern jam
Station A detects collision before ending transmission
Figure 4.6 Collision detection.
period of time. This doubling of the Manchester encoded signal’s voltage level is detected by station B at time t3, since station B is closer to the collision than station A. Station B then generates a jam pattern that is detected by station A.
Late Collisions
A late collision is a term used to reference the detection of a collision only after a station places a complete frame on the network. A late collision is normally caused by an excessive network segment cable length, resulting in the time for a signal to propagate from one end of a segment to another part of the segment being longer than the time required to place a full frame on the network. This results in two devices communicating at the same time never seeing the other’s transmission until their signals collide.
A late collision is detected by a transmitter after the first slot time of 64 bytes and is applicable only for frames whose lengths exceed 65 bytes. The detection of a late collision occurs in exactly the same manner as a normal collision; however, it happens later than normal. Although the primary cause of late collisions is excessive segment cable lengths, an excessive number of repeaters, faulty connectors, and defective Ethernet transceivers or controllers
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can also result in late collisions. Many network analyzers provide information on late collisions, which can be used as a guide to check the previously mentioned items when late collisions occur.