1. Ethernet By far, the dominant standard for guided media for the internet is Ethernet. How does it work?

3. Standard Ethernet The basis standard for all other ethernets. Data rate of 10Mbps. Ethernet frames have several components: Preamble - 56 bits alternating 1’s and 0’s SFD - 1 byte flag (10101011) Destination - MAC address of intended receiver Source - MAC address of sender Length/type - can be used to either designate the length of the data segment or the upper-level protocol used. Data - 46 to 1500 bytes CRC - 4 byte CRC-32 information

4. Minimum frame length is 64 bytes. –Required for proper use of CSMA/CD Maximum frame length is 1518 bytes –Keeps one station from hogging the link –Kept buffers small back when memory was expensive. MAC (or NIC) address is six bytes Three receiver modes: –Unicast: Frame only goes to one receiver –Multicast: Frame goes to many receivers –Broadcast: Frame goes to all receivers.

5. Uses 1-persistent CSMA/CD –Slot time is 2T p + jam sequence time –In Ethernet, this is defined as the time needed to send 512 bits (64 bytes); 51.2µs in standard Ethernet. –This implies a maximum network length. Theoretically it’s 5120m, but practically it’s about 2500m. At the physical layer, uses digital signals using Manchester encoding. –Specific physical medium used depends on topology.

6. Extensions of Ethernet What if we have a lot of devices on our network? Having one ethernet would lead to lots of collisions. Bridges allow for separating pieces of a network to reduce the frequency of collisions. Switches allow each device to be effectively put on its own logical network.

7. Fast Ethernet 100Mbps Backwards compatible with standard Ethernet. –Same addressing –Same frame format and min/max lengths. Only allows for star topologies. –Eliminates need for CSMA/CD overhead on full duplex links (like 10baseT). Adds autonegotiation, which allows a device and a hub to negotiate things like data rate.

8. Physical layer requires 100Base cable, which limits it to fiber (100BaseFX) or high end twisted pair (category 3 for full duplex or 5 for half-duplex). Manchester encoding requires too much bandwidth. Different schemes for different media: –100Base-FX uses 4B/5B and NRZ-I –100Base-TX uses 4B/5B and MLT-3 –100Base-T4 uses 8B/6T