CCN - Unit 3 - 7th ECE - VTU - Multiple Access - ramisuniverse

UNIT 3: Multiple accesses: Random access, Controlled access, Channelisation 6 Hours

Random access

Random access or contention methods:

No station: superior to other station

None of the station: assigned the control over the other station

No station permits, or not permits: other station to send

Each instance in: station uses the procedure defined by the protocol - to decide - to send or not send the data - if it has

the data to send

Decision of the station: depends on the status of the link (idle or busy) - it connected of

Each station: can transmit when it desires - on the condition that - it follows the procedure defined by the protocol -

including the testing of the state of the medium

Features that give its name:

There is no scheduled time, for a station to transmit - transmission is random among the stations - so, these

methods are - random methods

No rules specify, which station to send next - stations complete with one another, to transmit to the medium -

so, these methods are - contention methods

Random access method: each station has the right to the medium without being controlled by any other station

Collision (access conflict): which occurs when more than one station tries to send the data - if occurs, frames will be

either destroyed or modified

Collision to avoid or to resolve collision:

Each station follows the procedure - that answers the following questions:

When can the station access the medium?

What can the station do if the medium is busy?

How can the station determine the success or failure of the transmission?

What can the station do if there is an access conflict?

Random access methods evolution

came from ALOHA

ALOHA: simple procedure of multiple access (MA)

Carrier sense multiple access (CSMA): ALOHA been - improved by the addition of a procedure - that forces the station

to sense the medium before transmitting

CSMA two methods:

CSMA/CD: Carrier Sense Multiple Access/Collision Detection: tells the station - what to do when the

collision is detected

CSMA/CA: Carrier sense Multiple Access/Collision Avoidance: tries to avoid the collision

ALOHA

ALOHA: Earliest random access method - developed at the University of Hawaii - in early 1970

Designed for the radio (wireless) LAN - can be used on any shared medium

There are potential collisions: in this arrangement – medium, is shared between the stations

Station when sends the data - another station may attempt to do so at the same time - data from the 2 stations: collide

and garbled become

Types of ALOHA: pure ALOHA and slotted ALOHA

pure ALOHA

Original ALOHA - simple, elegant protocol

Idea: Each station sends a frame whenever it has a frame to send

Channel: since, only one - possibility of collision between frames from different stations - present

Fig. shows: ex. of frame collisions in pure ALOHA

Unrealistic assumption: 4 stations that contend with one another for access to the shared channel - Fig. shows: each

station sends 2 frames - total of 8 frames on the shared medium

Some frames collide because: multiple frames are in contention for the shared channel

Fig. shows: only 2 frames survive (frame 1.1 of station 1 and frame 3.2 of station 3) - if one bit of the frames coexists

on the channel with one bit of another frame: collision will and frames will destroy - Frame resending: of the destroyed

frames to be

pure ALOHA: relies on the acks from the receiver - Station when sends the frame, it expects the ack from the receiver

Methods to prevent congestion in pure ALOHA:

Ack if not arrived after a time-out period: each station waits for a random period - before resending its frame

Back-off period, TB: max. time within which ack had to back from receiver to transmitter - for the sent frame

Randomness: helps avoid more collisions

Prevent the congestion of the channel - with retransmitted frames

Max. no. of retransmission attempts, Kmax: max.: time up to which frame resending can be - to got ack of receipt of

that frame from the receiver - Kmax time: after station can give up resending of a frame, and try to later

Fig. shows: procedure for pure ALOHA - based on the above strategy

Max. round-trip propagation delay, 2Tp: amount of time required to send a frame - between the 2 most widely

separated stations

Time-out period: equal to max. no. of possible round-trip propagation delay

K: number of attempted unsuccessful transmissions

Back-off period, TB: is a random value - normally, depends on K - formula for TB - depends on implementation

Tp: max. propagation time

Tfr: avg. time required to send out a frame

Common formula for TB: Binary exponential back-off

For each retransmission: TB is the product of - a multiplier in the range of 0 to 2 power of (K-1), which is randomly

chosen and Tp or Tfr

Note: In this method - range of the random numbers increases - after each collision

