Chapter 3: Medium Access Control. Motivation The main question in connection with MAC in the...

Chapter 3:Medium Access Control

Motivation

The main question in connection with MAC in the wireless is whether it is possible to use complicated MAC schemes from wired networks.For example: CSMA/CDLet us consider carrier sense multiple access with collision detection (CDMA/CD) which works as follow:A sender senses the medium (a wire) to see if it is free. If the medium is busy, the sender waits until it is free.

Motivation

If the medium is free, the sender starts transmitting data and continues to listen into the medium.If the sender now detects a collision while sending it stops at once and sends a jamming signal. why does this scheme fall in wireless networks?CDMA/CD is not really interested in collisions at the sender, but rather in those at the receiver.The signal should reach the receiver without collisions.

Motivation

But the sender is the one detecting collisions.This is not a problem using a wire, and if a collision occurs somewhere in the wire, everybody will notice it.The situation is different in wireless networks.The strength of a signal decreases proportionally to the square of the distance to a sender.The sender may now apply carrier senses and detect an idle medium.Thus the sender start sending-but a collision happens at the receiver due to a second sender.

Motivation

This is hidden terminal problem. That is happen to the collision detection.The sender detects no collision assumes that the data has been transmitted without errors, but actually a collision.The destroyed the data at the receiver.Thus this very common MAC scheme from wire network fails in a wireless scenario.

Motivation - hidden and exposed terminals

Hidden terminals Consider the situation as show in figure. A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS

fails) collision at B, A cannot receive the collision (CD

fails) A is “hidden” for C


BA CBACBA


Exposed terminals Consider the situation as show in figure. B sends to A, C wants to send to another terminal

(not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore

waiting is not necessary C is “exposed” to B

Motivation - near and far terminals

Consider the situation as show in figure.A and B are both sending with the same transmission power.As the signal strength decreases proportionally to the square of the distance, B’s signal drowns out A’s signal.As a result, C cannot receive A’s transmission.The near/far effect is a several problem of wireless networks using CDM.All signals should arrive at the receiver with more or less the same strength.


A B C


E.g. A person standing closer to somebody could always speak louder than a person further away.Even if the sender were separated by code, the closest one would simply drown out the others.Thus, precise power control is needed to receive all senders with the same strength at a receiver.

SDMA

Space division multiple access (SDMA) is used for allocating a separated space to users in wireless networks.A typical application involves assigning a optimal base station to mobile phone user.The mobile phone may receive several base stations with different quality.A MAC algorithm could now decide which base station is best, taking into account which frequencies (FSM), time slots (TDM) or code (CDM) are still available.

SDMA

Typically, SDMA is never used in isolation but always in combination with one or more other schemes.The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division multiplexing (SDM).

FDMA

Frequency division multiple access (FDMA) comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM).Allocation can either be fixed (e.g.-radio station) or dynamic (e.g.- demand driven).Channels can be assigned to the same frequency at all times that is pure FDMA, or change frequencies according to a certain patter, that is FDMA combined with TDMA.

FDMA

The other example of many wireless systems to narrowband interference at certain frequencies known as frequency hopping.Sender and receiver have to agree on a hopping pattern otherwise the receiver could not tune to the right frequency.Thus hopping pattern are typically fixed, at least for a longer period.The fact that it is not possible to arbitrarily jump in the frequency space.

FDMA

Example : slow hopping (e.g., GSM), fast hopping (FHSS) Frequency Hopping Spread Spectrum)Furthermore, FDM is often used for simultaneous access to the medium by base station and mobile station in cellular networks. Here, the two partners typically establish a duplex channel.The two directions, Mobil station to base station and vice versa are now separated using different frequencies.

FDMA

This scheme is then called frequency division duplex (FDD).The two frequencies are also known as uplink, that is from mobile station to base station or from ground control to satellite and as downlink that is base station to mobile station or from satellite to ground control.As example for FDM and FDD show in next slide figure in that situation in a mobile phone network base on the GSM standard for 900 MHz.The basic frequency allocation scheme for GSM is fixed.

FDD/FDMA - general scheme, example GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

TDMA

Compared to FDMA, time division multiple access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocated certain time slots for communication, that is controlling TDM now tuning in a certain frequency in not necessary, that is the receiver can stay at the same frequency the whole time.Using only one frequency and thus every simple receivers and transmitters many different algorithms exist to control medium access.

