Information Technology Reference
In-Depth Information
operation, the mode on which traditional Ethernet is based, the size of your collision domain can be
limited by the physical limitations of the cabling utilized. Table 4-1 lists the collision domains for
10/100/1000 Mbps.
Table4-1
Examples of Traditional Ethernet and IEEE 802.3 Collision Domains
Traditional Ethernet and 802.3 Collision Domains
Signaling Speed
Network Diameter
10BaseX
About 280 meters (coax)
Ethernet
10/100BaseX
About 205 meters (twisted pair)
IEEE
802.3b
1000BaseX
About 20 meters
(fiber and copper)
IEEE
802.3z
The limitations of the cable itself can create even smaller boundaries.
Because the 64-byte slot time is consistent for 10/100/1000 transmission speeds, this severely limits the
scalability for 1000BaseX to operate in a network with a diameter of more than 20 meters. To overcome
this obstacle, use carrier extension bits in addition to the Ethernet frame size to extend the time that
transmits on the wire. This expands the network diameter to 100 meters per segment, like 100BaseT.
For this system to work, everyone must abide by the same rules. For CSMA/CD the rules are as follows:
Listen
—Stations listen for signals on the wire. If a signal is detected (carrier sense), then stations
should not attempt to transmit frame. If a station “hears” another signal on the wire while
transmitting the first 64 bytes of a frame, it should recognize that its frame has collided with another.
1.
Collision detect
—If a station detects a collision, it must back off from sending the frame using the
truncated back-off algorithm. The back-off algorithm counts the number of collisions, if any, to
determine how long a station must wait to retransmit the frame. This algorithm backs off each time
that a collision is detected. The goal of this method is to provide the system a way to determine how
many stations are trying to transmit simultaneously and then guess when it should be safe to try
again. The way that the truncated back-off algorithm tracks and adjusts timers is based on the value
of 2
n
, where
n
is the number of collisions encountered during transmission of the frame. The result
is a guess of how many stations may be on the shared channel. This result gets plugged in as a range,
counting from zero, for the number of slot times to wait. The algorithm randomly selects a value
from this range as shown in Table 4-2.
2.
Table4-2
Back-off Algorithm
2
n
value
1
Actions
2
0
-1
Stations either try to retransmit immediately or wait for one slot
time.
2
2
Stations randomly wait zero, one, two, or three slot times to
retransmit.
2
3
Stations randomly wait from zero to seven slot times.
2
4
. . . you get the point.
2
n
where
n
= the number of collisions
1.
Depending on the number of collisions the algorithm randomly selects to back off, a station could
potentially wait a while before retransmitting.