Kmax.: usual value is chosen as 15

Tfr: frame transmission time

Vulnerable time: Time in which - there is the possibility of collision

Assumption: stations send fixed length frames - with each frame taking Tfr s to send

Fig. shows - vulnerable time for station A

Station A: sends the frame at time t

Assumption: station B has already send a frame between t - Tfr and t

It leads to collision, between the station A and of B - end of B's frame collide with the

Beginning of A's frame - suppose: station C sends the frame between t and t+Tfr - there is a collision between the

frames from stations A and C - beginning of C's frame, collide with the end of A's frame

Fig. : in vulnerable time - in pure ALOHA - is 2 times the frame transmission tim

Vulnerable time of pure ALOHA: 2 * Tp

Throughput

G: average number of frames generated by the system during one frame transmission time

S: average number of successful transmissions for pure ALOHA - S= G * e(power of -2G)

Smax, max. throughput: Smax. = 0.184 for G=0.5

0.5 a frame is generated during one Tfr or 1 frame generated during 2Tfr, if generated at transmitter: then 18.4 percent

of these frames, reaches the receiver successfully - because, vulnerable time in pure ALOHA is 2Tfr

Station if generates only one frame in vulnerable time (no other stations generate their frames during this time): frame

will reach the destination successfully

For pure ALOHA: Throughput: S=G*e(power of -2G) and

Max. throughput: Smax=0.184, when G=0.5

slotted ALOHA

pure ALOHA: has the vulnerable time of 2Tfr - because no rule that when the station can send the data

Station may send the data soon after another station has started or soon before another station has finished

slotted ALOHA: invented to improve - the efficiency of pure ALOHA

Time is divided into slots of Tfr s - Forces the station to send only at the beginning of the time slot

Fig. shows - ex. of frame collisions in slotted ALOHA

Station is allowed to send only at the beginning of the synchronized time slot

If the station misses the moment: it must wait until the beginning of the next time slot - i.e., station which started at the

beginning of this time slot has already finished sending its frame

Still there is the possibility of collision: if the stations try to send at the beginning of the same time slot vulnerable time

in slotted ALOHA: reduced to half of pure ALOHA - equal to Tfr

Fig. shows - this situation

slotted ALOHA: vulnerable time= Tfr

Throughput

S: avg. no. of successful transmissions for slotted ALOHA - S=G*e (power of -G)

Smax, max. throughput of slotted ALOHA: Smax=0.368, when G=1

During Tfr - if one frame is generated in transmitted: then 36.8% of these frames reach to receiver successfully -

because vulnerable time for slotted ALOHA is Tfr

Station in generated one frame in vulnerable time (no other stations generates a frame during this vulnerable time) - the

frame generated will reaches the receiver successfully

For slotted ALOHA: Throughput: S=G*e(power of -G) and

Max. throughput: Smax=0.368, when G=1

Carrier Sense Multiple Access (CSMA)

Collision chances to minimize and to increase the performance: CSMA was developed

Collision chances can be reduced: if a station senses the medium before trying to use it

CSMA: require that each station - first listens to the medium or check the state of the medium - before sending - based

on the principle - "sense before transmit" or "listen before talk" can reduce the possibility of collision - but, it

cannot eliminate it - Fig. in space and time model of CSMA been shown - stations are connected - to a shared channel -

usually, a dedicated medium possibility of collision: still exists - because, of propagation delay - station when sends a

frame, it still takes time, although very short, for the first bit to reach every station and for every station to sense it -

station may sense the medium and find it idle, only because the first bit sent by another station has not yet been

received

At t1: station B senses the medium and finds it idle - so, it sends a frame

At t2 (t2>t21): station C senses the medium and finds it idle - because, at this time, the first bits from station B have

not reached station C and station C also sends a frame - the two frames collide and both frames are destroyed

Vulnerable time (Propagation time, Tp): it’s the propagation time, Tp in slotted ALOHA

Vulnerable time:time needed for the signal to propagate from one end of the medium to the other

When a station sends a frame - and any other stations send the frames at this time - collision will result

If the first bit of the frame reaches the send of the medium - every station will already have heard the bit and will

refrain from sending

Leftmost station A sends a frame at time t1: it reaches the rightmost station D at time t1+Tfr