TDMA

As already mentioned listening to different frequencies at the same time is quite difficult, but listening to many channels separated in time at the same frequency is simple.Now synchronization between sender and receiver has to be achieved in the time domain.This can be done by using a fixed pattern that is allocating a certain time slot for a channel or by using dynamic allocation scheme.

TDMA

Dynamic allocation schemes require an identification for each transmission as this is the case for typical wired MAC schemes (e.g.-sender address)Fixed schemes do no need an identification, but these are not as flexible considering.Varying bandwidth requirements, there are several examples for fixed and dynamic schemes as used for wireless transmission.Typically, those schemes can be combined with FDMA to achieve even greater flexibility.

Fixed TDM

The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern.This results in a fixed bandwidth and is the typical solution for wireless phone system.The fixed pattern can be assigned by the base station, where competition between different mobile stations that want to access the medium is solved.TDMA scheme with fixed access patterns are used for many digital mobile phone systems like IS-54,IS-136,GSM,DECT,PHS and PACS.

Fixed TDM

The next slide figure these fixed TDM patterns are used to implement multiple access and a duplex channel between a base station and mobile station.Assigning different slots for uplink and downlink using the same frequency is called time division duplex (TDD).As shown in the figure, the base station uses one out of 12 slots for the downlink, whereas the mobile station uses one out of 12 different slots for the uplink.Uplink and downlink are separated in time and each connection is allotted its own up and downlink pair.

TDD/TDMA - general scheme, example DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

What is Aloha?

Aloha, also called the Aloha method, refers to a simple communications scheme in which each source (transmitter) in a network sends data whenever there is a frame to send. If the frame successfully reaches the destination (receiver), the next frame is sent.There are different types of Aloha:

Classical AlohaSlotted Aloha

Classical Aloha

This is exactly what the classical Aloha scheme does, a scheme which was invented at the university of Hawaii and was used in the ALOHANET for wireless connection of several stations.Aloha neither co-ordinates medium access nor does it resolve contention on the MAC layer.Instead each station can access the medium at any time as shown in next slide figure.This is a random access scheme, without a central arbiter controlling access and without co-ordination, among the stations.

Classical Aloha

In two or more stations access the medium at the same time, a collision occurs and the transmitted data is destroyed.Resolving this problem is to retransmission of data.

Classical Aloha

sender A

sender B

sender C

collision

t

Slotted Aloha

"Slotted Aloha" reduces the chance of collisions by dividing the channel into time slots and requiring that the user send only at the beginning of a time slot. Aloha was the basis for Ethernet, a local area network protocol.In this case, all senders have to be synchronized, transmission can only start at the begin of a time slot as shown a figure.Under the assumption stated above, the introduction of slots raises the throughput from 18 to 36 percent that is slotting double the throughput.

Slotted Aloha

sender A

sender B

sender C

collision

t

Carrier sense multiple access (CDMA)

One improvement to the basic Aloha is sensing the carrier before accessing the medium.This is what carrier sense multiple access (CSMA) schemes generally do the sensing the carrier and accessing the medium only if the carrier is idle decreases the probability of a collision.But as already mentioned in the introduction, hidden terminals cannot be detected.Thus if a hidden terminal transmits at the same time as another sender, a collision might occur at the receiver.


Still, this basic scheme is used in most wireless LANs.Several versions of CSMA exist.Non-persistent CSMA:

The stations sense the carrier and start sending immediately if the medium is idle.If the medium is busy the station pauses a random amount of time before sensing the medium again and repeating this pattern.


P-Persistent CSMA:

In systems nodes also sense the medium, but only transmit with a probability of P, with the station reschedule to the next slot with the probability P that is access is slotted in addition.

I-Persistent CSMA:

in systems all stations wishing to transmit access the medium at the same time, as soon as it becomes idle.


CSMA with collision avoidance (CSMA/CD) is one of the access schemes used in wireless LANs following the standard IEEE 802.11.

Demand assigned multiple access (DAMA)

A general improvement of aloha access systems can also be achieved by reservation mechanisms and combinations with some (fixed) TDM patterns.These schemes typically have a reservation period followed by a transmission period.During the reservation period, stations can reserve future slots in the transmission period.

Demand assigned multiple access

While, depending on the scheme, collision may occur during the reservation period, the transmission period can then be accessed without collision or split into transmission periods with and without collision.In general these schemes cause a higher delay under a light load, but allow higher throughput.One basic scheme is demand assigned multiple access (DAMA) also called reservation Aloha, a scheme typical for satellite system.


