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Transcript of 04 Llc Mac Tiong 2011
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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
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Power_ ratio_ in_ dB =10log P1
P2
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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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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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