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Monday, November 11, 2013

Ethernet Concepts and Questions


Below are few questions on how ethernet operates under different circumstances.
1.    Fast ethernet at full duplex uses 2 pairs or the wire RX (wire 1 and 2) and TX (wires 3 and 6) . In case the Fast ethernet port is set/negotiated to half duplex will it continue to use only 1 pair to both send and receive and hence needing to do CSMA/CD to avoid collisions.
            
2.  A gigabit ethernet uses all 4 pairs of the wire if the Gig port is negotiated to 100 Mb speed will it start using only 2 pairs instead of 2 ?

3.  Many Gig ethernet ports are capable of 10/100/100 in-terms of speed and half and full in-terms of duplex. So is the half duplex possible only when the speed is 10 or 100. Or with a speed of 1000 also half duplex can be achieved ? If yes how many pairs of the wire will it use ?

4.  I case of Ethernet the speed negotiation happens by the exchange of FLP's which are electrical signals. What is it in case of fiber media ?

1. This would not be a correct assessment of the situation. FastEthernet always uses 2 pairs, one for Rx, the other for Tx. The duplex setting has no influence on what pairs are used. On a single TP cable, there is in fact no way of creating an electrical collision. However, if there was a hub, two stations operating in full duplex would cause the collision in the hub's circuitry. That is why even with half-duplex and 2 distinct pairs for Rx/Tx, Ethernet has to perform CSMA/CD to avoid collisions.
2. You mean to say "2 pairs instead of 4". Yes, that is correct. If a Gigabit NIC negotiates 1Gbps, it will use all 4 pairs. If it negotiates 100Mbps or 10Mbps, it will use 2 pairs.
3. The half duplex is a very rare thing with 1Gbps Ethernet. Theoretically, it is possible to operate in half duplex if you are using 1Gbps hubs. However, such hubs were extremely rare (I have not seen any in my life), you could not daisy chain them at all (you had to build your whole network using a single 1Gbps hub and that was it), and understandably, with all their disadvantages when compared to switches, I doubt that any networking vendor seriously considered manufacturing 1Gbps Ethernet hubs. Even with half duplex, a 1Gbps Ethernet would still use all 4 pairs in a TP cable.
4. To our best knowledge, fiber transceivers are always manufactured only for a single mode of operation (speed, duplex, wavelength, fiber parameters) of operation. For example:

Switch# show int te0/1 capabilities
TenGigabitEthernet0/1
  Model:                 WS-C3560E-24TD
  Type:                  10GBase-LRM
  Speed:                 10000
  Duplex:                full
  Trunk encap. type:     802.1Q,ISL
  Trunk mode:            on,off,desirable,nonegotiate
  Channel:               yes
  Broadcast suppression: percentage(0-100)
  Flowcontrol:           rx-(off,on,desired),tx-(none)
  Fast Start:            yes
  QoS scheduling:        rx-(not configurable on per port basis),
                         tx-(4q3t) (3t: Two configurable values and one fixed.)
  CoS rewrite:           yes
  ToS rewrite:           yes
  UDLD:                  yes
  Inline power:          no
  SPAN:                  source/destination
  PortSecure:            yes
  Dot1x:                 yes

Note there is no alternative listed in the highlighted lines.

Therefore, negotiation is not really an issue here. However, at least in 100Mbps and faster versions, these transceivers exchange continuous streams of symbols (either frames or IDLE symbols) to keep synchronized and to know there is a live device connected.


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