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2

Covered in TransmissionLine & Interface Design

unit

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Results from the losses in the transmissionmedium

Guided media Signal strength decays exponentially

May be expressed as a logarithmic power ratio

3

Power_ ratio_ in_ dB =10log P1

P2

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5

Addition or subtraction of dB yields the system loss/gain between 2pointsdB level at point 4=(-9)+(14)+(-3)=+2dBdB is a ratio and gives no indication of absolute power levels

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Delay effects Multipath propagation effects in wireless

Skew in parallel ports or busses

Signals consist of various frequency

components Each propagate at different speeds in guided

medium

Results in phase shift at the receiver

Intersymbol interference

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Various noise and disturbances may causeerrors in interpreting the received signal Thermal noise

Uniform distribution across the frequency spectrum

White noise Random errors

Does not (normally) effect the following bit interval

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Receiver must extract timing from theincoming signal

Allows sampling when SNR is at maximum

Maintain intersymbol spacing

Indicates start/end of each timing interval

Inclusion of error detecting/correcting

Introduce additional bits into the raw data stream

Channel line encoding

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Nyquist TheoremSample rate of at leasttwice the maximumfrequency component ofthe signal to bedigitisedAliasing occursQuantizing noise

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12

The end-to-end transfer of data from atransmitting application to a receivingapplication involves many steps

Each subject to error.

Errors can occur both at the bit and at the packetlevel.

At the bit level, the most common error is bitcorruption

At the packet level, we see errors such as packetloss, duplication, or reordering.

Error control is the process of detecting and

correcting both bit and packet errors.

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Bit-level error control usually involves addingredundancy to the transmitted data.

In some schemes, there is sufficient information for

the receiver not only to detect errors, but also to

correct most of them.

At the packet level, we assume that bit-level errorcontrol can detect all bit errors.

(Detectable but uncorrectable bit errors are treated

as a packet loss.)

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Packet-level error control mechanisms detectand correct packet-level errors such as loss,duplication, and reordering.

We typically implement bit-level error controlat the datalink layer of the protocol stack.

Packet-level error control is typically found atthe transport layer.

Thus, bit level error control is usually hop-by-hop,whereas packet-level error control is usually end-to-

end.

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Generally speaking, we prefer hop-by-hoperror control on links where the error rate ishigh (so-called lossy links) and end-to-enderror control when entire path is more or less

error free.

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Information is transmitted on a link byvarying the state of a signal.

On a digital link, each signal statecorresponds to one or more 0's and 1's-asignal that can take 2n, states represents nbits of information. To decipher a signal on adigital link, the receiver compares the

received signal with a set of predefinedreferences.

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We usually measure the error probability on adigital link in terms of the bit error rate orBER the ratio of the mean number of errors in any given

interval to the total number of bits transmitted inthat interval.

Typical fiber-optic links have a bit error ratio in therange of 10-18- 10-14, but copper links can have a

substantially higher bit error ratio, depending onshielding and the operating environment.

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The major causes of bit errors are Gaussianand non-Gaussian noise, loss of linesynchronisation, scramblers, protectionswitching, and, for cellular communication,

handoffs and fading

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Gaussian noise

A common assumption is that the noise amplitude is

described by a Gaussian (normal) distribution. We call

such noise Gaussian noise. Gaussian noise on a line

leads to uncorrelated and sporadic bit errors.

Non-Gaussian noise

Non-Gaussian noise, which refers to noise that does not

obey a Gaussian distribution, can lead to bursts of

errors. Common sources of non-Gaussian noise are

electrical impulses, such as lightning or electrical sparks.

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A third source of bit errors is loss of bitsynchronisation between the transmitter andthe receiver.

Receivers periodically sample the receivedanalog waveform to extract digitalinformation.

They typically use the transitions in the

transmitter's signal as the input to a phase-locked loop to determine the transmitter'sclock automatically.

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Handoffs and fading can cause error bursts incellular communication.

