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Extending LANs

Based on Chapter 11 in Computer Networks and Internets

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Size Limitations

• Recall that the combination of protocol and wiring scheme places size restrictions on a LAN – Ethernet using 10Base5 (thick) allows segments

up to 500 m– FDDI (Fiber Distributed Data Interface) allows

a single ring to be 100 kilometers (62 miles)

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Reason One

• Attenuation and interference: – The farther a signal travels, the more it

attenuates (weakens). If the amplitude is reduced, the signal-to-noise ratio may decrease. And the information may be lost in the noise.

– The farther a signal travels, the more likely it is to experience interference (noise) — other unwanted waves adding to it.

– Increasing the amplitude would help but that requires additional power and thus increasing cost.

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Reason Two

• Sharing Speed: – The longer a token ring (and the more

computers on it), the longer it takes before it is a given computer’s turn to transmit.

– The longer a bus (and the more computers on it), the more likely that two computers will transmit simultaneously causing a collision (CSMA/CD).

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Fiber-Optic Extensions• The attenuation/interference limitation can be

overcome by changing the medium: wire fiber optic cable.

• Recall fiber optic cable has a much higher bandwidth and is much less susceptible to interference.

• Building a LAN entirely of fiber is expensive.– One of the reasons FDDI did not become more widespread

is because of its cost.

• But fiber optic cable can be used to extend certain connections in a LAN.

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Fiber Optic Extensions (Cont.)

• The connection between the computer and the bus could be partially spanned by fiber optic cable.

• Electrical signal goes to a fiber modem. • The fiber modem converts the signal into an

optical form for transmission over the optical fiber.

• A second fiber modem receives the optical signal and converts it back to an electronic signal.

• The electronic signal is placed on the bus in the usual way.

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Fiber Extension (Fig. 11.1)

Note as shown above one is adding a single computer that is a large distance from the rest of the network. But this idea can be extended.

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Repeater

• Another solution to the attenuation/interference problem is to use a repeater.

• A repeater receives a signal, amplifies it, and then retransmits (forwards) it.

• Repeaters are used between segments of a local area network (LAN). – Segments have a fixed maximum length.

• Repeaters are also used in WANs, both wired and wireless.

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Repeater (Cont.)

• In its simplest version an analog repeater simply amplifies the signal it receives.

• Therefore, if any noise has corrupted the signal, that noise is also amplified. This is the best a repeater can do with an analog signal.

• It is possible for a repeater to clean up a digital signal provided it is not too noisy.

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It’s possible to clean up a digital signal

A noise ridden digital signal.

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It’s possible to clean up a digital signal

A cleaned-up digital signal.

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Repeater (Cont.) • While noise can in some sense be removed

from a digital signal by a repeater, digital signals tend to require more frequent repeating.

• Whereas analog signal amplifiers are spaced at 18,000 meter intervals, digital signal repeaters are typically placed at 2,000 to 6,000 meter intervals.

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Repeater (Fig. 11.2)

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Repeaters

• Repeaters allow the LAN size to be extended.• But the repeater solves only the attenuation-

interference problem and not the sharing speed problem.

• For this reason, some protocols place restrictions on the number or arrangement of repeaters.

• Ethernet standards allow only a maximum of 4 repeaters between any two nodes.

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Multiple Repeaters (Fig. 11.3)

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MAC Bridges• Like a repeater, a bridge (a.k.a. MAC bridge) connects

segments of a LAN, but a bridge is more “intelligent.” • Under “steady-state” conditions, a bridge only passes a

packet from segment A to segment B if the packet’s destination is on segment B or beyond.

• In other words, while a repeater works at the physical layer and sees the transmission only as a wave, a bridge operates at the data-link layer and understands the transmission as data, in particular its destination and source addresses.

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Bridge

• A bridge is a “computer” with more than one NIC card operating in promiscuous mode.– It will probably not be used for anything but this

purpose as the processor will be quite occupied with this task.

• Each NIC card is attached to a LAN segment.• All transmissions on these segments are read by

the bridge.

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Bridge (Cont.)• Most bridges are learning or adaptive bridges.• When such a bridge is first connected, it does not

know which computers are on which segment. • When a packet first arrives, the bridge “knows”

from which of its cards the message entered (i.e. what segment the message came from) and it also reads the source address.

• It has then learned the MAC address of a computer (the source) and which of its ports that computer is on.

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Bridges (Cont.)

• It does not yet know the side of the destination computer, so it must transmit (forward) the packet to all other ports.

