dB =10log P1 P2

! Results from the losses in the transmission 

medium 

! Guided media 

! Signal strength decays exponentially 

! May be expressed as a logarithmic power ratio 

Power _ ratio_in _ dB =10log P 1 

P 2 

3 

Covered in Transmission 

Line & Interface Design 

unit 

2 

4

Addition or subtraction of dB yields the system loss/gain between 2 

points 

dB level at point 4=(-9)+(14)+(-3)=+2dB 

dB is a ratio and gives no indication of absolute power levels 

! Various noise and disturbances may cause 

errors 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 

5 

! 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 

! Receiver must extract timing from the 

incoming 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 

7 8 

6

! The end-to-end transfer of data from a 

transmitting application to a receiving 

application involves many steps 

! Each subject to error. 

! Errors can occur both at the bit and at the packet 

level. 

! At the bit level, the most common error is bit 

corruption 

! At the packet level, we see errors such as packet 

loss, duplication, or reordering. 

! Error control is the process of detecting and 

correcting both bit and packet errors. 

Nyquist Theorem 

Sample rate of at least 

twice the maximum 

frequency component of 

the signal to be 

digitised 

Aliasing occurs 

Quantizing noise 

9 10 

11 

! Bit-level error control usually involves adding 

redundancy 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 error 

control can detect all bit errors. 

! (Detectable but uncorrectable bit errors are treated 

as a packet loss.) 

12

! Packet-level error control mechanisms detect 

and correct packet-level errors such as loss, 

duplication, and reordering. 

! We typically implement bit-level error control 

at the datalink layer of the protocol stack. 

! Packet-level error control is typically found at 

the transport layer. 

! Thus, bit level error control is usually hop-by-hop, 

whereas packet-level error control is usually end-toend. 

! Information is transmitted on a link by 

varying the state of a signal. 

! On a digital link, each signal state 

corresponds to one or more 0's and 1's-a 

signal that can take 2 n , states represents n 

bits of information. To decipher a signal on a 

digital link, the receiver compares the 

received signal with a set of predefined 

references. 

13 

15 

! Generally speaking, we prefer hop-by-hop 

error control on links where the error rate is 

high (so-called lossy links) and end-to-end 

error control when entire path is more or less 

error free. 

! We usually measure the error probability on a 

digital link in terms of the bit error rate or 

BER 

! the ratio of the mean number of errors in any given 

interval to the total number of bits transmitted in 

that interval. 

! Typical fiber-optic links have a bit error ratio in the 

range of 10 -18 - 10 -14 , but copper links can have a 

substantially higher bit error ratio, depending on 

shielding and the operating environment. 

14 

16

! The major causes of bit errors are Gaussian 

and non-Gaussian noise, loss of line 

synchronisation, scramblers, protection 

switching, and, for cellular communication, 

handoffs and fading 

! A third source of bit errors is loss of bit 

synchronisation between the transmitter and 

the receiver. 

! Receivers periodically sample the received 

analog waveform to extract digital 

information. 

! They typically use the transitions in the 

transmitter's signal as the input to a phaselocked 

loop to determine the transmitter's 

clock automatically. 

17 

19 

! 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. 

! Handoffs and fading can cause error bursts in 

cellular 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 which 

time the received signal is almost completely in error. 

18 

20

! Handoff errors can be substantially reduced 

by making the new connection before the old 

one ends. 

! However, this makes it necessary to overlap basestation 

ranges, which leads to less efficient spatial 

reuse of the radio spectrum. 

! 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 data 

flow 

! Invokes a control procedure known as an acknowledgement 

(ACK) 

! Two common methods 

! Stop and wait 

! Sliding window 

21 

23 

! If a mobile station does not receive a strong 

signal from a base station because of hills, 

buildings, or other obstacles, then its 

received signal is in error until the unit moves 

away 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 macroscopic 

environmental conditions, and short-term Rayleigh 

fading, primarily due to vehicle movement. 

