MAC

Computer Networks

Chapter 4

Medium Access Sublayer

Chap. 4- MAC 2

Chapter OverviewHere we want to know how to handle broadcast networks. As compared to

point to point networks, a major issue is handling arbitration when there is competition for the network.

This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.

4.1 The Channel Allocation Problem

How to allocate a single channel among multiple users.

4.2 Multiple Access Protocols

How to handle contention for the use of a channel.

4.3 IEEE Standards for LANsHow do the protocols of the last sections apply to real systems. Here we talk

about the actual standards in use.

4.4 BridgesWays of connecting networks together.

4.5 High Speed LANs

Directions in high speed networks.

Chap. 4- MAC 3

CHANNEL ALLOCATION

PROBLEM:

Overview

Or, how to allocate a single channel among competing users.



4.3 IEEE Standards for LANs

4.4 Bridges

4.5 High Speed LANs

Chap. 4- MAC 4

CHANNEL ALLOCATION

PROBLEM:

STATIC CHANNEL ALLOCATION IN LANs AND

MANS

The traditional (phone company) way of allocating a single channel is Frequency Division Multiplexing. (See Figure) FDM works fine for limited and fixed number of users.

Inefficient to divide into fixed number of chunks. May not all be used, or may need more. Doesn't handle burstiness.

T = mean time delayC = capacity (bps)

λ = arrival rate1/ µ = mean length

1T = ---------- µC - λ

Chap. 4- MAC 5

CHANNEL ALLOCATION

PROBLEM:

STATIC CHANNEL ALLOCATION IN LANs AND

MANS

Now divide this channel into N subchannels, each with capacity C/N. Input rate on each of the N channels is A/N. Now

1 NT(fdm) = ----------------- = ------------ = NT µ(C/N) - λ /N µC - λ

Chap. 4- MAC 6

CHANNEL ALLOCATION

PROBLEM:

DYNAMIC CHANNEL ALLOCATION

Possible underlying assumptions include:

Station Model -

Assumes that each of N "stations" (packet generators)independently produce frames. The probability of producing a packet in the interval dt is λdt where λ is the constant arrival rate. That station generates no new frame until that previous one is transmitted.

Single Channel Assumption -

There's only one channel; all stations are equivalent and can send and receive on that channel.

Collision Assumption -

If two frames overlap in any way time-wise, then that's a collision. Any collision is an error, and both frames must be retransmitted. Collisions are the only possible error.

Chap. 4- MAC 7

CHANNEL ALLOCATION

PROBLEM:

DYNAMIC CHANNEL ALLOCATION

Continuous Time -

There's no "big clock in the sky" governing transmission. Time is not in discrete chunks.

Slotted Time -

Alternatively, frame transmissions always begin at the start of a time slot. Any station can transmit in any slot (with a possible collision.)

Carrier Sense -Stations can tell a channel is busy before they try it. NOTE - this doesn't stop collisions. LANs have this, satellite networks don't.

Chap. 4- MAC 8

Multiple Access Protocols

Overview

How to handle contention for the use of a channel.4.1 The Channel Allocation Problem



4.4 Bridges

4.5 High Speed LANs

Chap. 4- MAC 9


MULTIPLE ACCESS PROTOCOLS

Collisions work well for low utilization (they're not likely to happen.) Arbitration, which we'll talk about later, works better at high utilization.

ALOHA:

Developed in Hawaii in the 1970s.

PURE ALOHA:

Every station transmits whenever it wants to. Colliding frames are destroyed. The sender knows if its frame got destroyed, and if so waits a random time and then retransmits.

1. ANY overlap is a collision.

2. Best efficiency if frames are same size.

3. BUT, what is the efficiency? Talk about stochastic.

Chap. 4- MAC 10



• S = frames to be transmitted. In units of frames per frame time so that 0 < S < 1. (What is the meaning of frame time as used here??)

• G = S + frames retransmitted due to previous collisions.• P0 = probability that a frame does NOT suffer collision.• S = P0 x GUse the Figure to determine collision vulnerability.

Pure Aloha Continued

Chap. 4- MAC 11



Probability that k frames are generated during a given frame time (Poisson distribution):

G k e-G Pr[k] = -------------- k!

