THE ETHERNET
- Developed at Xerox PARC in 1973.
- A standard for MAC (Media Access Control) sublayer.
- Data transmission rate : 10 Mbits/s (Fast Ethernet reaches 100 Mbit/s)
- High-speed driver on the computers connected by coax or UTP cables.
- Uses a bus (10base2, coax, max. distance 200 mt.) or a star (10baseT, UTP, max. distance 100 mt.) topology.
- Also uses optical fibres (10baseF , max. distance 2000 mt.).
- All nodes wanting to send message contention for the bus.
- Broadcast the frames to all stations on the network.
- All stations are continually listening to the bus looking for frames addressed to them.
- Variable length frames : 64 to 1518 bytes.
- Transmission time : 50 - 1200 microsec.
- Frame Format :
- Destination address (6 bytes)
- Source address (6 bytes)
- Type (2 bytes)
- Data (46 ... 1500 bytes)
- Check sequence/checksum (4 bytes).
- Length of the frame is not provided. Mandatory interval between sending frames. Last 4bytes are assumed to be the frame check sequence (checksum).
- Type field is sometimes used to store the length of the frame.
- Not more than 1024 nodes in a single Ethernet, but the address field is 6 bytes.
- US Institute for IEEE gives a unique id for each Ethernet card. Each node has a unique address.
- CSMA/CD : Carrier Sense Multiple Access with Collision Detection
- Packet Collisions : When multiple stations attempt to send a frame simultaneously. Three mechanisms to address this.
- Carrier Sensing : Listen for the presence of signal (carrier) in the cable. Wait until there is no signal before transmitting.
- Collisions may still occur. A sends a signal and B sends it right after that but before it has sensed A's signal.
- Collision Detection : Send the signal (output port) and listen (input port) and compare the two signals. If they are different then there is a collision. Stop sending the message but send a special signal to intimate all stations that collision has occurred.
- Retransmit after waiting for a random time (nT) : T : time taken by a signal to reach all stations, n < MAX: random number. If there is another collision, double MAX and so on. CSMA/CD
- Efficiency : Fraction of frames transmitted successfully.
- Possible to get 80 - 95 %. However, noticeable delay after 50%.
- Interconnecting Ethernets :
- Repeater : Analog device that simply retransmits the signal it receives.
- Bridge : Digital so less distortion, may be intelligent, it repeats frames passing only the addresses that are on the corresponding side of the bridge (filtering). Keep a list of addresses. (Note: Each ethernet address is unique).