A mobile unit is switched from one base station

to another. It is common for some information

from the mobile unit to be lost in this process.

Although voice callers may not notice this, data

sources (such as modems) can be badly hit.

Typical handoffs last for about 150 ms during whichtime the received signal is almost completely in error.

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Handoff errors can be substantially reducedby making the new connection before the oldone ends. However, this makes it necessary to overlap base-

station ranges, which leads to less efficient spatialreuse of the radio spectrum.

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If a mobile station does not receive a strongsignal from a base station because of hills,

buildings, or other obstacles, then its

received signal is in error until the unit movesaway from the obstacle.

We call this loss of signal strength fading, and it

causes long burst errors. There are two types of

fading: shadow fading, which is due to macroscopicenvironmental conditions, and short-term Rayleigh

fading, primarily due to vehicle movement.

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Two important functions Flow control

Error control

Communication systems have limitations

Speed at which they process incoming data Buffer space

Flow control enables the receiver to regulate dataflow Invokes a control procedure known as an acknowledgement

(ACK) Two common methods

Stop and wait

Sliding window

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Simple

Inefficient

Sender sends a frame and waits for an ACK

Must wait at least 2tprop plus tf (time toprocess the incoming frame)

No out of order frames

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May transmit several frames without receivingACK

A single ACK may acknowledge many frames

Utilises an identification scheme based on thesize of the window

Numbered modulo n, where n=window size-1

ACK is usually sent prior to window size

reducing to zero

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Refers to the method used to detect andcorrect errors that occur in the transmissionof frames

Retransmission of errored or dropped frames

Utilises ACK and NACK

Automatic Repeat Request (ARQ)

Three types

Stop and WaitGo-back N

Selective repeat

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Basically stop and wait flow control Extended to include NACK

Source sends a single frame

Waits for an ACK or NACK Timer counts down 2

tpropplus t

f

Very inefficient

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Based on sliding window AKA continuous ARQ

N specifies how many frames may be sent withoutan ACK or NACK

Receiver discards all incoming frames after anerror Waits to receive correctly the errored frame Sender must retransmit all frames after the one in

error, hence go back N

Also includes a timer More efficient and requires no re-ordering at the

receiver

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Only lost or damaged frames retransmitted More efficient

From the utilisation viewpoint

Complex implementation Re-ordering of retransmitted frames

Not often used

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Synchronous bit pipe Sending side of DLC supplies the sending side modem bits at a fixed

rate (one bit per T seconds)

Idle fill (dummy bits) when no data

Intermittent Synchronous bit pipe

Supplies synchronously when there is data to be sent

Sends nothing when no data

Receiver complications:

need to distinguish between 0, 1 and idle

Re-synchronize with sender at end of idle period

Asynchronous character pipe

Bits within a character sent at a fixed rate

Characters separated by variable delays

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In asynch serial communication, the electrical interface is held in the mark position betweencharacters. The start of transmission of a character is signaled by a drop in signal level to the spacelevel. At this point, the receiver starts its clock. After one bit time (the start bit) come 7 or 8 bits oftrue data followed by one or more stop bits at the mark level. The receiver tries tosample the signal in the middle of each bit time. The byte will be read correctly if the line is still inthe intended state when the last stop bit is read.

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We need to decide where a frame starts andends

Character based framing Uses special characters

SYN for idle and fill STX Start transmission

ETX End Transmission

Bit Oriented Framing Special string of bits 01111110

Start, end and fill

Length count Gives the frame length in a header field

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Frames consist of integer number of bytes Asynchronous transmission systems using ASCII to transmit printable

characters

Octets with HEX value

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When sending arbitrary data (not characters)control characters may appear in frame SOLUTION: Transparent Mode: Introduce DLE

Start of text: DLE STX, End of text: DLE ETX

If DLE appears in packet? Use DLE DLE (receiver strips first DLE of pair)

