Thin Ethernet (10B2 / IEEE 802.3a) Segment length 0.5 m Up to 30 attached nodes Cable flexible and cheap Integrated or external transceiver connected via a BNC 'T' connectortransceiver Used mainly for workgroups Difficult to manage (i.e. breaks in cable difficult to locate)

10BT/802.3i Connects the computer directly (i.e. using a point to point link) to a wiring hub. Segment length 100 m Cable flexible and very cheap Standard RJ-45 connector used (also can be used for telephone and other networks) Used mainly for work groups (requires a hub to connect to the LAN)LAN Easy to manage (also can be used for telephone and other networks)

Ethernet Repeaters and Hub operate at the Physical LayerPhysical Layer connect together one or more Ethernet cable segments to provide signal amplification and regeneration A network of repeaters and hubs is therefore called a "Shared Ethernet" or a "Collision Domain". Only one host is allowed to transmission within a Collision Domain An 3Com Office Hub (8 ports) Stack of several hubs to wire a building No (or little) processing (memory) unit all ports operate as one Ethernet LAN all connected NICs operate in the half- duplex (CSMA/CD) mode at the same transmission speedCSMA/CD

Ethernet Bridges & Switches bridge has to forward frames from one LAN to another Operates at the data link layer Bridges Must learn which addresses belong to the computers connected via each port. Separate Collision Domain

Ethernet Switches Solution to put 300 hosts in one Ethernet (in one building) Separate the Collision domain Contain a high-speed backplane Store-and-forward (inboard memory for each line) ranging from the simplest low cost devices to expensive high performance switcheshigh performance switches Cisco Catalyst 5000 Switch 3Com LinkSwitch 1000

Frame Format Preamble Dest Addr: 6-byte MAC address Source Addr: 6-byte MAC address Length: 1500 bytes maximum Data Pad: minimum 64 bytes –To distinguish valid fames and garbage –Transmission should last longer than round-trip delay. Checksum A sent at t0 B send around t0+ B realize collision and send noise burst A hear the noise burst at around t0+ 2 A must not conclude transmission success before t0+2

Round-trip Delay and Transmission Time With maximum length 2500 meters Four repeaters Round-trip time: 50 usec At 10 Mbps, one bit last 100 nsec Thus 500 bits are needed to last 50 usec Which is roughly 64 bytes

Binary Exponential Backoff How stations randomly wait after a collision –One slot is 51.2 usec (round-trip delay) –At first collision, each station waits 0 or 1 slots –After the second collision, each station waits either 0, 1, 2, or 3 slots randomly. –If a third collision occurs, wait 0 to 7 slots. –So on and so forth –Maximum slots to wait is 1023 after 16 straight collisions

Fast Ethernet and Gigabit Ethernet 100 Base-T4 100 Base-TX All gigabit Ethernet is point-to-point Full-duplex mode –Must use switch (not hub!) –No contention –No need for media sensing Half-Duplex mode –Connected with Hub –Collision is possible