Show in this next slide figure has two modes.ALOHA mode for reservation:competition for small reservation slots, collisions possible Reserved mode for data transmission within successful reserved slots (no collisions possible)During a contention phase following the slotted Aloha scheme, all stations can try to reserve future slots.


For example, different stations on earth try to reserve access time for satellite transmission.Thus collisions during the reservation phase do not destroy data transmission, but only the short requests for data transmission.If successful, a time slot in the future is reserved, and no other station is allowed to transmit during this slots.


Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t


Therefore, the satellite collects all successful requests and sends back a reservation list indicating access rights for future slots.All ground stations have obey this list.To maintain the fixed TDM pattern of reservation and transmission, the stations have to be synchronized from time to time.DAMA is an explicit reservation scheme.Each transmission slot has to be reserved explicitly.

Packet Reservation Multiple Access(PRMA)

An example for an implicit reservation scheme is packet reservation multiple access (PRMA).Here, slots can be reserved implicitly according to the following scheme.Show in the figure a certain number of slots forms a frame.The frame is repeated in time, that is a fixed TDM pattern is applied.The base station now broadcasts the status of each slot to all mobile stations.


frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-slot

collision at reservation attempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F Dt

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation


All stations receiving this vector will then know which slot is occupied and which slot is currently free.In the example, the base station broadcasts the reservation status ‘ACDABA-F’ to all stations, here, A to F.This means that slots one to six and eight are occupied, but slot seven is free in the following transmission.All stations wishing to transmit for this free slot in Aloha fashion.In the example shown, more than one station wants to access this slot, thus a collision occurs.


The base station returns the reservation status ‘ACDABA-F’ indicating that the reservation of slot seven failed and that nothing has changed for the other slots.Again stations can compete for this slot.Additionally, station D has stopped sending in slot three and station F in slot eight.This is noticed by the base station after the second frame.Before the third frame starts, the base station indicates that slots three and eight are now idle.


Additionally, station F has succeeded in reserving slot seven as also indicated by the base station.PRMA constitutes yet another combination of fixed and random TDM schemes with reservation compared to the previous schemes.As soon as a station has succeeded with a reservation, all future slots are implicitly reserved for this station.This ensures transmission with a guaranteed data rate.The slotted aloha scheme is used for idle slots only, data transmission is not destroyed by collision.

Access Method DAMA: Reservation TDMA

An even more fixed pattern that still allows some random access exhibited by reservation TDMA.Show in the figure.

N mini-slots N * k data-slots

reservationsfor data-slots

other stations can use free data-slotsbased on a round-robin scheme

e.g. N=6, k=2

Access Method DAMA: Reservation TDMA

every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up

to k data-slots using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots

according to a round-robin sending scheme (best-effort traffic)

MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet

CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

MACA-Collision Avoidance

Signaling packets contain sender address receiver address packet size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

MACA examples

MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B

A B C

RTS

CTSCTS

MACA-Collision Avoidance

MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot

receive CTS from A

A B C

RTS

CTS

RTS

MACA variant: DFWMAC in IEEE802.11

idle

wait for the right to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out; RTS

CTS; data

ACK

RxBusy

idle

wait fordata

RTS; RxBusy

RTS; CTS

data; ACK

time-out data; NAK

ACK: positive acknowledgementNAK: negative acknowledgement

RxBusy: receiver busy

time-out NAK;RTS


Show in the previous slide figure, to simplified state machines for a sender and receiver that could realize MACA.The sender is idle until a user requests the transmission of a data packet.The sender then issues an RTS and waits for the right to send.If the receiver gets an RTS and is in an idle state, it sends back a CTS and waits for would send an RTS again a time-out.


After transmission of the data the sender waits for a positive acknowledgement to return into an idle state.The receiver sends back a positive acknowledgement if the received data was correct.Otherwise, or if the waiting time for data is too long, the receiver returns into idle state.If the sender does not receive any acknowledgment or a negative acknowledgement, it sends an RTS and again waits for the right to send.


Additionally, a receiver could indicate that it is currently busy via a separate RxBusy.Real implementations have to add more states and transitions, e.g. in order to limit the number of retries.

Polling In the case where one station is to be heard by all

others, polling schemes as known from the mainframe/terminal world can be applied.

Polling is a strictly centralized scheme with one master station and several slave stations.