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Data to be sent

A DLE B ETX DLE STX E

After stuffing and framing

DLE DLE B ETX DLE DLE STXDLE STX A E DLE ETX

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Errors in header and packet caught by CRC Errors in DLE STX and DLE ETX

An entire frame is missing

Errors could causeDLE ETX

to appear inmiddle of frame Receiver interprets the bits following as CRC

So it would be dropped

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Frame delineated by flag character

HDLC uses bit stuffingto prevent occurrence of flag01111110 inside the frame

Transmitter inserts extra 0 after each consecutive five1s insidethe frame

Receiver checks for five consecutive 1s if next bit = 0, it is removed

if next two bits are 10, then flag is detected

If next two bits are 11, then frame has errors

Flag FlagAddress Control Information FCS

HDLC frame

any number of bits

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0110111111111100

Data to be sent

After stuffing and framing

0111111001101111101111100001111110

(a)

*000111011111-11111-110*

Data received

After destuffing and deframing

01111110000111011111011111011001111110

(b)

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DLL Protocols are sets of specifications usedto implement DLL.

Contain rules for line discipline, flow control,error handling, etc.

DLL Protocols comes in two broadgroups:

Asynchronous (treat each character in a bitstream independently)

Synchronous (takes the whole bit stream andchop it into characters of equal sizes)

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Developed over the last several decades XMODEM, YMODEM, ZMODEM, BLAST, Kermit

and Others

Used mainly in modems.

Due to its inherent slowness (stemming from

required addition of start and stop bits and

extended spaces between frames) asynchronous

transmissions at this level is being replaced byhigher speed synchronous mechanisms.

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Designed in 1979 by Ward Christiansen file transfer protocol for telephone line

communications between PCs.

Half duplex stop and wait ARQ protocol

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The 1st field is a one-Byte SOH (Start ofHeader).

The 2nd field is a 2-byte header. The first header byte is SN.

The second header byte is used to check thevalidity of the SN.

The fixed data field holds 128 Bytes (binary,ASCII etc).

The last field, CRC, checks for errors in datafield only.

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Transmission begins with sending of a NAK fromreceiver to sender.

Each time the sender sends a frame, it must waitfor an ACK before next frame is sent again.

A frame can also be resent if a response is notreceived by the sender after a specified amountof time.

Besides a NAK or an ACK, the sender can receive

a cancel signal (CAN), which aborts transmission. Slow but reliable

Necessary at that time

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This is similar to XMODEM, with followingmajor differences:

Data unit is 1024 bytes

Two CANs are sent to abort transmission

ITU-T CRC-16 is used for error checking

Multiple files can be sent simultaneously

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Zmodem is a newer protocol Combines Xmodem & Ymodem features

BLAST Full duplex

Sliding window flow control

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Currently the most widely used asynchronousprotocol. Similar in operation to XMODEM

sender waiting for a NAK before starts transmission.

Allows the transmission of control characters astext using two steps.

The control character is transformed to a printablecharacter by adding a fixed number to its ASCII coderepresentation.

The # character is added to the front of thetransformed character.

a control character is sent as two characters.

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When the receiver encounters a # character it knows that this must be dropped and that the

next character is a control character.

If sender wants to send a # character, it will send

two of them. Note that Kermit is a terminal emulation

program as well as a file transfer protocol.

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There are two broad categories of synchronousprotocols:

Character Oriented

interpret a transmission frame as a succession of characters,

each usually composed of one octet (8 bits). All controlinformation is in the form of an existing character encoding

system (e.g. ASCII characters).

Bit Oriented

interpret a transmission frame of packet as a succession of

individual bits made meaningful by their placement of theframe. Control information in a bit oriented protocol can be

one or multiple bits.

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Not as efficient as bit oriented protocols seldom used.

easy to comprehend and employ the same logic andorganization as the bit oriented protocols.

