Ethernet Networking

Ethernet is a contention media access method that allows all hosts on a network to share the bandwidth of a link, is specified at the Data Link layer, and uses specific physical layer cabling and signaling techniques.  Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to share the bandwidth without having two devices transmit at the same time on the network medium.  When a node transmits in a CSMA/CD environment, all the other nodes receive and examine the packet to see if it is destined for them, bridges and routers prevent the transmission from propagating throughout the internetwork.


  1. Carrier Sense – a workstation listens to the network to ensure that there aren't any other stations transmitting when it wants to transmit.
  2. If the network is quiet for a period of time called IFG (InterFrame Gap), the station may transmit.
  3. The network is continuously monitored if it is determined to be busy. Once the IFG is observed, the station may transmit.
  4. When two or more stations are listening at the same time and determine the network is free and then try to transmit at the same time, a collision will occur and destroy both packets. The network is monitored during transmissions to detect collisions.
  5. Transmission is stopped immediately if the station detects a collision on the network during its transmission. A signal is sent on the network to indicate that a collision has occurred and informs all stations to disregard all corrupted packets they may have been receiving.
  6. A random backoff algorithm is applied to determine how long each station that had a collision during its transmission will have to wait to retransmit.
  7. Starts over at step one to repeat the transmitting process.

Since Ethernet networks are connected with repeaters to extend the network, whenever a collision occurs it is repeated on each adjoining segment.
How some LAN Hardware treats collisions:

  • A repeater receives and immediately retransmits each bit; it doesn’t depend on any particular protocol, it simply duplicates everything, including the collisions.
  • A bridge receives the entire message into memory. If the message was damaged by a collision or noise, it is discarded.  Otherwise, the message is queued up and will be retransmitted onto another Ethernet cable. The bridge has no address. Its actions are transparent to the client and server workstations.
  • A router acts as an agent to receive and forward messages. The router has an address and is known to the client or server machines. Typically, machines directly send messages to each other when they are on the same cable, and they send the router messages addressed to another zone, department, or sub-network.

IEEE Specifications for Ethernet
  • 802.3 - Ethernet LAN Specifications
    Two new specifications, both run on twisted pair and fiber optic.
  • 802.3u - Fast Ethernet
  • 802.3z - Gigabit Ethernet
Ethernet Speeds at Cisco Hierarchical Layers
  • Access - 10Mbps switches for clients, 100Mbps for Servers.
  • Access to Distribution - use 100Mbps.
  • Distribution to Core - use 100Mbps or 1000Mbps, with redundant links.

Cable Specifications
Name Class Length Host per Segment Topology
10base2 50ohm Coax 185m 30 Phys. + logical Bus
10base5 50ohm Coax 500m 208 Phys. + logical Bus
10baseT Cat 3,4,5 UTP 100m 1 Phys.. star + logical bus
100baseTx Cat 5,6,7 UTP 100m 1 Phys. star + logical bus
100baseFX Multimode Fiber 400m 1 point to point
1000baseCX STP 25m 1 Point to point
1000baseT Cat 5 UTP 100m 1 Phys. star + logical bus
1000baseSX Multimode Fiber 260m    
1000baseLX Singlemode Fiber 3 - 10K    

802.3u Fast Ethernet Features and Benefits

  • 100baseT was adopted by IEEE as the 802.3u standard in 1995.
  • 100baseT is ethernet operating at 10 times the speed of regular ethernet.
  • Just like 10baseT, it can be used in a shared or switched environment.
  • Can operate in full-duplex without collisions.
  • Use twisted pair or fiber.
  • The 100BaseT maximum network diameter is 205 meters, which is approximately 10 times less than 10-Mbps Ethernet.
  • 100BaseT networks support an optional feature, called autonegotiation, that enables a device and a hub to exchange information (using 100BaseT FLPs) about their capabilities, thereby creating an optimal communications environment.  Autonegotiaton supports a number of capabilities, including speed matching for devices that support both 10-and 100-Mbps operation, full-duplex mode of operation for devices that support such communications, and an automatic signaling configuration for 100BaseT4 and 100BaseTX stations.
  • The IEEE 802.3u specification for 100BaseTX networks allows a maximum of two repeater (hub) networks and a total network diameter of approximately 200 meters.  A link segment, which is defined as a point-to-point connection between two Medium Independent Interface (MII) devices, can be up to 100 meters.