! Simple 

! Inefficient 

! Sender sends a frame and waits for an ACK 

! Must wait at least 2t prop plus t f (time to 

process the incoming frame) 

! No out of order frames 

22 

24

! Refers to the method used to detect and 

correct errors that occur in the transmission 

of frames 

! Retransmission of errored or dropped frames 

! Utilises ACK and NACK 

! Automatic Repeat Request (ARQ) 

! Three types 

! Stop and Wait 

! Go-back N 

! Selective repeat 

! May transmit several frames without receiving 

ACK 

! A single ACK may acknowledge many frames 

! Utilises an identification scheme based on the 

size of the window 

! Numbered modulo n, where n=window size-1 

! ACK is usually sent prior to window size 

reducing to zero 

25 26 

27 

! 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 tprop plus t f 

! Very inefficient 

28

! Based on sliding window 

! AKA continuous ARQ 

! N specifies how many frames may be sent without 

an ACK or NACK 

! Receiver discards all incoming frames after an 

error 

! 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 

29 

! Only lost or damaged frames retransmitted 

! More efficient 

! From the utilisation viewpoint 

! Complex implementation 

! Re-ordering of retransmitted frames 

! Not often used 

31 32 

30

! 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 

! We need to decide where a frame starts and 

ends 

! 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 

33 

35 

• In asynch serial communication, the electrical interface is held in the mark position between 

characters. The start of transmission of a character is signaled by a drop in signal level to the space 

level. At this point, the receiver starts its clock. After one bit time (the start bit) come 7 or 8 bits of 

true data followed by one or more stop bits at the mark level. The receiver tries to 

sample the signal in the middle of each bit time. The byte will be read correctly if the line is still in 

the intended state when the last stop bit is read. 

! Frames consist of integer number of bytes 

! Asynchronous transmission systems using ASCII to transmit printable 

characters 

! Octets with HEX value

! 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) 

Data to be sent 

A DLE B ETX DLE STX E 

After stuffing and framing 

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

HDLC frame 

Flag 

Address Control Information FCS 

any number of bits 

! Frame delineated by flag character 

! HDLC uses bit stuffing to prevent occurrence of flag 

01111110 inside the frame 

! Transmitter inserts extra 0 after each consecutive five 

1s inside the 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 

37 

Flag 

(a) 

! Errors in header and packet caught by CRC 

! Errors in DLE STX and DLE ETX 

! An entire frame is missing 

! Errors could cause DLE ETX to appear in 

middle of frame 

! Receiver interprets the bits following as CRC 

! So it would be dropped 

Data to be sent 

0110111111111100 

After stuffing and framing 

01111110 01101111101111100001111110 

(b) 

Data received 

01111110000111011111011111011001111110 

After destuffing and deframing 

*000111011111-11111-110* 

38

! DLL Protocols are sets of specifications used 

to implement DLL. 

! Contain rules for line discipline, flow control, 

error handling, etc. 

! DLL Protocols comes in two broad 

groups: 

! Asynchronous (treat each character in a bit 

stream independently) 

! Synchronous (takes the whole bit stream and 

chop it into characters of equal sizes) 

! Designed in 1979 by Ward Christiansen 

! file transfer protocol for telephone line 

communications between PCs. 

! Half duplex stop and wait ARQ protocol 

41 

43 

! 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 by 

higher speed synchronous mechanisms. 

! The 1 st field is a one-Byte SOH (Start of 

Header). 

! The 2 nd field is a 2-byte header. 

! The first header byte is SN. 

! The second header byte is used to check the validity 

of the SN. 

! The fixed data field holds 128 Bytes (binary, 

ASCII etc). 

! The last field, CRC, checks for errors in data 

field only. 

42 

44

! Transmission begins with sending of a NAK from 

receiver to sender. 

! Each time the sender sends a frame, it must wait 

for an ACK before next frame is sent again. 