The master can poll the slaves according to many schemes: Round robin & randomly, According to reservations, etc.

Example: Randomly Addressed Polling Base station signals readiness to all mobile

terminals.

Polling

Terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address).

The base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) .

The base station acknowledges correct packets and continues polling the next terminal.

This cycle starts again after polling all terminals of the list.

ISMA (Inhibit Sense Multiple Access)

Another combination of different schemes is represented by inhibit sense multiple access (ISMA).

In this scheme Current state of the medium is signaled via a “busy tone” The base station signals on the downlink (base

station to terminals) if the medium is free or not. Terminals must not send if the medium is busy. Terminals can access the medium as soon as the

busy tone stops.

ISMA (Inhibit Sense Multiple Access) The base station acknowledges successful

transmission, a mobile station detects a collision only via the missing positive acknowledgement.

In case of collisions, additional back-off and retransmission mechanisms are implemented.

This Mechanism used, e.g., for Cellular Digital Packet Data (CDPD) in the Advance mobile phone system (AMPS) mobile phone system, also known as digital sense multiple access (DSMA).

ISMA (Inhibit Sense Multiple Access)

Fig: Inhibit sense multiple access using a busy tone

What is CDMA?

CDMA is spread spectrum based fastest growing digital wireless technology.

It is an advanced digital technology that can offer about 7 to 10 times the capacity of analog technologies and up to 6 times the capacity of digital technology such as TDMA.

How CDMA Work

The words code and division are important parts of how CDMA works.

CDMA uses codes to convert between analog voice signals and digital signals.

CDMA also uses codes to separate voice and control data in to data stream called channels.

CDMA Signal Generation

There are five steps in CDMA signal generation. Analog to Digital Conversion Vocoding Encoding and Interleaving Channelizing the signals Conversion of the digital signal to a Radio

Frequency (RF) signal. The use of codes is a key part of this process.


A/D Conversion: The first step of CDMA signal generation is analog

to digital conversion, sometimes called A/D conversion.

CDMA uses a technique called Pulse Code Modulation (PCM) to accomplish A/D conversion.

Voice compression: The second step of CDMA signal generation is

voice compression. CDMA uses a device called a vocoder to

accomplish voice compression.


The term “vocoder” is a contraction of the words “voice” and “code”.

Vocoders are located at the base station controller (BSC) and in the phone.

Variable length vocoders: A CDMA vocoder varies compression of the voice signal

into one of four data rates based on the rate of the user’s speech activity.

The four rates are: full, 1/2,1/4, and 1/8. The vocoder uses its full rate when a person is taking

very fast. It uses the 1/8 rate when the person is silent or nearly so.


Encoding & interleaving: Encoders and interleaves are built into the base

transceiver station (BTS) and the phones. The purpose of the encoding and interleaving is

to build redundancy into the signal so that information lost in transmission can be recovered.

Advantages CDMA

Coverage: CDMA’S features result in coverage that is

between 1.7 and 3 times that of TDMA. Power control helps the network dynamically

expand the coverage area. Coding and interleaving provide the ability to

cover a larger area for used in other systems.

Advantages CDMA

Capacity: CDMA capacity is ten to twenty times that of analog

systems, and it’s up to four times that of TDMA. Reasons for this include: CDMA users are separated by codes, not frequencies. Power control minimizes interference, resulting in

maximized capacity. CDMA’s soft handoff also helps increase capacity. This is because a soft handoff requires less power.

Advantages CDMA Clarity: Often CDMA systems can achieve “wireline” clarity

because of CDMA’s strong digital processing. The rake receiver reduces errors. The variable rate vocoder reduces the amount of data

transmitted per person, reducing interference. The soft handoff also reduces power requirements and

interference. Power control reduces errors by keeping power at an

optimal level. CDMA’s wide band signal reduces fading. Encoding and interleaving reduce errors that result

from fading.

Advantages CDMA

Cost: CDMA’s better coverage and capacity result in

cost benefits: Increased coverage per base transceiver station

(BTS) means fewer are needed to cover a given area.

This reduces infrastructure costs for the providers. Increased capacity increases the service provider’s

revenue potential. CDMA costs per subscriber has steadily declined

since 1995 for both cellular and PCS applications.

Advantages CDMA

Compatibility: CDMA phones are usually dual mode. This

means they can work in both CDMA systems and analog cellular systems.

