• CSMA/CD Ethernet VII

Ethernet originated in the 70's from efforts at Xerox's PARC to create a new method for connecting computers together for sharing programs, data and information

The first version of Ethernet was soon replaced by Version II that was developed by a joint effort between Digital Equipment Corp, Intel Corp and Xerox Corp. This is also know as the DIX Ethernet standard

• Ethernet Version II has the following properties
  • uses thick coaxial cable
  • operates at 10mb
  • allows unrepeated cable runs up to 500m
  • allows 100 devices per cable segment
  • uses CSMA/CD for controlling access to the shared media
    • has a two byte protocol type field following the mac source address
      • minimum data length of 64 bytes and a maximum length of 1500 bytes
      • systems running Ethernet VII cannot talk to 802.3 systems unless they support both frame types concurrently

• CSMA/CD 802.3
  • The IEEE eventually ratified a slightly different version of Ethernet under the 802.3 standards track.
  • The original 802.3 standard has the following properties
    • uses thick coaxial cable
    • operates at 10mb
    • allows unrepeated cable runs up to 500m
    • allows 100 devices per cable segment 2.5 meters apart
    • uses CSMA/CD for controlling access to the shared media
    • is also known as (aka) 10Base5
    • has a two byte length field following the mac source address
    • uses the 802.2 SAP fields to denote the protocol in the frame
      • minimum data length of 61 bytes and a maximum length of 1496 bytes when using 802.2 LLC2 ABME
    • systems running 802.3 cannot talk to Ethernet VII systems unless they support both frame types concurrently

• CSMA/CD 802.3 additional media types
  • 802.3 has been revised many times to add new physical layer media
    • 10Base2 has the following properties
      • standard for thinner coax cable
      • allows cable runs up to 185m
      • cabling runs from station to station in a chain, no concentrators
      • aka cheapernet
    • 10baseF (aka foirl) has the following properties
      • standard for optical fiber
      • allows significantly longer cable runs
        • cabling is usually connected to a hub in a star configuration
    • 10baseT has the following properties
      • standard for utp
      • allows cable runs up to 100m
        • cabling is usually connected to a hub in a star configuration
    • 100baseF/T has the following properties
      • operates at 100mb
      • allows 100m cable runs on UTP (100baseT)
        • allows significantly longer cable runs on fiber (100baseF)
        • cabling is usually connected to a hub in a star configuration
    • The IEEE is currently working on a new version of 802.3 that will operate at 1 gigabit
    • Cisco recently purchased Granite Systems to obtain their gigabit ethernet technology

• CSMA/CD physical connectivity options

• CSMA/CD physical summary

• As a rule it is possible to connect 5 10Base5 500 meter cable segments together. However stations can only attach to three of the segments. It is fairly unusual to see end stations directly attached to 10Base5 segments in this day and age. However inter-connectivity devices such as routers and bridges are still occasionally connected to a 10Base5 backbone.

• CSMA/CD simple view of how it works

• PC A listens to see if any other station is already transmitting prior to sending a frame. In the example above the network was available so PC A transmitted its frame. PC B is also listening. PC B sees the frame that PC A transmitted and checks the destination MAC address to ss if the frame is addressed to itself. Note that the transmission flows in both directions simultaneously. The cable terminators at the end of the cable absorb the signal and prevent it from echoing back down the cable. The arrow from the cable back to the PCs is intended to indicate that the PCs are listening even when they are transmitting to detect collisions.

• CSMA/CD a collision in action

In this example both PC A & PC B believed that the network was available and started to transmit nearly simultaneously. Neither PC heard the others transmissions right away due to the propagation delay of the bits traveling on the cable. This causes a collision. In this case PC B will detect the collision. PC B stops transmitting and sends a JAM signal telling all devices on the cable to back off and try again. PC A hears the JAM signal and stops transmitting. Each device waits a period of time based on the standardized back off algorithm and tries again.

The minimum packet length on an ethernet network is 512 bits (64 bytes). This minimum length ensures that a stations is still transmitting a frame when all other stations on the ring have had an opportunity to see the frame and raise a collision alert if necessary. Collisions raised by other stations tell a transmitting station that its frame was not delivered and it will have to try again.

• CSMA/CD a JAM signal in action

• Eventually both stations detect the collision and send a JAM signal to
• indicate that a collision has occurred. Both stations wait a random amount
• of time as calculated by the backoff algorithm and try again.

