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Token Ring Lans:

IEEE Standard 802.5

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Like Ethernet, the token ring is a MAC protocol sitting betweenthe logical link control and the physical layer.

Data rates are listed at 4 Mbps and 1 Mbpsbut IBM token rings can run at 4, 16, or 100Mbps.

Differential Manchester Encoding is used.

Devices are configured in a ring.

Any particular device can send only to its neighbors, in most casesonly one neighbor as data flow is usually unidirectional.

Unlike Ethernet, collisions do not occur in token ring networks,contention is controlled by use of a special packet called a token.

Only one token circulates the ring, and a device wishing to send musthave possession of the token in order to send.

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In some token ring configurations, the failure of a single device

can cause failure of the entire network.

Each device, in turns checks the destination address of a data frame,

If the address matches the device address, the frame is copied, a few bits

changed and the frame is passed to the next station. If the address does

not match, the frame is simply passed to the next station.

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The token is a special frame.

Both token and frame formats are shown below:

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Each frame has a starting delimiter SD and an endingdelimiter ED.

The SD has a special signal pattern:

JK0JK000How to you send a J or K?By violating differential Manchester signal rules;

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Sending a J or K

We simply violate differential Manchester encoding rules;

remember, both Manchester and differential Manchester encoding require a signal transition in the middle of the bit period:

To send J: start out like you are sending a 0, ie, transition at thebeginning of the bit period, but do not transition in the middle of thebit period.

To send K: start out like you are sending a 1, ie, no transition at thebeginning of the bit period, but do not transition in the middle. Thesignals, J and K, will not appear naturally in data, so there is noconfusion. 1 0 J 1 K 1

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Notice that the ending delimiter sequence is:

JK1JK11E, the “E” bit is an error bit. Remember that the frame

check sequence using CRC uses modulo 2 arithmetic to check for errors,

since this is just the EXOR of the received bits and the divisor bits,

AND, since the error check operation can begin as soon as a string of

bits equal in length to the divisor function has been received and the

next step in the operation is performed as soon as the next single bit

arrives, by the time the last bit arrives, the error check is complete and it

is a simple matter to set the E bit, error bit, to indicate whether or not an

error has occurred.

The frame status field, FS has two bits which can be set by the receiving

station: address recognized bit and frame copied bit.

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Access Control Field AC

p p p t m r r r

the p bits refer to priority

the t bit t = 0 this is a token frame t = 1 this is a data frame

m monitor bit

r bits are used for reservations

All of these functions are necessary to efficient operationof a token ring LAN

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Reservations:

Tokens can only be claimed in a round robin fashion by stations unlessWe implement a reservation system.

We prioritize devices on the ring, and assign a priority to the token

This permits devices to capture the token in a different order.

A device can only claim the token if it has a sufficient priority.

Who defines the token’s priority.

Initially a device sends a token with priority 0.

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Example:

A token arrives with a priority higher than the device’s priority. The deviceCannot claim the token but it may be able to make a reservation.

It would examine the reservation bits, if the current reservation is lowerThan that devices priority, it replaces the reservation with its priority.

If the current reservation is higher than the priority of that device, it cannotMake a reservation at that time.

When a device drains a frame from the ring, it creates a new token.It examines the reservation bits and creates a new token with that Priority.It then stores the old priority and the new one (current) on its stack locally,It becomes “a stacking station” and only it can restore the token to itsOriginal priority.When the new token begins to travel the ring, it is claimed by the firstStation with an equal or higher priority.

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The device that originated a frame is the one which mustRemove it and put a new token on the ring. What priority shouldThe new token have?

Two possibilities:One: Some device with a higher priority than the frames’s priorityHas made a reservation. The device would give the new token aPriority equal to the reservation value and make new reservation value= 0 to give stations a chance to make a new reservation.

Any device with raises the token priority has the responsibility to lowerIt, this is why the station (stacking station) stores the old and currentPriority.

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Second case: reservation on incoming frame was lower than priority of incoming framePriority.

New token’s priority must be lowered, but by whom? If the current deviceDid not raise the priority, it cannot lower it. Presumably the token wouldEventually reach the stacking station and thus be lowered.