In all DLL protocols, control information isinserted into the data stream either as separatecontrol frames or as additions to existing dataframes. In character oriented protocols this information is in the

form of code words taken from existing character sets

The best known is IBMs binary synchronouscommunications (BSC)

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Usable in both Point to point and multi pointconfigurations Supports half duplex transmissions using stop and wait

ARQ flow control and error correction.

BSC protocol divides transmission into frames.

Two types, Data and Control.

Data frames are used to transmit information but maycontain control information applicable to thatinformation.

Control frames are used to exchange informationbetween communicating devices. (e.g. establish initialconnection, control the flow of the transmission, requesterror corrections, disconnect the devices).

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SYN is used by the receiving device tosynchronize its timing with the sendingdevice.

STX tells the user that the next byte starts the

data (variable length) until ETX is reached.

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Problems Require addresses

SN, at least 0 or 1 for stop and wait

The probability of error in the block of text

increases with the length of the block. a message is often divided between several blocks.

Each block except the last one, starts with an STX andends with an ITB (Intermediate Text Block).

The last block starts with an STX and ends with an ETX.

Immediately after an ITB or ETX there is a BCC field.

If a retransmission is required, the entire frame isrequired to be transmitted.

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Entire message in one frame Several frames can carry continuations of a single

message.

The ETX in all frames but the last is replaced by ETB

(End of Transmission Block). The receiver will acknowledge each frame separately but

cannot take control of the link until it receives the ETX

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Control frames are not control characters. They are used to send commands to or solicit

information from another device.

Such a frame contains no data but it carries informationspecific to the function of the DLL itself.

Control frames serves in establishing connections,maintaining flow, and error control during datatransmission and terminating connections.

BSC was originally designed to transport textual

messages a user is just as likely to send binary sequences that

contain non textual information and commands.

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Byte oriented protocols bits are grouped into predefined patterns (characters)

Bit-oriented protocols

more information into shorter frames

Avoid the transparency problems of character oriented

protocols.

Broadly categorized into

SDLC (Synchronous Data Link Control) HDLC (High-level Data Link Control)

LAPs (Link Access Protocols)

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In 1975 IBM pioneered development of SDLC and lobbied ISOto make SDLC the standard.

In 1979, ISO answered with HDLC, which was based on SDLC.

Adoption of HDLC by ISO led to its adoption and extension by

other organizations.

ITU-T was one the first organizations that embraced HDLC.

LAPs (LAPB, LAPD, LAPX, etc), all based on HDLC.

Other protocols such as Frame relay and PPP developed both by

ITU-T and ANSI also derive from HDLC, as do most LAN access

control protocols.

In short all bit oriented protocols in use today either derive from

or are sources for HDLC.

Thus, through HDLC we can obtain a basic understanding for

others.

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HDLC (ISO 3009, ISO 4335) is a bit oriented DLLprotocol designed to support both half duplex and

full duplex communication over point to point and

multi-point links.

Systems using HDLC can be characterized by theirstation types, configurations and their response

modes.

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HDLC differentiate between three types of stations: Primary:

This has complete control of the links in point to point and

multi point line configurations. Frames issued by primary are

called commands

Secondary:

Primary issues commands to secondary and secondary sends

responses. Primary maintains a separate logical link with each

secondary station on the line.

Combined:

This can both command and respond and behaves either as a

primary or as a secondary depending on the nature and

direction of the transmission.

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Primary, Secondary and Combined can be connectedin three different configurations supporting half and

full duplex.

Unbalanced configuration

also called master-slave is one in which one device is the primary

and the others are secondary.

These can be point to point but more often they are multipoint,

with one primary controlling several secondaries.

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Symmetrical configuration is one in which each physical

station on a link consists of two logical stations (one a primary

and the other a secondary)

Separate lines link the primary aspect of one physical station to the

secondary aspect of another physical station. A symmetrical

configuration behaves like an unbalanced configuration except that

control of the link can shift between the two stations.

Balanced configuration is one in which both stations

in a point to point topology are of combined type.

The stations are linked by a single line that can be

controlled by either station.