! A frame can also be resent if a response is not 

received by the sender after a specified amount 

of 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 

! Zmodem is a newer protocol 

! Combines Xmodem & Ymodem features 

! BLAST 

! Full duplex 

! Sliding window flow control 

! This is similar to XMODEM, with following 

major 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 

45 46 

47 

! Currently the most widely used asynchronous 

protocol. 

! Similar in operation to XMODEM 

! sender waiting for a NAK before starts transmission. 

! Allows the transmission of control characters as 

text using two steps. 

! The control character is transformed to a printable 

character by adding a fixed number to its ASCII code 

representation. 

! The # character is added to the front of the 

transformed character. 

! a control character is sent as two characters. 

48

! 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. 

! Not as efficient as bit oriented protocols 

! seldom used. 

! easy to comprehend and employ the same logic and 

organization as the bit oriented protocols. 

! In all DLL protocols, control information is 

inserted into the data stream either as separate 

control frames or as additions to existing data 

frames. 

! In character oriented protocols this information is in the 

form of code words taken from existing character sets 

! The best known is IBMs binary synchronous 

communications (BSC) 

49 

! There are two broad categories of synchronous 

protocols: 

! Character Oriented 

! interpret a transmission frame as a succession of characters, 

each usually composed of one octet (8 bits). All control 

information 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 the 

frame. Control information in a bit oriented protocol can be 

one or multiple bits. 

51 52 

50

! Usable in both Point to point and multi point 

configurations 

! 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 may 

contain control information applicable to that 

information. 

! Control frames are used to exchange information 

between communicating devices. (e.g. establish initial 

connection, control the flow of the transmission, request 

error corrections, disconnect the devices). 

! 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 and 

ends 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 is 

required to be transmitted. 

53 

55 

! SYN is used by the receiving device to 

synchronize its timing with the sending 

device. 

! STX tells the user that the next byte starts the 

data (variable length) until ETX is reached. 

! 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 

54 

56

! 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 information 

specific to the function of the DLL itself. 

! Control frames serves in establishing connections, 

maintaining flow, and error control during data 

transmission 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. 

! In 1975 IBM pioneered development of SDLC and lobbied ISO 

to 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. 

57 

59 

! 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) 

! HDLC (ISO 3009, ISO 4335) is a bit oriented DLL 

protocol 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 their 

station types, configurations and their response 

modes. 

58 

60

! 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. 

! 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. 

61 

! Primary, Secondary and Combined can be connected 

in 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. 

63 64 

62

! HDLC has three modes of data transfer: 

! NRM (Normal Response Mode) 

! Used in unbalanced configurations. The primary may initiate 

data transfer to secondary. But a secondary may transmit only in 

response to a command. 

! ARM (Asynchronous Response Mode) 

! A secondary may initiate a transmission without permission 

from primary whenever channel is idle. All transmission from 

secondary (even to another secondary on same link) must still 

be relayed through primary. 

! ABM (Asynchronous Balanced Mode) 

! All stations are equal and therefore only combined stations 

connected in point to point are used. Either combined station 

may initiate transmission with the other combined station 

without permission. 

65 

67 

! 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) 

! 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. 

66 

68

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

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

! The control fields of all three types of 

frames contain a bit called poll/final (P/F) 

bit. 

! U-frames have neither N(S) nor N(R) fields 

and are not designed for user data exchange 

or 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. 

69 

! An I-frame contains 3-bit flow and error control 

sequences 

! 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 used 

to carry coded flow and error control information. 

71 72 

70

! The P/F field is a single bit with a dual 

purpose. 

! It has meaning only when it is set and can mean 

poll or final. 

! It means poll when the frame is sent by primary 

station to secondary 

! It means final when frame is sent by secondary to a 

primary. 

73 74 

75 

! 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. 

76

! HDLC operation consists of exchange of Iframes, 

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. 

! 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 asking 

the 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. 

77 

79 

! 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 whether 

3 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. 

! Disconnect: 

! Either HDLC module can initiate a disconnect. 

! HDLC issues a disconnect by replying with a DISC 

(disconnect) frame. 

