Extending LANs 1 Motivation Recall Each LAN technology

Extending LANs 1

Motivation • Recall – Each LAN technology has a distance limitation – Example: CSMA/CD cannot work across arbitrary distance • However – Users desire arbitrary distance connections – Example: two computers across a corporate campus are part of one workgroup 2

Extension Techniques • Must not violate design assumptions • Often part of original design • Example technique – Use connection with lower delay than copper 3

Illustration Of Extension For One Computer • Optical fiber – Has low delay – Has high bandwidth – Can pass signals within specified bounds 4

Repeaters • • Hardware device Connects two LAN segments Copies signal from one segment to the other Connection can be extended with Fiber Optic Intra-Repeater Link 5

Repeaters (Con’t) • Amplifies signals from one segment and sends to the other • Operates in two directions simultaneously • Propagates noise and collisions • Each repeater and segment increase the delay – Limit of four repeaters • Drawbacks: – Do not understand complete frames – Does not distinguish between the signals that correspond to a valid frame and other electrical signals 6

Repeaters And The Original Ethernet Wiring Scheme • Designed for office • Only two repeaters between any pair of stations 7

Hub • Physically – Small electronic device – Has connections for several computers (e. g. , 4 or 20) • Logically – Operates on signals – Propagates each incoming signal to all connections – Similar to connecting segments with repeaters – Does not understand packets • Extremely low cost 8

Bridges • • • Hardware device Connects two LAN segments Forwards frames Does not forward noise or collisions Learns addresses and filters Allows independent transmission 9

Bridge Algorithm • • • Listen in promiscuous mode Watch source address in incoming frames Make list of computers on each segment Only forward if necessary Always forward broadcast/multicast 10

Illustration Of A Bridge 11

Extending A Bridge • Typically optical fiber • Can span buildings 12

Satellite Bridging • Can span arbitrary distance 13

Apparent Problem • Complex bridge connections may not be apparent • Adding one more bridge inadvertently introduces a cycle • Consider a broadcast frame 14

An Example of bridges connected in a cycle 15

Spanning Tree Algorithm • Allows cycles • Used by all bridges to – Discover one another – Break cycle(s) • Known as Distributed Spanning Tree (DST) 16

Spanning Tree Algorithm • Problem: loops • Bridges run a distributed spanning tree algorithm – select which bridges actively forward – developed by Radia Perlman – now IEEE 802. 1 specification 17

Algorithm Overview • Each bridge has unique id (e. g. , B 1, B 2, B 3) • Select bridge with smallest id as root • Select bridge on each LAN closest to root as designated bridge (use id to break ties) • Each bridge forwards frames over each LAN for which it is the designated bridge 18

Algorithm Details • Bridges exchange configuration messages – id for bridge sending the message – id for what the sending bridge believes to be root bridge – distance (hops) from sending bridge to root bridge • Each bridge records current best configuration message for each port • Initially, each bridge believes it is the root 19

Algorithm Detail (cont) • When learn not root, stop generating config messages – in steady state, only root generates configuration messages • When learn not designated bridge, stop forwarding config messages – in steady state, only designated bridges forward config messages • Root continues to periodically send config messages • If any bridge does not receive config message after a period of time, it starts generating config messages claiming to be the root 20

Switch • Electronic device • Physically similar to a hub • Logically similar to a bridge – Operates on packets – Understands addresses – Only forwards when necessary • Permits separate pairs of computers to communicate at the same time • Higher cost than hub 21

Conceptual Switch Function • Conceptual operation – One LAN segment per host – Bridge interconnects each pair of segments • NOT an actual implementation 22

Switched Ethernet A simple example of switched Ethernet. {each plug-in card forms its own collision domain, card to card communication via high speed backplane. } 23

Interconnection Devices • Repeaters – – Understand Volts, do not understand frames, packets, or headers Physical Devices that are connected to two cable segments • Hub – Multiport • Bridges – – Connect two LANs Route on frame addresses (e. g. , Extract 48 -bit destination address in Ethernet) • Switches – – Similar to bridges in that both route on frame addresses Most often used to connect computers • Router – Network Layer Device • Transport Gateway – Connect two computers that use different connection-oriented transport protocols. For example, TCP > connection oriented ATM 24

Interconnection Devices 2 25

Summary • LANs – Have distance limitations – Can be extended • Fiber can be used between computer and LAN • Repeater – Connects two LAN segments – Repeats and amplifies all signals – Forwards noise and collisions 26

Summary (continued) • Bridge – Connects two LAN segments – Understands frames – Uses addresses – Does not forward noise or collisions – Allows simultaneous transmission on segments 27

Summary (continued) • Hub – Central facility in star-shaped network – Operates like a repeater • Switch – Central facility in star-shaped network – Operates like a set of bridged segments 28