• CSMA/CD out of spec cable causes problems

In this example PC A has just finished transmitting a 64 byte frame onto the cable. PC B did not detect PC A's frame even though it met the minimum size requirement. This can be caused by an out of spec cable plant. Normally a 60 byte frame would have filled up the entire cable and PC A would have still been transmitting when PC B attempted to transmit. A collision would have occurred and PC A would have known that it would have to retransmit its frame.

In this case PC A has finished transmitting the frame and removed it from its hardware buffers before PC B started transmitting its own frame. Eventually PC B will detect PC A's frame and a collision will result. PC A will detect the collision signal but will not know that it was referring to the frame it just sent. PC A will assume another station was the cause of the collision. As far as PC B is concerned the frame it detects is just another normal collision. PC B has no knowledge that this was in fact a frame that PC A thought it had transmitted successfully.

In this example both PC A has transmitted the first 60 bytes of its frame on the cable. As PC A has not heard a collision for this frame yet it will continue sending the balance of the frame. However PC B has not yet heard this frame. Possibly due to an out-of -spec cabling system. PC B begins transmitting its own frame. PC B will soon discover its mistake when it hears the frame from PC A on the cable. PC B immediately issues a JAM signal. PC A will hear the JAM and stop transmitting. PC A will log a Late Collision because it should not have been able to get 60 bytes of a frame onto the cable before hearing a collision.

• CSMA/CD minimum frame size and cable length

In fact the time it takes to transmit a 64 byte frame onto a cable has to be greater than propagation delay of sending a bit from from one end of the cable plant to the other and back. This is because a station at one end may start sending a frame while a station at the other end may also start sending a frame at the same time. Each station must detect the other stations' frame and send a JAM signal that must reach the originating station while it is still transmitting its frame.

In other words a cable plant can only be <32bits long in order to work successfully.

• 802.5 Token Ring
  • IEEE 802.5 and Token Ring as described by the IBM Token Ring Architecture Reference are very similar and essentially compatible
    • For this presentation Token Ring will be used to mean either standard
  • 802.5 does not specify a physical layer technology like 802.3 does
    • Who or what does?
  • The original IBM specification called for Shielded Twisted Pair (STP) wiring however IBM has now issued guidelines for UTP and Fiber
    • It is possible to connect Token Ring networks with the following cable types
      • STP
      • CAT 3/4/5 UTP
      • SM or MM Fiber
    • Token Ring operates at either 4mb or 16mb regardless of media type in use
      • Not all vendors' CAT 3/4 UTP equipment supports 16mb properly
  • Full Duplex 32mb (16mb each way, concurrently) Token Ring is now a standard IEEE ?????

• Token Ring physical connectivity options

• IBM Originally used what is referred to as a "hermaphroditic" connector.
• This connector could connect to other connectors simple by flipping one
• of the two connectors over and clicking them together. No gender

• benders were necessary. This made it easy to make a STP patch cable longer. This diagram shows a classic IBM Token Ring network. The Ring In of one MAU is connected to the Ring Out of the other MAU and vice versa thus creating a physical ring.

• Token Ring has a concept of ring repair similar to FDDI although not as sophisticated. In Token Ring the end station cannot wrap a ring link and FDDI station can. A broken Token Ring must be wrapped by a device like an intelligent hub. The next diagram shows how an intelligent hub wraps a ring.

• Token Ring physical layer issues

• Although the ring length is in fact extended to the end stations the four wires are not labeled the same as they are when interconnecting hubs. In the case of end systems the wires are primary transmit and receive and backup transmit and receive.

• Although Token Ring is in fact a "ring" it is wired in a star configuration as depicted here. Remember from the previous diagram that the "ring" extends out to each active end station.

• The five phase insertion process for connecting to an active token ring
  • Lobe Test

The station sends frames to itself without inserting itself into the ring. The frames are looped at the MAU. If the test passes the station sends a 5 volt signal down the lobe cable. This opens the relay and inserts the station onto the ring.

• Active Monitor Check

If no active monitor is present this station will become the active monitor.

• Duplicate Address Check

Duplicate MAC addresses are not allowed on the same ring. If the station detects another station with the same address already on the ring it will not insert itself into the ring.

• Neighbor Notification

This is the process where each station on the ring discovers its upstream and downstream neighbors. This information is used for isolating fault domains.

• Request Initialization from RPS if present
  • If there is an RPS on the ring it will provide information about the ring to a station in the RPS response. The RS can also prevent stations from inserting by implementing security restrictions configured by an administrator.

• Other Token Ring physical connectivity options

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