78 

80

81 82 

83 

! Several protocols under the general category of LAP 

have 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 that 

provides only the ABM. 

! LAPB was issued by ITU-T as part of x.25 packet switching 

network interface standard. Its frame format is same as 

HDLC 

! Thus it provides only the functions required for communication 

between a DTE and DCE. 

84

! 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 

asynchronous–synchronous conversion, error detection, and 

retransmission. 

85 

87 

! 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. 

! 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 the 

source 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. 

86 

88

! A LAN is a community of users who share their 

interconnecting 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 

89 

91 

! 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 L p 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 P n(t) of exactly n packets arriving during time interval t is 

given by 

( ) 

Pn ( t) 

= e!"t "t 

n! 

! Where ! is the average packet arrival rate 

! Throughput S is the measure of successful traffic 

! S often shown as normalised 

! L p=packet length 

! R= Transmission rate 

! Throughput is expressed in terms of offered load G 

! Normalised offered load is 

G = ! T LP R 

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

! Latency is the time lapse between the generation of 

the first bit and the receipt of the last bit at the 

destination 

n 

S = !L P 

R 

90 

92

! Queuing delay 

! As the traffic increases packets have to wait longer 

before they can be sent 

! Transmission delay 

! Packet transmission time T f is the time it takes to 

transmit a unit of data T f=L p/R 

! Propagation delay 

! Dependant on the media 

! 3x10 8 m/s in space 

! 2.3x10 8 m/s in copper 

! 2x10 8 m/s in optical fibre 

! 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 

93 

95 

! 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 T f 

! Transmission may only occur at the start of a slot 

! No partial overlapping of packets 

! Doubles the efficiency of pure Aloha to about 37% 

! 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 

94 

96

! Transmit and wait (2T prop ) for an acknowledgement 

! Drops packets after K attempts 

! Assuming 

! All packets are the same length 

! Each requires t p (slot) for transmission 

! Vulnerable period = 2t p 

! S=Throughput, G=offered traffic 

! Assume the probability p k of k transmissions follows a 

Poisson distribution then 

p k = Gk e !G 

k! 

S is just the offered load times 

p success or p 0! 

S = Gp 0 

97 98 

99 

! Where p 0, probability that no packet is 

generated in 2t p 

! P 0=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% 

100

! All users are synchronised to time slots 

! Vulnerable period is now t p 

! p 0 =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 

! Three possibilities when channel is sensed idle 

CSMA Protocol Characteristics 

Nonpersistent 

1-Persistent 

p-Persistent 

If medium is idle - transmit 

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

If medium is idle - transmit 

If busy-continue listening, transmit once idle 

If medium is idle - transmit with probability p 

Listen until channel idle, transmit with probability p 

101 102 

103 

! S is expressed in terms of G and a 

! “a” is a Non dimensional parameter 

a = 

propagation _ delay 

packet _ transmission _ time 

This equates to the vulnerable period! 

104

105 

107 

! Scaling to t p, 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! 

! 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 

106 

108

! 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 

! 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 0"r#2 k 

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

presently attempted transmissions 

109 

! 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 time 

prior to attempting re-transmission 

! Ethernet uses truncated binary exponential backoff 

! Transmission retries continue until success or 16 

attempts are made 

! Packet is then discarded 

111 112 

110

Reprinted with permission from Takagi and Kleinrock,17 © 1987, 115 IEEE! 

113 114 

! Deterministic 

! Signals are used to grant permission to 

transmit 

! No collisions 

! Control may be centralized or distributed 

! Polling is a centralized method 

116

! Eliminates collision detection timing 

restrictions 

! 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 

117 

! 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 no 

traffic to transmit 

! Stations wishing to transmit, remove the token 

! All stations are responsible for identifying and 

accepting messages addressed to them 

! Additionally they must forward all other messages 

! Once a station has finished transmitting, it replaces 

the token into circulation 

119 120 

118

dB =10log P1 P2

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