• Later when a packet arrives having as a destination the previous packet’s source address, the bridge knows whether the packets must be forwarded or not.

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Bridge (Cont.) • The bridge develops a table of MAC addresses, and

after a time reaches its “steady state” in which it knows the addresses of most of the active computers.– Tables are refreshed periodically in case a computer is

moved.

• It only transmits packets– That are broadcast– That are multicast – That are unicast and have source and destination on

different ports of the bridge.

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Repeater vs. Bridge• Repeaters lead to identical traffic on the connected

segments. • Bridges reduce the amount of traffic on the

segments, freeing up the transmission line for increased traffic provided a reasonable amount of the traffic is intra-segmental (within a segment). – Increases throughput

• When designing a multi-segmented network, one wants to maximize intra-segmental communication.

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Bridge

• Recall that error-checking takes place at the data-link layer.

• Consequently, a bridge transmits only packets thought to be error free.

• Collisions, noise, interference are not transmitted across a bridge. – Repeaters do transmit error ridden packets.

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Bridge vs Router

• A restriction on a bridge is that the connected segments utilize the same protocol.

• A router serves a somewhat similar purpose but acts at a higher level (the network layer) and is “more intelligent.”

• Occasionally a bridge and router are combined in a product called a brouter.

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Long-distance bridge

• One can combine the ideas of the fiber extension and the bridge to achieve a bridge that extends over a long distance.

• The computer connected to the first LAN (segment) is given a second NIC card so that it can serve as a bridge to a second LAN (segment).

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Long-distance bridge (Fig. 11.6)

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Even Greater Distances (Fig. 11.7)

2 half bridges

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Bridged Networks

• A bridged network does not necessarily just form a long line with the end of one segment bridged to the end of another segment and a given segment bridged to at most two others.

B B B

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Bridged Networks (Cont.)

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Possible Problem

Segment X

Segment Y

Bridge 1 Bridge 2

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Possible Problem (Cont.)

• If a node on Segment X unicasts a message to a node on Segment Y, then (in steady state) Bridge 1 will forward it to Segment Y as will Bridge 2. It will arrive twice.

• If a node on Segment X broadcasts a message, then Bridge 1 will forward it to Segment Y, then the message will reach Bridge 2 and be forwarded to Segment X, where it will reach Bridge 1 and be forwarded to Segment Y, where ….

• We have an infinite loop (actually two counter rotating infinite loops).

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Logical vs. Physical again

• Physically, loops are good because they can provide a backup route should one route fail.

• Logically, loops are bad, they lead to an infinite cycling of messages.

• So long as the network is logically loop-less (that is, a tree), it is OK.

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STP• Spanning Tree Protocol (part of the IEEE 802.1

standard) allows for a bridged network that has physical loops but is logically a tree.

• STP puts the bridges that lead to a loop into a standby or blocked state (forming a logical tree).

• However, it stores alternate logical trees “in the event that one bridge is unable to perform its duties”– Path redundancy

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STP (Cont.)• Of all the logical trees, one wants the best, that is

the cheapest.• There will be some “cost function” which will

depend on the throughput of the various connections, the typical traffic patterns on those connections and so on.

• STP will select the “minimal” tree, but that could change, which is another reason that a blocked bridge may later be activated.

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May the circle be unbroken• Similar to the way one can have more than one

logical tree in case a bridge goes out, one can have various logical rings in case a connection within the ring is broken.

• If the network’s physical topology is a ring and it is broken, then it goes down.

• However, if the network’s physical topology is a star but it’s logical topology is a ring, then a new logical ring can be formed should a connection or node go down.

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FDDI Hub

• FDDI Hub contains the electronic circuitry necessary to detect a broken link and reconfigure the network.

• The FDDI logical network topology is a ring, but the physical topology is star.

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Switch

• A switch is an intelligent hub.• The hub operates at the physical layer,

forwarding an incoming signal to all other ports. • A switch operates at the data-link layer,

forwarding a (unicast) message only to the designated port.

• A switch is like a many-ported bridge with only one computer on each segment.

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Switch vs. Hub• For a given message a hub should be faster. • With increased traffic, switches should

improve throughput, like bridges different signals can be simultaneously transmitted on the various segments. – It allows different messages to be transmitted

“in parallel” — a given message is still sent serially.

• Hubs are cheaper.

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Switch vs. Router• Routers operate at a higher layer (the

network layer), so they are:– more “intelligent” improving throughput for

heavier traffic – slower for handling a given packet– More expensive

• There’s an intermediate device known as an “IP switch”

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Other references

• http://www.whatis.com

• http://www.webopedia.com