If the current station is the stacking station, it creates a new token withA lower priority, but what priority?

The stacking station compares the incoming reservation value with itsOld stacked priority and chooses the greater value

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Wireless Networks

IEEE Standard 802.11

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The previous examples represent LANs in which the devices are

Actually physically connected. It is often desirable to implement LANs

That are not physically connected.

These are Wireless LANs WLAN

Usually, you will want to have contact with a traditional wired LAN, this

Is accomplished through an “access point”

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Wireless LANs use either of two modes of communication:

Infrared waves

Radio waves

Infrared devices use light emitting diodes or laser diodes

The beams can be aimed directly at the receiver (point to point)Or they can be reflected off the ceiling or walls, the latter method isEasier.

One advantage of IR systems is that, since the signals do not penetrateWalls, infrared signals are not regulated by the FCC.For this reason, the same IR frequencies can be used in different areasOf a building without interferenceIR signals are not affected by radio interference, though bright sunlightOr heat sources can be a problem.

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Systems that use radio waves usually have built-in transmitters And receivers.

Broadcasting on a single frequency can cause problems fromInterference with competing devices or simple electromagneticNoise, motors, etc.

Using a single frequency makes the signals easier to jam and

makes it easier for some third party to intercept the signals.

To overcome these problems, standard 802.11 uses “spreadspectrum”. This technology is also used in cordless and cellularTelephones.

Instead of using a narrow frequency band, spread spectrumtransmission spreads the signal over a wider range of frequencies a wider bandwidth.

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This improves the noise immunity, since noise is usually confined toa small range of frequencies. It is more secure, since the data is spread among a number of frequencies, an eavesdropper on one frequency would hear only a small part of the message which mightappear to be noise.

How is this accomplished?

FREQUENCY HOPPING SPREAD SPECTRUM FHSSUsing a pseudo random number generator, we “hop” among aselected range of frequencies, sending only a few bits on each frequency.This may be as little as one signal element or even a part of one signalelement (fast FHSS)

Both the sender and receiver use the same pseudo-random numbergenerator algorithm and the same seed value. To an outsider, the sequencewould appear to be completely random, but if both parties use the samealgorithm and the same seed value, the sequence is deterministic.

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SLOW

FAST

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DIRECT SEQUENCE SPREAD SPECTRUM

DSSS works in a different way.

FHSS uses, over time, a wide frequency band but at any given instantis transmitting over a very narrow band.

DSSS expands a single data bit into many bits. This increases the required data rate and forces the transmitter to operate at a higher bitrate which, in turn, requires a higher bandwidth.

DSSS uses a “chipping sequence” of length N.The chipping sequence is determined, just as in FHSS, by a pseudo-random sequence generator, again with both sender and receiverusing the same generator algorithm and the same seed value, so thatthe sequence of values generated is deterministic.

Each data bit is combined, EXOR, with the chipping sequence toproduce a “chipping code” of length N for each data bit.

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If you use a chipping code of length N, then each data bit wouldrequire transmission of ten code bits, obviously greatly increasingthe number of bits that must be transmitted.

Anyone intercepting this transmission would not be able to determinewhat is being transmitted unless they have the chipping sequence.

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On the receiving end, use the exact same chipping sequence anddecryption procedure

receiver chipping sequence 0 0 1 0 1 1 0 1 1 0 0 0 1 1 0 1

receiver data stream 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1

actual data 0 1 0 1

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We saw that with Ethernet and Token Ring wired LANs, we neededsome method to allocate access to the medium, to determine whogets to transmit.

We have the same problem with wireless LANs. How do we handlecontention for access to the network?

A contention protocol such as the CSMA/CD used by Ethernet will notwork for a wireless LAN, you can not assume that you can listen to themedium to sense when another device is transmitting or to hear collisions that might occur.