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HDLC has three modes of data transfer: NRM (Normal Response Mode)

Used in unbalanced configurations. The primary may initiatedata transfer to secondary. But a secondary may transmit only inresponse to a command.

ARM (Asynchronous Response Mode) A secondary may initiate a transmission without permission

from primary whenever channel is idle. All transmission fromsecondary (even to another secondary on same link) must stillbe relayed through primary.

ABM (Asynchronous Balanced Mode)

All stations are equal and therefore only combined stationsconnected in point to point are used. Either combined stationmay initiate transmission with the other combined stationwithout permission.

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HDLC defines three types of frames Information frames (I-frames)

used to transport user data and related control

Supervisory frames (S-frames)

used only to transport control

Unnumbered frames (U-frames)

reserved for system management (managing the link)

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The flag field is an 8-bit sequence

01111110

The second field is the address field.

Contains the address of the secondary station is to receive

the frame. Not needed for point to point links; however, it is always

included for uniformity. (PPP)

The address field can be one or several bytes long.

If address field is several bytes, all bytes but the last one

will end with a 0 only the last will end with a 1. Ending the

intermediate byte with a 0 indicates to the receiver that

more address bytes exist.

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Control field One or two byte segment of frame used for flow

management.

If the first bit of the control field is a 0 the frame

is an I-frame.

If the 1st two bits are 10 then it is a S-frame.

If both 1st and 2nd bits are 1 then it is a U-frame.

The control fields of all three types offrames contain a bit called poll/final (P/F)

bit.

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An I-frame contains 3-bit flow and error controlsequences

N(S) (for SN)

N(R ) (for RN) flanking P/F bits.

Thus, N(R ) is the acknowledgement field. The control field of an S-frame contains an N(R)

field but not an N(S) field.

S-frames do not transmit data and hence do not require

N(S). The two bits preceding P/F bits in an S-frame are usedto carry coded flow and error control information.

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U-frames have neither N(S) nor N(R) fieldsand are not designed for user data exchangeor acknowledgement. U-frames have two code fields

one 2-bits and the other 3-bits flanking the P/F bit. These codes are used to identify the type of U-frame

and its function.

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The P/F field is a single bit with a dualpurpose.

It has meaning only when it is set and can mean

poll or final.

It means poll when the frame is sent by primarystation to secondary

It means final when frame is sent by secondary to a

primary.

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Information field is present only in I-frames and U-frames The field can contain any sequence of bits but must

consist an integral number of octets.

The length of the information field is a variable up to some

system defined maximum.

FCS (Frame Check Sequence) field is an error detecting

code calculated from the remaining bits of the frame

(exclusive of flags).

The normal code is the 16 bit CRC-CCITT.

An optional 32-bit FCS using CRC-32 may be employed if the

frame length or the line reliability dictates this choice.

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HDLC operation consists of exchange of I-frames, and some U-frames.

Operation of HDLC involves three phases.

First one side or another initializes the Data Link so that

frames may be exchanged in an orderly fashion. During this phase, the options that are to be used are agreed

upon.

Then the two sides exchange user data and the control

information to exercise flow and error control. Finally, one of the two sides will signal termination of

operation.

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Initialization: This may be requested by either side.

Initialization procedure signals the other side that

initialization is requested and specifies which of the

three mode (NRM, ARM, ABM) is requested and whether3 or 7 bit sequence numbers are to be used.

If the other side accepts then the HDLC Module at that

end will send an UA (unnumbered acknowledgement)

frame back to initiating side.

If request is rejected then DM (Disconnected Mode)

frame is sent.

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Data Transfer: Once initialization has taken place a logical connection is

established.

Both sides may begin to send user data in I-frames

SNs will be either modulo 8 or modulo 128 depending on

whether 3 or 7 bit sequences are used. RR (received ready) frame is used when there is no reverse

user data to carry acknowledgement.

RNR (Receiver Not Ready) acknowledges I-frames askingthe peer entity to suspend transmission of I-frames.

