Thin Ethernet 10 B 2 IEEE 802 3

Thin Ethernet (10 B 2 / IEEE 802. 3 a) • • Segment length < 185 m and > 0. 5 m Up to 30 attached nodes Cable flexible and cheap Integrated or external transceiver connected via a BNC 'T' connector • Used mainly for workgroups • Difficult to manage (i. e. breaks in cable difficult to locate)

10 BT/802. 3 i • 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) • Easy to manage (also can be used for telephone and other networks)

Ethernet Repeaters and Hub • operate at the Physical 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 3 Com Office Hub (8 ports) • No (or little) processing (memory) unit • all ports operate as one Ethernet LAN • all connected NICs operate in the halfduplex (CSMA/CD) mode at the same transmission speed Stack of several hubs to wire a building

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 switches 3 Com Link. Switch 1000 Cisco Catalyst 5000 Switch

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 t 0 B send around t 0+ B realize collision and send noise burst A hear the noise burst at around t 0+ 2 A must not conclude transmission success before t 0+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-T 4 • 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