Grey area: shows the vulnerable area in time and space

Persistence methods: stations do what, if channel is busy and channel free: is answered by persistence methods

Persistence methods types: 1-persistent method, non-persistent method, p-persistent method

Fig. shows behavior of 3 persistent methods - when a station finds a channel busy

1-persistent method: simple and straightforward

After a station finds the line idle: it sends its frame immediately (with probability 1)

Has the highest chance of collision - because, 2 or more stations may find the line idle and send their frames

immediately - Ethernet: uses this method

Non-persistent method: station that has the frame to send - senses the line

If the line is idle: it sends immediately

If the line is not idle: it waits a random amount of time and then senses the line again

Reduces the chance of collision - because, it is unlikely that 2 or more stations will wait the same amount of time and

retry to send simultaneously

Reduces the efficiency of the network - because, medium remains idle when there may be stations with frames to send

p-persistent method: used if the channel has the time slots with a slot duration >= max. Tp

Combines the advantages of other 2 strategies

Reduces the chance of collision and improves the efficiency

After the station finds the line idle - it follows these steps:

1. With probability p: station sends the frame

2. With probability q=(p-1): station waits for the beginning of the next time slot and checks the line again

a. If the line is idle: it goes to step 1

b. If the line is busy: it acts as though a collision has occurred and uses the back-off procedure

CSMA: does not specify the procedure - following a collision

Types of CSMA: CSMA/CD and CSMA/CA

Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

Augments the algorithm to handle the collision

Station monitors the medium after it sends a frame to see if the transmission was successful

If so - station is finished - If there is a collision, frame is sent again

Understanding of CSMA/CD: First bits transmitted by the 2 stations involved in the collision been important

Each station continues to send bits in the frame until it detects the collision if the first bits collide - shown in fig.

Stations A and C: frame been in collision

At t1: station A has executed its persistence procedure - and, starts sending the bits of its frame

At t2: station C has not yet sensed the first bit sent by A

Station C executes - its persistence procedure and starts sending the bits in its frame - which propagates both to the left

and the right

After time t2: collision also occurs - station C detects the collision at time t3, when it receives the first bit of A's frame

- station C immediately (or after a short time, but assumption we that it is immediately) aborts the transmission

Station A detects collision at time t4: when it receives the first bit of C's frame - it also immediately aborts the

transmission

A transmits for the duration: t4-t1

C transmits for the duration: t3-t2

Protocol to be operated for: length of any frame divided by the bit rate must more than t4-t1 and t3-t2 durations

At t4: transmission of A's frame - though incomplete, is aborted

At t3: transmission of B's frame - though incomplete, is aborted

Fig. shows more complete graph - for the durations for the 2 transmissions

Minimum frame size

CSMA/CD to work: restriction on frame size is needed

Before sending last bit of the frame - sending station must detect a collision, if any, and aborts the transmission -

because station once the entire frame is sent, does not keep a copy of the frame and does not monitor the lien for

collision detection- so, min. Tfr = 2 *max.Tp to understand let the worst case scenario been considered -

If the 2 stations involved in the collision are max. distances apart - signals from the first takes time Tp to reach the

second and the effect of the collision takes another time Tp to reach the first - so, requirement is that the first station

must still be transmitting after 2Tp

Procedure

Fig. shows the flow diagram for CSMA/CD - is similar to one of ALOHA protocol, but with few differences

CSMA/CD differences compared when with ALOHA

1. Addition of the persistence processes:

Needs to sense the channel before - start sending the frame - by using one of the persistent processes corresponding box

can be replaced by one of the persistent processes shown in Fig.