Some CDMA phones are dual band as well as dual mode.

They can work in CDMA mode in the personal communication service (PCS) band, CDMA mode in the cellular band, or analog mode in an analog cellular network.

Advantages CDMA

Customer Satisfaction: CDMA results in greater customer satisfaction

because CDMA provides better: Voice quality Longer batter life due to reduced power

requirements No cross-talk because of CDMA’s unique

coding Privacy- again, because of coding.

Disadvantages CDMA

Higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal).

All signals should have the same strength at a receiver.

CDMA in theory

Sender A sends Ad = +1, key Ak = 010011 (assign: “0”=

-1, “1”= +1) sending signal As = Ad * Ak = (-1, +1, -1, -1,

+1, +1) Sender B

sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1)

sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

CDMA in theory

Both signals superimpose in space interference neglected (noise etc.) C=As + Bs = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A apply A’s code for despreading : c*Ak= Ae = (-2, 0, 0, -2, +2, 0) *(-1, +1, -1, -1, +1, +1) = 2 + 0 + 0 + 2 + 2 + 0 = 6 result greater than 0, therefore, original bit was

“1”

CDMA in theory

receiving B Be = (-2, 0, 0, -2, +2, 0) (-1, -1, +1, -1, +1,

-1) = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0”

CDMA on signal level I

The next slide figure shows a sender A that wants to transmit the bits 101.

The key of A is shown as signal and binary key sequence Ak.

After spreading, i.e., XORing Ad and Ak the resulting signal is As.

CDMA on signal level I

data A

key A

signal A

data key

key sequence A

Real systems use much longer keys resulting in a larger distance between single code words in code space.

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

CDMA on signal level II

Next slide figure shows: The same happens with data from sender B,

here the bits are 100. The result of spreading with the code is the

signal Bs. As and Bs now superimpose during

transmission. Thus the resulting signal is simply the sum

As+Bs as in next slide figure.

CDMA on signal level II

signal A

data B

key B

keysequence B

signal B

As + Bs

data key

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

CDMA on signal level III

Next slide figure shows: A receiver now tries to reconstruct the original

data from A, Ad. Therefore the receiver applies A’s key, Ak to

the received signal and feeds the result into an integrator.

The integrator adds the products and a comparator then has to decide if the result is a 0 or 1 as shown in next figure.

CDMA on signal level III

Ak

(As + Bs) * Ak

integratoroutput

comparatoroutput

As + Bs

data A

1 0 1

1 0 1 Ad

CDMA on signal level IV

Next slide figure shows: A receiver wants to receive B’s data and the

comparator can easily detect the original data. Looking at (As+Bs)*Bk on can also imagine

what could happen if A’s signal was much stronger.

The little peaks which are now caused by A’s signal would be much higher, and thus the result of the integrator would be wrong.

CDMA on signal level IV

integratoroutput

comparatoroutput

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

CDMA on signal level V Next slide figure shows: What happens if a receiver has the wrong key or is

not synchronized with the chipping sequence of the transmitter.

The integrator still presents a value after each bit period but now it is not always possible for the comparator to decide for a 1 or a 0 as the signal rather resembles noise.

Even if the comparator could detect a clear 1 this could still not reconstruct the whole bit sequence transmitted by a sender.

comparatoroutput

CDMA on signal level V

wrong key K

integratoroutput

(As + Bs) * K

As + Bs

(0) (0) ?

Comparison SDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMA Idea segment space into

cells/sectors segment sending time into disjoint time-slots, demand driven or fixed patterns

segment the frequency band into disjoint sub-bands

spread the spectrum using orthogonal codes

Terminals only one terminal can be active in one cell/one sector

all terminals are active for short periods of time on the same frequency

every terminal has its own frequency, uninterrupted

all terminals can be active at the same place at the same moment, uninterrupted

Signal separation

cell structure, directed antennas

synchronization in the time domain

filtering in the frequency domain

code plus special receivers

Advantages very simple, increases capacity per km²

established, fully digital, flexible

simple, established, robust

flexible, less frequency planning needed, soft handover

Dis-advantages

inflexible, antennas typically fixed

guard space needed (multipath propagation), synchronization difficult

inflexible, frequencies are a scarce resource

complex receivers, needs more complicated power control for senders

Comment only in combination with TDMA, FDMA or CDMA useful

standard in fixed networks, together with FDMA/SDMA used in many mobile networks

typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)

still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA

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