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Hidden Station Problem

Scenario One:Assume stationsA and B are infrareddevices, the wall prevents either devicefrom sensing theother’s signal, so bothcould transmit to thereceiver simultaneouslywith realizing it andcausing a collision

Scenario Two:All stations are switched to radio waves, so the wall is no longer aproblem. However, let us assume, as indicated, that stations A and Care on opposite sides of the access point receiver, both A and C arewithin range of the receiver, but not within range of each other, bothcould transmit to the receiver without being able to hear each other,so ordinary CSMA/CD would not work in either case.

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How do we manage contention on Wireless LANs?IEEE Standard 802.11 includes a MAC layer protocol called

DISTRIBUTED COORDINATION FUNCTION DCF

this implements “Carrier Sense Multiple Access with Collision Avoidance”CSMA/CA

Just like CSMA/CD it does not eliminate all collisions, but it does reduce them.

THE CSMA/CA PROTOCOL:•Source wants to send. It first senses the medium, if busy, it waits until clear

using a persistence strategy as described for CSMA/CD. If clear, senderwaits an additional amount of time. The extra time is used to prioritizeactivities. Additional wait times are defined by a short interframespace (SIFS) and a DCF interframe space (DIFS) that is longer than SIFS.When a device is contending for the medium frame, it waits for a DIFS.After the DIFS wait, if medium is still clear, it sends a Request to Send(RTS) frame to destination, this frame also contains a duration value

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• The destination point receives the RTSD frame and mustrespond by sending a Clear to Send (CTS) frameback to source. However, it must also contend forthe medium since other sources may wish to send.These other sources may not have heard the RTS, thedestination proceeds as in the previous case, exceptwhen the medium is clear, it only waits a DIFS which isshorter than SIFS. The effectively gives the destinationhigher priority than other sources wishing to initiate atransmission with an RTS since they must wait an SIFS.

•When the source receives the CTS frame, it has permission to senddata. If two RTS frames collide, the sender will not know, butthe destinations will, so no destination will send a CTS, the senders, recognizing that they received not CTS in a specifiedamount of time, will assume a collision has occurred, both will wait a random amount of time and try again.

Other devices will receive the CTS frame along with the durationof the impending transmission and will wait the necessary timeperiod before attempting to send. Effectively giving the devicewhose RTS evoked the CTS, exclusive access to the medium.

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•Once the source has received the CTS frame, it sends a data frame collisions should not occur because other devices know not to send.

•When the destination receives the data frame, it returns an ACK, so the source knows the data has been received.

This is a simplified discussion of CSMA/CD. There is another featurecalled Point Coordination Function (PCF) used on systems requiringsome measure of coordination and control. The availabletransmission time is divided into “frames” much like slotted Aloha.

We will not discuss further here.

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ADDRESSING:

The 802.11 frame format specifies four address fields for each frame, why?

Wireless LAN Infrastructure

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A set of wireless devices with which a single AP communicatesdefines a basic service set (BSS)

There may be multiple APs and there may also be a distributionsystem (DS) to which the BSSs are connected.

The DS may be a LAN or a collection of LANs, but in all cases it allowsa device in one BSS to communicate with a device in another BSS or toaccess printers, servers, etc.

So when a device sends a frame, there are four possibilities:Case Description Address1 Address 2 Address3 Address4

(receiver) (Transmitter)1. X sends frame to Z, frame stays Z X BSS1 ------ within BSS12. X send frame to Y, Frame first AP1 X Y ------- goes to AP13. API send frame originating at AP2 AP1 Y X X to AP2 over a wireless DS Frame is destined for Y4. AP2 send frame originating at X to Y Y AP2 X ----

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All four cases have two things in common:

Address1 always specifies the device that should receive the frame.Address 2 always specifies the device that transmitted the frame

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Security:

In wireless networks, security is obviously a problem.

802.11 standard includes a security protocol called:Wired Equivalent Privacy (WEP)

The encryption algorithm uses a 40 bit secret key and appends a 24 bit initialization vector to create a 64 bit key, newer equipmentuses 128 bits

There are several variations of 802.11, Wi-Fi (802.11b) uses radiofrequency waves in the 2.4 GHz range and is rated at 11 Mbps, compared to 1 or 2 Mbps for 802.11.Uses on DSSS since the higher rate could not be achieved usingFHSS