• Named after the "ether", a term describing the medium through which scientists once thought electromagnetic radiation flowed.
• Ether refers to the cable. Typical wire is 10Base-T (twisted pair), 10Base-2 (thin coax), 10Base-5 (thick coax). 10 refers to Mbps.
• Uses digital signal (+.85v, -.85v, 0v for idle), with Manchester encoding: each bit has a signal transition (low-to-high or high-to-low) in middle, to facilitate synch. Baseband.
• Transmission rate 10Mbps; Fast Ethernet is 100Mbps.
• Characterized by bus topology and CSMA/CD for coordinating transmission.
• CSMA/CD stands for:
• Carrier Sense : station ready to transmit senses the state of the carrier signal on the bus. If idle, begin transmitting, otherwise wait.
• Multiple Access : all stations use same bus, and "hear" all transmissions
• Collision Detection : if transmission from one station collides with transmission from another, both can detect collision and try again.

Consider how collisions can occur, since a station will not begin transmitting until it senses an idle carrier.

If collision occurs, how to avoid repeating it, since both stations will try again? Solution is to have each station wait a random amount of time before retrying. The two stations will likely select a different random value.

Specific Ethernet solution:

1. Specify fixed time slot (512 bits or 51.2 microseconds at 10Mbps)
2. station can begin transmitting only at beginning of time slot
3. after i’th collision involving a frame, station waits between 0 and 2ⁱ-1 time slots, selected randomly, before trying again. called Binary Exponential Backoff.

NOTE: B.E.B. is compromise solution to conflicting situation:

if range of time slots too small, high probability of repeated collide

if range of time slots too large, throughput suffers (wait too long to retransmit)

Ethernet uses 48-bit (6 byte) addresses. Each frame contains both source and destination address.

Recall that frames are broadcast.

• NIC reads destination address as frame comes by.
• If it matches the NIC i.d. number, the frame is saved and passed on to the O.S. of its station.
• *Otherwise, remainder of frame is ignored. Destination address is at the beginning of the frame. NIC handles all this without bothering its attached station.

Generic: consists of frame header part, with addressing and type information, and frame data part with the bits/bytes being transmitted.

Note: an octet is the same thing as a byte

Ethernet frame

Links courtesy of Purdue University web site for textbook: Computer Networks and Internets, by Douglas Comer.

NIC. Network Interface Card. Interface between computer and network. Handles network transmission so that the main CPU need not bother. Consider it a peripheral, like a disk or CD-ROM. Better reasons for NIC:

1. general purpose CPU is not fast enough to handle network speed traffic

2. CPU speeds vary from one computer to the next, whereas network operates at fixed speed

3. OS scheduling policies cannot guarantee the real-time response that network traffic processing requires

Last reviewed: 11 March 1998