REJ (Reject) indicates the Go Back n ARQ. SREJ (Selective Reject) is used to request retransmission of a

single frame.

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Disconnect: Either HDLC module can initiate a disconnect.

HDLC issues a disconnect by replying with a DISC(disconnect) frame.

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Several protocols under the general category of LAPhave been developed. Each of these is a subset of HDLC tailored for a specific

purpose.

LAPB: Link Access Procedure Balanced is used only for connecting

a station to a network and is a subset of HDLC thatprovides only the ABM.

LAPB was issued by ITU-T as part of x.25 packet switchingnetwork interface standard. Its frame format is same as

HDLC Thus it provides only the functions required for communication

between a DTE and DCE.

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LAPD: Link Access Procedure D channel is another simplified subset of

HDLC issued by ITU-T as part of its recommendation on ISDN.

LAPD provides DLC over channel D.

There are several key differences between LABD and HDLC.

LAPD is restricted to ABM and always use 7-bit SN. The FCS of LAPD is always 16 bit CRC.

The address field of LAPD is a 16 bit field that actually contains

two sub addresses.

LAPM:

Link Access Procedure for Modems is designed to do

asynchronoussynchronous conversion, error detection, and

retransmission.

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LLC: Logical Link Control is a part of IEEE 802 family of

standards for controlling operation over a LAN.

LLC is lacking some features found in HDLC and

also has some features not found in HDLC. The most obvious difference is in the frame format.

Link control functions in the case of LLC are divided

between two layers:

A MAC and the LLC layer.

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The previous shows the structure of the combined MAC/LLC frame.

Shaded portion corresponds to fields produced by LLC.

The rest are added by the MAC.

Two addresses are required since there is no concept of primary and

secondary.

Error detection is done at the MAC layer by 32-bit CRC.

At the LLC layer there are the destination and source services Access

Points (DSAP and SSAP) identifying the logical user of LLC at thesource and destination systems.

LLC control field has the same format as the HDLC limited to 7 bit

SNs.

Operationally LLC offered three forms of services.

The connection-oriented mode service is the same as ABM of HDLC.

The other two services, unacknowledged connectionless and

acknowledged connectionless.

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A LAN is a community of users who share theirinterconnecting medium

The Media Access Control (MAC) protocol

handles this function

Two basic types of LAN

Broadcast

All users receive transmitted information

Random access and controlled access contention methods Switched

Employs forwarding logic and switching tables

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Usually average packet delay vs throughput Based on a queuing model

Node is represented by a single server queue

Node has a single buffer

Packets of length Lp arrive and queue up to be processed

Packets arrive according to Poisson distribution Not true - traffic is bursty, therefore a self similar distribution should be

used

The probability Pn(t) of exactly n packets arriving during time interval t is

given by

Where is the average packet arrival rate

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Pn t( ) =e

- l t l t( )n!

n

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Throughput S is the measure of successful traffic S often shown as normalised

Lp=packet length

R= Transmission rate

Throughput is expressed in terms of offered load G Normalised offered load is

Max throughput (capacity) is found by maximising S WRT G

Latency is the time lapse between the generation ofthe first bit and the receipt of the last bit at thedestination

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S=l L

P

R

G =lTL

P

R

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Queuing delay As the traffic increases packets have to wait longer

before they can be sent

Transmission delay

Packet transmission time Tf is the time it takes totransmit a unit of data Tf=Lp/R

Propagation delay Dependant on the media

3x108

m/s in space 2.3x108 m/s in copper

2x108 m/s in optical fibre

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Stations contend for time on the network No control mechanisms

Aloha

Any station transmits whenever they have data

Maximum of 18% efficient (Proof to follow)

Slotted Aloha

Same as Aloha with the addition of time slots

Each slot is the same length as Tf Transmission may only occur at the start of a slot

No partial overlapping of packets

Doubles the efficiency of pure Aloha to about 37%

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CSMA- an improvement Carrier Sense Multiple Access