Probability of no traffic initiated during the vulnerable period:

P0 = e-2G so

Throughput per frame time is:

S = G e -2G See Figure 4.3

Pure Aloha Continued

Chap. 4- MAC 12



SLOTTED ALOHA:

Doubles efficiency by dividing time into "ticks". Sends occur only at the start of a tick. Vulnerable period is 1/2 of pure Aloha case, so

S = G e-G

See throughput on the last page. Best throughput is at G = 1 when

S = 0.37; empty slots = Pr[0] = 0.37; collisions = G - S = 0.26

Chap. 4- MAC 13


CARRIER SENSE MULTIPLE ACCESS PROTOCOLS:

This is where the sender listens before ejecting something on the wire. Collision occurs when a station hears something other than what it sent.

PERSISTENT AND NONPERSISTENT CSMA:

1-persistent CSMA

Station listens. If channel idle, it transmits. If collision, wait a random time and try again. If channel busy, wait until idle.

If station wants to send AND channel == idle then do send.

Success here depends on transmission time - how long after the channel is sensed as idle will it stay idle (there might in fact be someone else's request on the way.)

Nonpersistent CSMA (equivalent to 0-persistent CSMA)

Same as above EXCEPT, when channel is found to be busy, don't keep monitoring to find THE instant when it becomes free. Instead, wait a random time and then sense again.

Leads to 1) better utilization and 2) longer delays than 1 - persistent. (why?)

Chap. 4- MAC 14



Nonpersistent CSMA (equivalent to 0-persistent CSMA)

Same as above EXCEPT, when channel is found to be busy, don't keep monitoring to find THE instant when it becomes free. Instead, wait a random time and then sense again.

Leads to 1) better utilization and 2) longer delays than 1 - persistent. (why?)

p-persistent CSMA [For slotted channels.] If ready to send AND channel == idle

then send with probability p, and

with probability q = 1 - p defers to the next slot.

Interpret the chart for these shown in the Figure.

Chap. 4- MAC 15



CSMA WITH COLLISIONS DETECTION:

CSMA/CD - used with LANs.

When a station detects a collision, it stops sending, even if in mid-frame. Waits a random time and then tries again.

What is contention interval -- how long must station wait after it sends until it knows it got control of the channel? It's twice the time to travel to the furthest station.

Chap. 4- MAC 16


COLLISION-FREE PROTOCOLS

How long is a packet (or how long a wire is needed to contain a packet) of length 1500 bytes on a 100 Mbps ethernet?

As cables become longer and faster, the above methods become less efficient. So, . . . .

Bit map protocol - A "contention slot", subdivided into bits, allows each station to announce that it wants to

send. After the announcement, all stations can send in priority order, and there will be no fighting over the channel. Called "reservation protocol".

What are pros and cons of this method? Analyze at low and high loads.

Binary Countdown -

In the contention slot, each station places its ID. They all get or’d on top of each other. A particular station knows if it won because no wanting-to-send station had a higher number than it did in the slot. ( For instance, 101101 OR 110011 : The 101101 station knows it lost by the time it sends its second bit - it sees a “1” on the wire when it just sent out a “0”, so it knows the game is up.

Chap. 4- MAC 17


LIMITED-CONTENTION PROTOCOLS:

Collision techniques work well for low utilization (they're not likely to happen.) Arbitration, which we'll talk about later, works better at high utilization. This method provides best of these techniques.

Divide the stations up into groups. Stipulate that only members of group 0 can arbitrate for slot 0, members of group 1 for slot 1, etc. Works because it cuts down on the contention felt by any particular station.

Want a method that will have many members per group at low contention, and few (or one) member at high contention. Can use a binary search to do this.

Chap. 4- MAC 18


Wireless LAN ProtocolsIEEE 802.11

Wide Range of uses:• Infrared signals within a building• Mobile computing• Network of low flying satellites

Physical Properties:The spec allows running over three possible media

• Radio using frequency hopping• Radio using direct sequencing• Infrared over short distances ( 10 meters )

The explanation of this requires a detour into “spread spectrum radio”

Chap. 4- MAC 19



Spread Spectrum Radio• Purpose is to spread signal over a wider frequency range so as to

minimize interference ( military uses this for anti-jamming ).• So the signal can be in a very noisy environment and still get through.