2. Frame transmission:

In ALOHA: first transmission of entire frame will be and then wait for the ack will be

In CSMA/CD: transmission and collision detection is a continuous process

Entire frame will not send and then look for the collision

Station transmits and receives continuously and simultaneously (using 2 different ports) - loop been shown to have

transmission is a continuous process

Constant monitoring will to detect one of two conditions - either transmission is finished or a collision is detected -

either event in transmission is stopped

Loop from when came out - if the collision is not been detected - then, it means that transmission is complete; the entire

frame is transmitter otherwise the collision - has occurred

3. Sending of a short jamming signal:

Enforces the collision in case other stations have not yet sensed the collision

Energy level

Level of energy in the channel: can have 3 values

1. Zero: at this level, channel is id

2. Normal: at this level, station has successfully captured the channel and is sending its frame

3. Abnormal: at this level, there is a collision and the level of the energy is 2 the normal level

Station that has a frame to send or is sending a frame: needs to monitor the energy level - to determine if channel is

idle, busy, or in collision mode

Throughput

Max. throughput: occurs at different value of G and based on the persistence method and the value of p in the p-

persistent approach

Max. throughput in 1-persistent method: is around 50% when G=1

Max. throughput in non-persistent method: can go up to 90% when G is between 3 and 8

Max. throughput in p-persistent method: depends on p value

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

CSMA/CD basic idea: station need to be able to receive - while transmitting to detect a collision

When no collision - station receives one signal - its own signal

When there is a collision - station receives 2 signals - its own signal and a signal transmitted by a second station

Distinguishing between above 2 cases: received signals in these 2 cases must be different

Signal from the second station can be made: needs to add a significant amount of energy to the one created by the first

station

In wired network:

Received signal: has almost the same energy as the sent signal

Because, either the length of the cable is short or there are repeaters that amplify the energy between the sender and the

receiver - it means that, in a collision detected energy almost doubles

In a wireless network:

Much of the sent energy is lost in transmission

Received signal has very little energy - so, collision may add only 5 to 10% of additional energy - this is not useful for

effective collision detection

Collisions in wireless network - need to be avoided, since, they can't be detected - CSMA/CA been invented for this

CSMA/CA: Collisions are avoided here - by 3 strategies:

1. InterFrame Space (IFS)

2. Contention window

3. Acknowledgement

Fig. shows the 3 strategies of CSMA/CA

1. IFS (InterFrame Space):

Collisions are avoided: by deferring transmission even if the channel is found idle

Channel when idle found: station does not send immediately - stations wait for a period of time called IFS

Channel even though appeared idle - when it is sensed - a distant station may have already started transmitting - distant

stations frame might not yet reached this station

If IFS time later: channel is still idle - stations can send the frame - but only after waiting a time called contention time

Priorities of stations and frame types can be made: by varying the IFS variable

Ex.: station with shorter IFS, has a higher priority

IFS can be used define the priority of a station or a frame

2. Contention window:

Amount of time divided into slots

Station that is ready to send chooses a random number of slots as its wait time

Number of slots in the window: changes according to the binary exponential back-off strategy - that is, it is set to one

slot for the first time and then doubles each time the station cannot detect an idle channel after the IFS time

Is similar to p-persistent method - except, that a random outcome defines the number f slots taken by the waiting station

needs to sense the channel after each time slot

If the station finds the channel busy - it does not restart the process - it just stops the timer and restarts it when the

channel is sensed as idle - it gives priority to the station ith the longest waiting time

Contention window: in CSMA/CA, if the station finds the channel busy, it does not restart the timer of the contention

window; it stops the timer and restarts it when the channel becomes idle

3. Acknowledgemnt:

Data may be corrupted during transmission - from that collisions still might be, which destroy the data

Positive acks and time-out timer: can help guarantee that the receiver has received the frame

Procedure: Fig. shows the procedure

Note:

Channel needs to be sensed before and after the Ifs

Channel also needs to be sensed during the contention window

For each time slot of the contention window: channel is sensed –

If channel is found idle: the timer continues

If channel is found busy: timer is stopped and continues after the timer becomes idle again

CSMA/CA and wireless networks

CSMA/CA: Mostly intended for use in wireless networks

Procedure above: not enough sophisticated to handle wireless networks of hidden terminals and exposed terminals

problems - these problems can be solved by using hand-shaking features

Controlled Access

Controlled access: stations consult one another - to find which station has the right to send

Station cannot send: unless it has been authorized by other stations

Popular controlled access method:

1. Reservation

2. Polling

3. Token passing

1. Reservation

Reservation: station need to make a reservation before sending data - time is divided into intervals