Multiple Access, obvious

Carrier Sense

Listen to the media and transmit only if no transmission is

already taking place CSMA/CD

Adds in collision detection

Listens to the channel

If a collision is detected, send a jamming signal and cease

transmission Wait a random amount of time before attempting to

retransmit

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CSMA/CA (802.11) Collision avoidance

Details in the wireless unit

CDMA (Mobile comms)

Code division multiple access

May cover this briefly

Frequency hopping

Direct sequence

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Transmit and wait (2Tprop) for an acknowledgement Drops packets after K attempts

Assuming

All packets are the same length

Each requires tp (slot) for transmission

Vulnerable period = 2tp

S=Throughput, G=offered traffic

Assume the probability pk of k transmissions follows a

Poisson distribution then

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pk =Gke- G

k!

S is just the offered load times

psuccess

or p0

S=Gp0

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Where p0, probability that no packet isgenerated in 2tp

P0=e-2G so that S=Ge-2G

The maximum value of S occurs at G=0.5

where S=1/2e or 0.184

Thus the maximum throughput of pure aloha

is 18%

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All users are synchronised to time slots Vulnerable period is now tp p0 =e

-G which leads to

S=Ge-G

Maximum throughput is where G=1 so

S=1/e=0.368 twice that of pure aloha

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Three possibilities when channel is sensed idle

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CSMA Protocol Characteristics

NonpersistentIf medium is idle - transmit

If busy, wait a random time and re-sense channel

1-PersistentIf medium is idle - transmit

If busy-continue listening, transmit once idle

p-PersistentIf medium is idle - transmit with probability p

Listen until channel idle, transmit with probability p

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S is expressed in terms of G and a a is a Non dimensional parameter

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a = propagation_ delaypacket_ transmission _ time

This equates to the vulnerable period

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Scaling to tp, At t=0,channel is sensed idle Takes time a for all other stations to be aware of

transmission

So if no other transmission in time a, we have success

Busy period for the channel is 1+a which is the

propagation delay+transmission time

Busy period is followed by an idle, thus a cycle is

busy+idle time

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Ethernet/ 802.3 uses a deference mechanism Even if no packet waiting, station monitors the

media

When a packet is available and channel idle

Packet transmitted if non-persistent or 1-persistent

P-persistent, packet is sent with probability p or is

delayed by the propagation delay

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If busy The packet is backed off and the algorithm is repeated

(for non-persistent)

Station defers until channel idle and immediately

transmits for 1-persistent The p-persistent, the station defers until the channel

is idle then follows the channel idle procedure

MAC layer adds inter frame spacing

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Collisions may still occur In that case, station aborts transmission

Sends jamming signal of duration b

Dimensionless, analogous to a

Stations involved in collisions wait a random timeprior to attempting re-transmission

Ethernet uses truncated binary exponential backoff

Transmission retries continue until success or 16

attempts are made Packet is then discarded

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Packet unsuccessfully sent n times Next transmission attempt is delayed r times

the base backoff

Base backoff is usually twice the round trip

propagation delay

Uniformly distributed integer 0r2k

Where k=min(n,10), k is the minimum number of

presently attempted transmissions

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116Reprinted with permission from Takagi and Kleinrock,17 1987, IEEE

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Deterministic Signals are used to grant permission to

transmit

No collisions

Control may be centralized or distributed

Polling is a centralized method

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Distributed (token) control Commonly ring topology, but may be bus

Tokens are special bit patterns within packets

Tokens continue to circulate even if there is notraffic to transmit

Stations wishing to transmit, remove the token

All stations are responsible for identifying andaccepting messages addressed to them

Additionally they must forward all other messages

Once a station has finished transmitting, it replacesthe token into circulation

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Eliminates collision detection timingrestrictions

Micro segmentation

Two major components in a switch

Forwarding logic I/O ports

IEEE 802.1 defines forwarding logic Examines incoming frame

Transfers it to the correct port More details later

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