Frequency Hopping:• Transmit the signal over a pseudo-random sequence of frequencies.

1st one frequency, then a second, etc. The sender and receiver are using the same random number generator so they can stay in sync.

• The spec calls for using 79 different 1 MHz wide bandwidths.

Direct Sequence:• Each bit of data is replaced by multiple bits in the signal.• Transmitter sends the exclusive or of the data, PLUS n random bits.• Again, both the sender and receiver know the random sequence.

Chap. 4- MAC 20



Direct Sequence Example:

A. Data Stream 1 0 1 0

B. Random Sequence 0100101101011001

C. XOR of the two: 1011101110101001

A.

B.

C.

Chap. 4- MAC 21



Collision Avoidance:• Similar to Ethernet, but not quite the same.• But more complicated because all nodes don’t see each other.

Example of the problem:• A and C send a signal to B.• But A and C aren’t aware of each other’s signals.• Signals collide at B.• But A and C don’t know they collided so don’t go into collision avoidance.A and C are “hidden nodes”

Example of another problem:• B is sending to A.• C can hear this signal from B• C assumes it can NOT transmit• But C could in fact transmit to DThis is called the “exposed node problem”

A. D.B. C.

C can “see” this range

Chap. 4- MAC 22



Collision Avoidance:Problem is solved by using a protocol Multiple Access with Collision

Avoidance ( MACA).

Sender and receiver exchange control frames so the transaction goes like this:• Sender (A) does Request to Send (RTS) to receiver(B)• B sends back Clear to Send (CTS)• A sends packet.• B sends an ACK after receiving the frame.

Logic:• If a node hears the CTS, it knows it is near the receiver - so don’t transmit.• If a node hears the RTS but not the CTS, it’s not near the receiver so it can start its own transaction.

RTS and CTS contain length of packetto be sent so others know how long to wait.

A. D.B. C.

C can “see” this range

Chap. 4- MAC 23



Distribution System:

Some nodes roam (A - H in the figure.)Some nodes are wired together -- Access Points (AP in the figure). These

“AP’s” are called the distribution system.

A.

D.

B.

C.

Distribution System

AP-1AP-2 AP-3

H.

E.

F.G.

The three regions shown are like cells in a cell phone system.

Two nodes ( A & B) could communicate with each other directly, but in practice they go thru the AP’s.

The path from A to E is: A > AP-1 > AP-3 > E.

Nodes

Chap. 4- MAC 24



Distribution System:

Protocol for how a node finds an access point:

A.

D.

B.

C.

Distribution System

AP-1AP-2 AP-3

H.

E.

F.G.

1. The node sends a Probe frame.

2. All AP’s within range respond with a “Probe Response” frame.

3. The nodes selects one of the AP’s & sends that AP an Association Request frame.

4. That AP replies with an Association Response frame.

AP’s also send a Beacon frame that advertises they are available.

Chap. 4- MAC 25



Frame Format:

Contains the following fields:Control - is the frame carrying data or is it RTS or CTS or is it forwarding data.Payload - up to 2312 bytes of dataCRC - checksum of the packet.Addr(I) - It’s possible that the packet needs to be sent across the distribution

system in which case we keep track of the original sender and the original receiver, but we also want to know intermediate senders and receivers.

Control Duration Addr1 Addr2 Addr3 SeqCtrl Addr4 Payload CRC

16 16 48 48 48 16 48 0-2313 32

Chap. 4- MAC 26

IEEE Standard 802 For LANs and

MANs

Overview

How do the protocols of the last sections apply to real systems. Here we talk about the actual standards in use.

802.2 Describes the upper part of the data link layer, the LLC (Logical Link Control).

Descriptions of the physical and lower part of the DLL are:

802.3 Is CSMA/CS LAN802.4 Is Token Bus802.5 Token Ring




4.4 Bridges

4.5 High Speed LANs

Chap. 4- MAC 27

IEEE Standard 802IEEE STANDARD 802.3: ETHERNET

This is a 1-persistent CSMA/CD LAN. Originated in Aloha.