Each interval in: a reservation frame precedes the data frames sent in that interval

N stations: if in the system - N minislots in the reservation frame - each minislot - belongs to a station

Station when needs to send a data frame: it makes a reservation in its own minislot

Stations made reservations: can send their data frames after the reservation frame

Fig. shows the situation with 5 stations and 5 minislot reservation frame

First interval in: only stations 1, 3, and 4 have made reservations

Second interval in: only station 1 has made a reservation

2. Polling

Polling: works with topologies in which one device is designed as a primary station and the other devices, the

secondary stations

Data exchanges: all to be from primary device - even destination when is the secondary device

Primary device: controls the link - determines which device is allowed to use the channel at a given time - is the

initiator of a session

Secondary devices: follow its instruction

Poll: Primary: if wants the data - it asks the secondary, if they have anything to send

Select: Primary if wants to send data: it tells the secondary to get ready to receive

Select:

Used whenever - primary device has something to send

Primary - controls the link

Primary if neither sending the data nor receiving the data: it knows the link is available

Primary does not know: however, whether the target device is prepared to receiver - so,

Primary alert to the secondary to the upcoming transmission - and wait for the ack of the

secondary ready status

Primary: prior to send the data - transmits a select (SEL) frame - which contains will be the address of the secondary

Poll:

Used by the primary: to solicit the transmissions from the secondary devices

Primary when ready: to receive the data - it ask (poll) each device in turn – whether they had any data for it

First secondary when approached: it responds with NAK - if no data is there for it to primary

with data frame: if it has the data for primary

Response if NAK: primary polls the next secondary - in the same manner - until it finds the secondary having data for it

Primary: reads the frame - when the response is positive (a data frame) - and returns an ACK frame - verifying its

receipt

Token Passing

Token passing: Stations in a network - are organized in a logical ring

For each station: there is a predecessor and a successor

Predecessor: station which is logically before the station in the ring

Successor: station which is logically after the station in the ring

Current station: station which the one being accessing the channel

Channel accessing right: passed from predecessor to the current station

Channel accessing right: passes from current station to the successor - when current station has no more data to send

Token: special packet circulates through the ring possession of it - gives the station - right to access the channel, and

send its data

Station: When has some data to send - it waits until it receives the token from its predecessor then holds the

token - and sends its data

When no more data to send - it releases the token - passes the token to next station

Cannot send data - until it receives the token again in the next round

When receives the token - no data to send - then, it transfers token to next station

Token management

Need for token management:

For accessing the token

Stations: must be limited in the time - they can have possession of the token

Token: must be monitored - to ensure it has not been lost or destroyed

Ex.: If a station, holding the token fails - token will disappear form the network

Priorities to be assigned - to the stations and to the type of data being transmitted

Maintenance of priorities - to make low-priority stations release the token to high-priority stations

Logical ring

Token-passing network: stations do not have to be physically connected in a ring stations can be connected in the

logical ring form

Fig. shows 4 different physical topologies - that can create a logical ring

Physical ring topology:

When the station sends the token: to its successor - token cannot be seen by other stations - successor is next one in the

line - token does not have to have the address of the next successor

Problem: if one of the links (medium between two adjacent stations) fails - the whole system fails

Dual ring topology:

Uses a second (auxiliary) ring - which operates in the reverse direction compared with the main ring

Second ring: used in emergency

Main ring: in one of the links - failed if - system automatically combines - the two rings - to form a temporary ring

Failed link - when restored - auxiliary ring becomes idle again

Note: This topology to work - each station needs to have 2 transmitter ports and 2 receiver ports - CDDI and FDDI:

high-speed token ring networks - use this topology

Bus ring topology (Token bus):

Bus: stations are connected to single cable

Stations make: a logical ring - since, each station knows the address of its successor and also the predecessor for token

management purposes

Station when finished the data sending: it releases the token - inserts the address of its successor in the token - station:

with the address - matching with the address in the toke - gets the token - to access the shared media

Toke bus LAN: standardized by IEEE - uses this topology

Star ring topology:

Star: is the physical topology

Hub: acts as a connector - wiring inside the hub, makes the ring - stations are connected to this ring through 2 wire

connections

Advantage: less-prone to failures - if a link goes down, it will be bypassed by the hub - and the rest of the stations can

operate - adding and removing of stations - is easier

IBM Token ring LAN: uses this topology

CHANNELIZATION

Channelization: Multiple access method - in which the available bandwidth of a link is shred in time, frequency, or

through code - between different stations

Three channelization protocols:

1. FDMA

2. TDMA

3. CDMA

Frequency-Division Multiple Access (FDMA)

FDMA: Available bandwidth is divided into frequency bands

Each station is allocated a band to send its data

Each band is reserved for a specific station

Each band belongs to the station all the time

Each station also uses a band pass filter to confine the transmitter frequencies

Guard bands: used to prevent interferences - separate allocated bands from one another

Fig. shows the FDMA idea

FDMA: Available bandwidth of the common channel is divided into bands that are separated by guard bands

Specifies a predetermined frequency band for the entire period of communication

Stream data (continuous flow of data that may not be packetized): can easily be used with FDMA

FDMA Vs FDM

Both seem conceptually similar - but, they have differences between them

FDM: Physical layer technique

Combines the load from low-BW channels and transmits them by using a high-BW channel

Channels combines: are low pass

Multiplexer: modulates the signals - combines them - creates a band pass signal

BW of each channel is shifted by the multiplexer

FDMA: An access method in the data link layer

DLL in each layer: tells its physical layer - to make a band pass signal from the data passed to it

Signals must be created in the allocated band

There is no physical multiplexer at the physical layer

Signals created at each station are automatically band pass-filtered - they are mixed when they are sent to the

common channel

Time-Division Multiple Access (TDMA)

TDMA: Stations share the BW of the channel in time

Each station is allocated a time slot - during which it can send data

Each station transmits - its data in is assigned time slot

Fig. shows the idea behind TDMA

Synchronization: main problem in TDMA - between different stations

Each station: needs to know the beginning of its slot and location of its slot - is difficult because of propagation delays -

introduced in the system if the stations are spread over a large area

Guard times: can be inserted to compensate for the delays

Synchronization - normally, accomplished by having some synchronization bits

Synchronization bit: Preamble bit: is added at the beginning of each slot

TDMA: BW is just one channel - timeshared between different stations

TDMA Vs TDM

TDMA and TDM: conceptually seem the same - but, there are differences between them

TDM: Physical layer technique

Combines the data from slower channels and transmits them by using a faster channel

TDM uses: a physical multiplexer - that interleaves data units from each channel

TDMA: An access method in the DLL

DLL in each station: tells its physical layer - to use the allocated time slot

NO physical multiplexer - is present at the physical layer

Code-Division Multiple Access (CDMA)

CDMA: Was conceived several decades back

Implementation made possible: by recent advancements in electronic technology

Differs from FDMA: because only one channel occupies the entire BW of the link

Differs form TDMA: because all sections can send data simultaneously there is no time sharing

CDMA: one channel carries all transmission simultaneously

Analogy: CDMA: simply means - communication with different codes

Ex.: In a room: 2 can talk in English, if they only know English, and no others there - 2 can talk in Chinese, if they only

know Chinese, and no others there

Room: here, is common channel - can allow communication between several couples – but in different languages

(codes here)

Idea:

Assumption: 1 2 3 and 4 stations - connected to same channel

Station 1: d1 - data and c1 - code




Assumption: Assigned codes have 2 properties

1. If we multiply each code by another: we get 0

2. If we multiply each code by itself: we get 4 (the number of stations)

Fig. shows how the 4 stations can send data using the same common channel

Station 1: multiples its data by its code to get: d1.c1




data on the channel: sum of the data from all the terms above calculated

Any station that wants to receive data from one of the other 3: multiples the data on the channel by the code of the

ender

Ex.: Station 1 and 2: are talking to each other

Station 2: wants to hear what station 1 is saying

Station 2: multiplies the data on the channel by c1(code of station 1)