100BaseT supports three media types at the OSI physical layer (Layer 1): 100BaseTX, 100BaseFX, and 100BaseT4.
Characteristics of 100BaseT Media Types
Characteristics 100BaseTX 100BaseFX 100BaseT4
Cable Category 5 UTP, or Type 1 and 2 STP 62.5/125 micron multi-mode fiber Category 3, 4, or 5 UTP
Number of pairs or strands 2 pairs 2 strands 4 pairs
Connector ISO 8877 (RJ-45) connector Duplex SCmedia-interface connector (MIC) ST ISO 8877 (RJ-45) connector
Maximum segment length 100 meters 400 meters 100 meters
Maximum network diameter 200 meters 400 meters 200 meters

Half-Duplex Ethernet

  • Defined in 802.3 ethernet, usually runs on 10baseT.
  • Uses only one wire pair with signals running in both directions on the wire, stations either transmit or receive not both.
  • Uses CSMA/CD protocol to detect collisions and retransmit if they occur.
  • If you attach a hub to a switch, the switch must run in half-duplex so the end stations can detect collisions.

Full-Duplex Ethernet

Full-duplex provides the means of transmitting and receiving simultaneously on a single wire. Full-duplex is typically used between two endpoints, such as between switches, between switches and servers, between switches and routers, and so on. Full-duplex has allowed bandwidth on Ethernet and Fast Ethernet networks to be easily and cost-effectively doubled from 10 Mbps to 20 Mbps and 100 Mbps to 200 Mbps, respectively.

  • Data can be simultaneously transmitted and received, doubling the nominal throughput.
  • Uses two pairs of wire, one pair sends and the other receives data.
  • Can't run on COAX, must be twisted pair or fiber.
  • Uses a point-to-point connection between transmitting and receiving devices.
  • When powered on, it negotiates with the other end of the link (auto-detect mechanism), it first checks for available speed 10 or 100mbps, it then checks to see if it can run in full-duplex. If it can't, it will run in half-duplex.


  • Uses RJ-45 connector with up to four pairs of twisted wire.
  • Twisted wires eliminate crosstalk between the wire and the more twists in the wire the higher the category rating and the quality.
    Straight Wired
  • The wires are in the same order at both ends of the RJ-45s.
    Used For
    • Router to hub or switch.
    • Server to hub or switch.
    • Workstation to hub or switch.
  • Two pairs of the wires are crossed at one end of the cable.
  • Switch the first and third and second and sixth wires on one end.
    Used For
    • Uplinks between hubs and switches.
    • Hubs to switches.
    • Router to router.
    • Connecting two PCs without a hub or switch.

CAT 5 UTP Wiring Order for Patch and Crossover Cables

  1. To make your own CAT 5 (patch/crossover) cable you will have to first cut the wire to the needed length and strip about 3/4 of an inch of the outer jacket off both ends.
  2. Next arrange the eight individual wires in the following order:
    Patch Cable Wiring Order    [ Left to Right ]
    Wire Color orange/white orange green/white blue blue/white green brown/white brown
    Wire Number 1 2 3 4 5 6 7 8

  3. Next, holding the wires together, trim them so the ends are all flush. Wire number 1 will stay on the left side.
  4. Next turn an RJ-45 jack upside down {locking tab down} and slide the wires in until they align below the brass connectors. Make sure they are still in the same order as above.
  5. Now, holding the wires firmly in the jack, place the jack in the pair of crimpers and proceed to firmly crimp them in place.
  6. Once done crimping, pull gently on the wires to verify that they were securely crimped; if they aren't secure, cut the bad end off and go back to step 1.
  7. If that side is done, you can now proceed to step 8 to make a crossover cable (the other side is done the same except wires 1 and 3 and 2 and 6 are switched) or if you want to make a patch cable (connects PC to hub) follow steps 1 through 6 for both sides of the cable.
  8. Only one end of a crossover cable is switched, and is in the following order:
    Crossover Cable Wiring Order    [1 + 3 and 2 + 6 switched from patch cable order]
    Wire Color green/white green orange/white blue blue/white orange brown/white brown
    Wire Number 1 2 3 4 5 6 7 8

  9. Now follow steps 3 - 6 to create the crossover cable.