WIRES:

Name Cable Max Segment Nodes/seg. Advantages10 Base 5 Thick Coax 500 m 100 Good for Backbones10 Base 2 Thin Coax 200 m 30 Cheapest System10 Base T Twisted Pair 100 m 1024 Easy Maintenance10 Base F Fiber Optics 2000 m 1024 Best between buildings

Chap. 4- MAC 28

IEEE Standard 802IEEE STANDARD 802.3: ETHERNET

Wiring

Repeaters - Multiple cables can be connected. From software point, a repeater is transparent.

Chap. 4- MAC 29

IEEE Standard 802Manchester Encoding

Life would be easy if:

binary 0 = 0 volts

binary 1 = 5 volts

But there's no way to distinguish a 0 from nothing-happening. Need to know when is middle of bit WITHOUT a clock.

Chap. 4- MAC 30

IEEE Standard 802802.3 MAC SUBLAYER PROTOCOL

Preamble == 7 bytes of 10101010

Start == 1 byte of 10101011

Dest == 6 bytes of mac address

multicast == sending to a group of stations.

broadcast == (dest. = all 1's) to all stations on network

Source == 6 bytes of mac address

Length == number of bytes of data

Data == comes down from network layer

Pad == ensures 64 bytes from dest addr thru checksum.

The pad ensures transmission takes enough time so it's still being sent when the first bit reaches the destination. The frame needs to still be going out when the noise burst from another stations collision detection gets back to the sender.

checksum == 4 bytes of CRC.

Packet Definition

Chap. 4- MAC 31


Packet Definition

Why you need minimum Packet Size.

Chap. 4- MAC 32


BINARY EXPONENTIAL BACKOFF ALGORITHM:

After a collision, station waits 0 or 1 slot. If it collides again while doing this send, it picks a time of 0,1,2,3 slots. If again it collides the wait is 0 to 23 -1 times. Max time is 210 -1 (or equal to 10 collisions.) After 16 collisions, an error is reported.

Slot is determined by the worst case times; 500 meters X 4 repeaters = 512 bit times = 51.2 microseconds.

500 m Rep 500 m Rep 500 m Rep 500 m Rep 500 m

Algorithm adapts to number of stations.

Chap. 4- MAC 33

IEEE Standard 802802.3 PERFORMANCE

Note that channel efficiency depends on F -- frame length, B -- network bandwidth, L -- cable length c -- speed of signal propagation e -- optimal number of contention slots per frame. (512 bits = 64 bytes means a

64 byte frame has value == 1.) BUT, this is not the optimal value.

1channel efficiency = --------------------- 1 + 2 B L e / c F

Note: Efforts focus on improving both B and L, both of which will decrease efficiency.

Note on traffic patterns; arrivals are not Poisson, but self similar. This means that fluctuations occur on any observation scale (kind of like fractals.)

Chap. 4- MAC 34

IEEE Standard 802Switched 802.3 LANs

Uses 10Base-T to each of the hosts. And a high speed backplane between the connectors. Works because the assumption is that many requests can be routed within the switch. Relieves congestion on the hub.

Routing -

Local (on-switch) destinations are sent there directly. Off-switch are sent to the backplane.

Collision Detection - The connections on the switch

form their own LAN and do collision handling as we've just seen. The switch buffers the transmission and ensures no collisions occur.

Chap. 4- MAC 35

IEEE Standard 802IEEE STANDARD 802.4: TOKEN BUS:

Need a mechanism to handle real-time, deterministic requirements. 802.3 could contend forever and this is often not acceptable.

A ring, with stations taking turns is deterministic. Uses logical ring on linear cable.

Mechanism -

o All stations numbered; station knows # of its neighbors.

o A token, required in order to send, is initialized by the highest number station.

o A station, receiving the token, does a send if it has a request, then sends the token to its logical (not necessarily physical) neighbor.

Activation -

o Stations can come and go on the bus, without breaking mechanism.

Cabling -

o Uses 75 ohm coax. Speeds are 1, 5, 10 Mbps.

Chap. 4- MAC 36


TOKEN BUS MAC SUBLAYER PROTOCOL:

Station has 4 possible priorities, 0, 2, 4, 6; station maintains 4 queues for requests.

