Chapter 11 Extending LANs

1. Distance limitations of LANs2. Connecting multiple LANs together3. Repeaters4. Bridges 5. Filtering frame6. Bridged network7. Broadcast cycles8. Switching and Switched LANs

Note: Sections 11.3, 11.8, 11.9, 11.10 & 11.15 in the text will not be covered

Introduction

• LAN technologies are designed with constraints of speed, distance and costs

• Typical LAN technology can span, at most, a few hundred meters

• How can a network be extended to cover longer distances?

LAN design for distance• Many LANs use shared medium - Ethernet, token ring • Length of medium affects fair, shared access to

medium – set max length to ensure no significant delay – CSMA/CD (time to reach every part of Ether) & Token

passing time proportional to network size

• Length of medium affects strength of electrical signals and noise immunity – signal becomes weaker as it travels

LAN extensions

• Several techniques extend connectivity of LAN medium

• Most techniques use additional hardware to relay LAN signals between LAN segments

• Resulting mixed technology stays within original engineering constraints while spanning greater distance

Repeaters• A device that joins 2 cables to extend LAN medium

– continuously monitors signal on cables

– amplifies and transmit signal to other cable

• One repeater can effectively double the length of an LAN segment

Limits on repeaters• A pair of repeaters can be used to connect 3 segments (see fig 11.3 in your text)• Can't extend Ethernet with repeaters indefinitely • CSMA/CD requires low delay; if medium is too long, CSMA/CD won't work

• Ethernet standard includes limit of 4 repeaters between any two Ethernet stations

Characteristics of repeaters• Very easy to use - just plug in

• Simply copy signals between segments – Do not understand frame formats

– Do not understand hardware addresses

• Limitations: Repeaters may re-transmit unwanted signals – Collisions affect entire network

– Transient problems - noise - propagates throughout network

Bridges• Also an electronic device that connects two LAN segments

• Retransmits frames from one segment on other segment(s)

• Handles complete frame

– Performs some processing on frame

– listen to frame traffics on each segment

– verify frame arrived intact; ensure no interference

– forward frame to other segment

Bridged LAN segments• 2 connected segments behave like a single LAN• computers on segment 1 can send frames to computers in segment 2 and vice versa

Characteristics of bridges

• Relatively easy to use - just plug in

• more popular than repeaters

• does more than just forwarding frames

• Isolate collisions, noise

• typical bridge may consist of a conventional

computer

Filtering bridges• Bridges can do additional processing

– Don't forward collisions, noise – Only forward frames where necessary

• Bridge performs frame filtering and forwards frames along LAN segments to destination – don’t forward frame to next segment if

destination in the same segment

Frame filtering

• Bridge checks destination of each incoming frame • Looks up destination in list of known stations

– Destination not on same LAN: Forwards frame to next LAN segment on path to destination

– Doesn't forward frame if destination on same LAN segment from which frame was received

• Bridge needs to know location of each computer attached to the LANs that it is connected to

How does bridge set up table?

• Learns location of stations by listening to frames in promiscuous mode (examines all frames regardless of whether they are unicast, multicast or broadcast frames)

• Bridge examines source address in each frame – Adds entry to list for LAN segment from which frame was

received

• examines destination address in each frame – use destination address to determine whether to forward

frame to next LAN segment

Filtering example

Startup behavior of filtering bridges

• Initially, the forwarding tables in all bridges are empty

• First frame from each station on LAN is forwarded to all LAN segments

• After all stations have been identified, frames are only forwarded as needed

• May result in burst of traffic after, e.g., power failure

Multiple Bridges• Can use multiple bridges to interconnect many LAN segments • Station of segment c sends frames to station on segment g through B2, B1, B3 and B6

Bridges and cycles

Broadcast cycles• Broadcasts are forwarded through all bridges

• Recall that bridge use destination address to determine whether to forward frame (see slide 14)

• Bridge always forward a frame sent to a broadcast address

• Problem: Bridged network with cycles may have broadcast cycles.

• Eg, computer on LAN segment a sends a broadcast frame– B1 & B2 forward frame to segments b and c respectively

– B4 forward frame to d; B3 forward another copy of the frame to d

– Segment d received 2 copies

– B4 forward B3’s copy to segment c; B3 forward B4’s copy to segment b

– … and the cycle continues

Eliminating broadcast cycles• To prevent infinite loops:

– Bridges must cooperate to broadcast frames exactly once on each segment

– prevent bridged network from forming cycles

• Solution from graph theory - spanning tree used to determine which bridges will forward broadcasts

• As each bridge joins the network, it communicates with other bridges on special hardware (typically multicast) address

– Learns network topology

– Performs spanning tree computation (called distributed spanning tree algorithm)

– Determines if bridge will form a cycle

Switching

• Device that connects to 1 or more computers & allow them to send/receive data• Effectively a separate LAN segment for each port • Similar to hub - hub consists of single box with multiple ports each attach to a computer• Switched LAN: consists of a single electronic device that transfers frames among many

computers

Switching

Switches and hubs• Switches are more expensive per port • May make more sense economically to use hubs for some

stations and switches for others • hub simulates a single shared medium

– at most 2 computers can communicate at any time– only one computer can send at a time

• switch simulates a bridged LAN with one computer per segment– parallelism: With switching, multiple stations can transmit

simultaneously – Provides much higher aggregate bandwidth: up to one-half

of the computers can send data simultaneously

Summary• Repeater acts as amplifier and retransmits analog signals

• Bridge accepts entire incoming frame and retransmits

– Doesn't forward collisions

– Avoids collisions on destination segments

• Filtering bridge forwards frames only as needed

– Allows simultaneous use of LAN segments for local transmission

– Forwards all broadcast and multicast packets

• Switches provide full LAN speed to each port by simulating separate LAN segments