Troubleshooting ethernet
Table Of Contents
Full-Duplex Operation 10/100/1000
Troubleshooting Ethernet
Ethernet was developed by Xerox Corporation's Palo Alto Research Center (PARC) in the 1970s. Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in 1980. Shortly thereafter, Digital Equipment Corporation, Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification (Version 2.0) that is substantially compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest market share of any local-area network (LAN) protocol. Today, the term Ethernet is often used to refer to all carrier sense multiple access collision detect (CSMA/CD) LANs that generally conform to Ethernet specifications, including IEEE 802.3.
When it was developed, Ethernet was designed to fill the middle ground between long-distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distances. Ethernet is well suited to applications on which a local communication medium must carry sporadic, occasionally heavy traffic at high peak data rates.
Ethernet and IEEE 802.3
Ethernet and IEEE 802.3 specify similar technologies. Both are CSMA/CD LANs. Stations on a CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations "listen" to the network to see if it is already in use. If it is, the station wanting to transmit waits. If the network is not in use, the station transmits. A collision occurs when two stations listen for network traffic, "hear" none, and transmit simultaneously. In this case, both transmissions are damaged, and the stations must retransmit at some later time. Back-off algorithms determine when the colliding stations retransmit. CSMA/CD stations can detect collisions, so they know when they must retransmit. This access method is used by traditional Ethernet and IEEE 802.3 functions in half-duplex mode. (When Ethernet is operated in full-duplex mode, CSMA/CD is not used.) This means that only one station can transmit at a time over the shared Ethernet.
This access method was conceived to offer shared and fair access to multiple network stations/devices. It allows these systems fair access to the Ethernet network through a process of arbitration by dictating how stations attached to this network can access the shared channel. It allows stations to listen before transmitting and can recover if signals collide. This recovery time interval is called a slot time and is based on the round-trip time that it takes to send a 64-byte frame the maximum length of an Ethernet LAN attached by repeaters. Another name for this shared LAN is a collision domain. For half-duplex 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.
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:
1. 
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.
2. 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 2n , 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.
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2n value1
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Actions
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20-1 |
Stations either try to retransmit immediately or wait for one slot time. |
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22 |
Stations randomly wait zero, one, two, or three slot times to retransmit. |
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23 |
Stations randomly wait from zero to seven slot times. |
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24 |
. . . you get the point. |
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1 2n where n = the number of collisions |
Depending on the number of collisions the algorithm randomly selects to back off, a station could potentially wait a while before retransmitting.
The algorithm collision counter stops incrementing at 10, where the penalty wait time is selected from a range of 0 to 1023 slot times before retransmission. This is pretty bad, but the algorithm will attempt to retransmit the frame up to 16 collisions. Then it just gives up, and a higher-layer network protocol such as TCP/IP will attempt to retransmit the packet. This is an indication that you have some serious errors.
When a station successfully sends a frame, the collision counter (penalty) is cleared (for that frame) and no loner must wait for the back-off time. ("Interface" statistics are not cleared, just the timer is). Any stations with the lowest collisions will be capable of accessing the wire more quickly because they do not have to wait.
Both Ethernet and IEEE 802.3 LANs are broadcast networks. In other words, all stations see all frames, regardless of whether they represent an intended destination. Each station must examine received frames to determine whether the station is a destination. If it is a destination, the frame is passed to a higher protocol layer for appropriate processing.
Differences between Ethernet and IEEE 802.3 standards are subtle. Ethernet provides services corresponding to Layers 1 and 2 of the OSI reference model, whereas IEEE 802.3 specifies the physical layer (Layer 1) and