Within each station,

• Token comes first to priority 6 queue. Sends occur until nothing to send OR timer expires.

• Token goes next to priority 4 queue. Sends occur until nothing to send OR timer expires.

• And so on . . . .

Proper setting of the various timers ensures that high priority requests happen first.

Chap. 4- MAC 37


The frame format. Fields are:

Preamble - used to synchronize receiver clock.

Start/End Delimiter - contains a non-data (illegal) Manchester Encoding.

Frame control - shows control or data. shows priority of data packets. flag requiring ACK from receiver. shows type of control frame (more later).

Destination Address - (same as 802.3) - usually 6 bytes.

Source Address - (same as 802.3) - usually 6 bytes.

Data - BIG - 8182 or 8174 bytes (note no length field - why not?)

Checksum - (Same as 802.3)

Chap. 4- MAC 38

IEEE Standard 802TOKEN BUS:

LOGICAL RING MAINTENANCE

control frames for ring maintenance.

SOLICIT_SUCCESSOR -

Gives sender’s address and the current successor's address. Stations not in the ring, with address between these two are invited to bid to be inserted.

• No response within given time ==> go on as before.

• One response ==> newcomer is inserted; becomes new successor.

• Two or more responses ==> answers collide so garbled.

Chap. 4- MAC 39

IEEE Standard 802TOKEN BUS:

LOGICAL RING MAINTENANCE

control frames for ring maintenance.RESOLVE_CONTENTION -

Causes responding stations to NOT immediately try to be successors, but use binary countdown by 0, 1, 2, or 3 slots. Mechanism also ensures that traffic isn't slowed down by solicitation.(limited to less frantic times.)

SET_SUCCESSOR -

Used by a leaving station. Sent to the predecessor to say the leaver's successor is now the predecessor's successor.

WHO_FOLLOWS -

The token sender listens to make sure the successor got and then passed on the token. If doesn't happen, it sends a WHO_FOLLOWS and failed station's successor sends a SET_SUCCESSOR to the failed one's predecessor.

SOLICIT_SUCCESSOR_2 -

The token sender can't find the successor and there's no response from WHO_FOLLOWS; This causes ALL stations to once again bid for a place in the ring - this is like starting from scratch.

CLAIM_TOKEN -

If the token holder crashes, then nothing appears on the ring. All station's timers go off and the contention algorithm determines who gets to generate the token.

Chap. 4- MAC 40

IEEE Standard 802IEEE STANDARD 802.5: TOKEN RING

• Not broadcast but point to point.

• All digital rather than analog (such as used by 802.3 for collision detection.)

• Chosen by IBM for its LAN; included by IEEE as Token Ring.

Calculate the number of bits on the ring at any one time:

• At R Mbps, a bit is emitted every 1/R microseconds (usecs).

• At a speed of 200 m/usec, each bit occupies 200/R meters of the ring. So a 1 Mbps ring, with circumference 1000 meters has only 5 bits on it at any one time.

In addition, there's a 1 bit delay at each station. (Data bit can be modified before being forwarded.)

Token is 3 bytes. Must be sufficient delay on the ring so that the whole token is there. Why?? Stations may be powered down, etc. - no guarantee that stations are adding delay. So may need to add artificial delay.

Chap. 4- MAC 41


• Arbitration -• Must hold the token in order to transmit.

• Listen mode - • Input just copied to output.• • Transmit mode - • Seize the token and put own data on ring. As sender's data comes back around, it

removes data. At end of transmission, stick token back on. Receiver can ACK receipt by flipping a bit on end of packet.

• Efficiency is excellent: At high usage, with many stations transmitting, they get token one after the other.

Chap. 4- MAC 42


Wires - Shielded twisted pair/ 1 or 4 Mbps.

Differential Manchester encoding.

Reliability -- Star Shaped Ring --

Chap. 4- MAC 43

IEEE Standard 802TOKEN RING MAC SUBLAYER

PROTOCOL:

Frame Structure Components -

SD, ED Delimiters - have illegal encoding so not confused as data.

AC Access control, containing bits for:

The token bit - flip this bit and it’s a data preamble

Monitor bit,

Priority bits,

Reservation bits

Frame control Provides numerous control options.