(c1.c1) = 4, (c2.c1) = 0, (c3.c1) = 0, (c4.c1) = 0

Station 2: divides the result by 4 - to get the data of station 1

Data = (d1.c1 + d2.c2 + d3.c3 + d4.c4).c1 = d1.c1.c1+d2.c2.c1+d3.c3.c1+d4.c4.c1 = 4.d1 + 0 + 0 + 0 =4.d1 and station

2: divides result by 4: 4.d1/4 = d1 (data of station 1)

Chips

CDMA: based on coding theory

chip: each station is assigned a code - which is a sequence of numbers

Fig. shows the codes using for previous ex. - c1, c2, c3, and c4 of

Fig. chip sequences - chip sequences are carefully selected

Orthogonal sequences:

Chip sequences carefully selected - which obey the properties given below:

1. Each sequence is made of N elements - N, number of stations

2. Multiplication of a sequence by a scalar: Number with if, a sequence is multiplied - every element in the sequence is

multiplied by that element 2.[+1 +1 -1 -1]=[+2 +2 -2 -2]

3. Inner product of 2 equal sequences: 2 equal sequences: if, multiplied - element by element -and added - N is result -

N: number of elements in each sequence – [+1 +1 -1 -1].[+1 +1 -1 -1]=1+1+1+1 = 4

4. Inner product of 2 different sequences: two different sequences - if multiplied - element by element – and added -

result is 0 and [+1 +1 -1 -1].[+1 +1 +1 +1]=1+1-1-1=0

5. Two sequences of addition: adding the corresponding elements result - is another sequence

[+1 +1 -1 -1]+[+1 +1 +1 +1]=[+2 +2 0 0]

Data representation

Encoding rules: Station if sending 0: -1 is encoding

Station if sending 1: 1 is encoding

Station if not sending data: 0 is encoding

Encoding and Decoding:

Ex.: 4 stations share - the link during a 1-bit interval

Procedure: can easily be repeated for additional intervals

Assumption: Stations 1 and 2 are sending a 0 bit - channel 4 is sending a 1 bit - station 3 is silent

Data at sender site: are translated to -1, -1, 0, and 1

Each station: multiplies the corresponding number (chip: it's orthogonal sequence) - which is unique for each station -

Result: is a new sequences at the same time

Sequence on the channel: sum of all 4 sequences -as defined before

Fig. shows - the situation

Assumption: station 3 - which was silent earlier - is listening station 2

Station 3: multiplies the total data on the channel by the code for station 2

[-1 -1 -3 +1].[+1 -1 +1 -1] = -4/4 = -1 i.e., the bit 1

Signal Level

Fig. shows: Digital signal produced by each station and data recovered at the destination

Fig. also shows: corresponding signals - for each station (by NRZ-L for simplicity) and also the signal that is on the

common channel

Fig. shows: Station 3 detection of data of station 2 - using code of station 2

Station 3: Total data on the channel: multiplied (inner product operation) - by station 2 chip code

then integrates and adds the area under the signal(Here: -4 is the result) - divides the result by 4

If answer is -1: data is interpreted as 0

If answer is 1: data is interpreted as 1

If answer is 0: data being not sent

Sequence Generation

Walsh table: Used to generate the chip sequences

2-D table - with an equal number of rows and columns

Fig. : shows the Walsh table

Fig. : General rule and examples of creating Walsh tables

Walsh table: each row is a table of chips

W1: is used for - one-chip sequence - having 1 row and 1 column

Walsh: according to - if Walsh table (WN) for N sequences known - for 2N sequences table (W2N) can be created

Fig. shows this

WN bar - is the complement for WN: here, each 1 is changed to -1 and vice versa

Fig. shows: how W2 and W4 - can be created using W1

W1 and W2: can be made from 4 W1's - with the last 1 complement of W1

W4: can be generated - by 4 W2's and by complementing the last one

W8: composed of 4 W4s and can be repeated this procedure for WNs

Note: WN formed later - each station is assigned a chip - corresponding to a row

Number of sequences needs to be: power of 2 - it must obey N=2(power of m)

Number of sequences in the Walsh table: N = 2 (power of m)

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CCN - Unit 3 - 7th ECE - VTU - Multiple Access - ramisuniverse

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stations station

receiver station

station b h

aloha aloha

frame resending

frame collisions

multiple frames

random methods