Source/Destination addresses/checksum

same as 802.3 & 802.4.

Chap. 4- MAC 44


PROTOCOL:

Frame Structure Components -Frame status

A bit - the intended receiver saw the packet

C bit - the receiver copied the packet into its buffers.

Serves as acknowledgment.

Priorities -

Token gives priority of that token - a sender must wait for token of correct priority. The access control byte (of the token or data frame) has reservation bits. As frame goes by, a requester can say it wants the token at that priority the next time around.

Chap. 4- MAC 45


PROTOCOL:

RING MAINTENANCE:

Monitor station oversees the ring, but on failure any station can become monitor.

CLAIM_TOKEN is a request to become the new monitor.

Monitor oversees:• .Lost token management - If timer says token not seen in a while, produce new

one.• .Orphan frames - (Frame on ring, but sender crashes before draining frame.) Sets

"monitor" bit in access control byte. If this bit seen as set the next time around, then something is wrong.

• .Garbled frame - Monitor drains the frame and issues new token.• .Delay time - Ensures enough delay so whole token fits on ring.

Broken rings handled by any station who thinks neighbors unreachable. Uses BEACON control type.

Token management handled by the monitor so not de-centralized. Management easier, but susceptible to berserk behavior.

Chap. 4- MAC 46

IEEE Standard 802COMPARISONS OF 802.3, 802.4,

AND 802.5:

In great scheme of things, differences are small. All three have approximately same technology and speed.

POSITIVES NEGATIVES

802.3 Large installed base. Has analog requirements.Simple protocol. Must detect possible weak remote station.Good configurability.Passive and cheap cable. Minimum size = 64 bytes.Low latency (no waiting Non-deterministic/no priorities.

for token.) Short cable length.Efficiency drops at higher speeds.

802.4 Highly reliable hardware. Lots of analog.More deterministic except Complex protocol.

when token is lost. Delay at low load waiting for token.Supports priorities.Good throughput and Small installed base.

efficiency.Cable can support

multiple channels.

802.5 Connections are Centralized control meansPoint-to-point. critical component.

Simple engineering. Delay at low load waiting for token.Fully digital.Use many media.Priorities possible.Short & long frames

possible.Good throughput and

efficiency.

Chap. 4- MAC 47

IEEE Standard 802IEEE 802.2: Logical Link Control

LLC

For when a reliable error-controlled flow-controlled data link protocol is required.

Also hides differences inherent in the 802.3/4/5 from the network layer.

Three possible options:

o Unreliable datagram service.

o Acknowledged datagram service.

o Reliable connection-oriented service.

DestinationAddress

SourceAddress

Control Information

LLCProtocol

Entity

LLCProtocol

Entity

Set mode/Information/Acknowledge/Poll

Set mode/Information/Acknowledge/Poll

Chap. 4- MAC 48

BRIDGESOverview

This is one way that networks are connected together.

Bridges operate in the data link layer, and so don’t have the intelligence to do much address resolution.

What we will talk about here -

Translation from one LAN type to another. Given a MAC address, how does a packet get to its destination.




4.4 Bridges

4.5 High Speed LANs

Chap. 4- MAC 49

BRIDGESThe Big Picture

Hub or repeater just electronic amplification.

Bridges operate with active Data Link Layer. Can convert between different physical/data link types. Way to connect multiple LANs.

Routers operate at Network layer - they read and depend on a specific protocol.

Protocol Converters are able to convert from one network layer type to another.

Detour on why to have multiple LANs -

Organizations have different LANs (802.3/4/5) to meet various needs.

• Cost - may make the cabling less expensive.

• To carry a combined load heavier than any one LAN could do.

• Total distance more than 2.5 Km.

• Bridges can act as firewalls, to partition against errant hardware.

• LANs broadcast everything on the LAN to all stations. May want to prevent this from happening for some data. A bridge partitions off these messages.

Chap. 4- MAC 50

BRIDGESThe Big Picture

How they work -

Chap. 4- MAC 51

BRIDGES From 802.x to 802.y

Issues -• Each LAN type has its own frame format. Bridges take off one type and put on

another. <<< Figure 4.36 >>>• LANs don't necessarily run at the same speed, so must reject or buffer the data• Two input LANs feeding one output LAN.• Each LAN type has it's own maximum data length. So bridges must do framing in

order to translate.• Network layers may time out because they expect the destination to ACK within a

given time; all this translating slows down the transmission.• All LAN types don't carry the same information:

o Priority bits.

o Acknowledgment bits.

In essence, the LAN standards are incompatible.

Chap. 4- MAC 52

BRIDGES Bridge Types

In addition to translating packets, bridges also route packets between source and destination. It’s this function we now turn to.

Transparent Bridges and Source Routing Bridges are two competing and mutually exclusive ways of routing packets.

Chap. 4- MAC 53


TRANSPARENT BRIDGES:Also called Spanning Tree Bridges -

Goals:• "Perfect" transparency. No one needs to do anything. It just works.• No hardware or software configuration required.• No switches, no routing tables.• Stateless (or as stateless as possible.)

Promiscuous mode:

• Accepts all packets from all LANs attached to the bridge.

• If destination is on incoming LAN, discard the packet.

• Otherwise, forward the packet.

• Use table (hashed) in bridge to determine choice of the LAN for forwarding.

Chap. 4- MAC 54


Parallel Redundant Bridges:

Here two or more bridges are used for reliability.

Problems with infinite flooding.

Solution is to overlay the Backward Learning policy with a virtual loop-free topology.

A Spanning Tree Bridge does this.

Note how paths are reduced.

Chap. 4- MAC 55


Parallel Redundant Bridges

(cont):Algorithm is as follows:

• Choose one bridge to be root of tree (lowest unique serial number wins.)

• Each bridge determines cost of the path from the root bridge to each of its ports. (The root path cost.) Cost determined by number of segments and the bit rate of those segments.

• Determine the root port - for a bridge, which of its ports has the lowest root path cost.

• Determine the designated bridge - the bridge that will handle requests for a particular LAN (even though that LAN may have several bridges attached to it.) Selection based on smallest path cost from the segment to the root bridge.

• Tree includes every LAN but not necessarily every bridge.

• Continues to check for topology changes.

Chap. 4- MAC 56


SOURCE ROUTING BRIDGES:

Used by IBM/rings. Here the sender holds ALL knowledge of how a packet should be routed.

The sender of a frame:

• Knows if destination is on its own LAN.

• Sets a bit alerting bridges destination NOT on its LAN.

• Places, in header, the exact path frame will follow.

Chap. 4- MAC 57


SOURCE ROUTING

BRIDGES(cont):In the figure above, the Path from A to D is L1, B1, L2, B2, L3.

The Bridge looks for bit set.

• Scans route - is the incoming LAN number followed by the number of the bridge doing the looking? If so, forward the frame, otherwise reject it.

• Can be done in software, hybrid, hardware.

• If the source doesn't know the route, it sends a "discovery frame" that goes to every LAN in the network. The destination replies and each bridge along the way puts its ID in that reply. The source then knows all that it needs. This discovery produces lots of excess packets.

Chap. 4- MAC 58


COMPARISON OF 802 BRIDGES

Chap. 4- MAC 59

High Speed LANsOverview

xxxx4.1 The Channel Allocation Problem



4.4 Bridges

4.5 High Speed LANs

Chap. 4- MAC 60

SUMMARY

2.1 Theoretical Basis For Data Communication

What every sophomore EE knows !!! How much data can be put on a wire? What are the limits imposed by a medium?

2.2 Transmission Media

Wires and fibers.

2.3 Wireless TransmissionRadio, microwave, infrared, unguided by a medium.

2.4 The Telephone System

The system invented 100 years ago to carry voice.

2.5 Narrowband ISDN

Mechanisms that can carry voice and data.

Here we want to know how to handle broadcast networks. As compared to point to point networks, a major issue is handling arbitration when there is competition for the network.

This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.


How to allocate a single channel among multiple users.


How to handle contention for the use of a channel.

4.3 IEEE Standards for LANsHow do the protocols of the last sections apply to real systems. Here we talk

about the actual standards in use.

4.4 BridgesWays of connecting networks together.

4.5 High Speed LANs

Directions in high speed networks.

Internet

MAC