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© UNI Hannover, Institut für Allgemeine Nachrichtentechnik

Institut für Kommunikationstechnikwww.ikt.uni-hannover.de

Protocolls of the OSI-layer 2Link Layer

MAC (Medium Access Control)Kapitel 7.1

Netze und ProtokolleDr.-Ing. J. Steuer

Literatur:

[Boss99] M.Bossert, M.Breitenbach, Digitale Netze, 1999, B.G. Teubner, Stuttgart, Leipzig, ISBN 3-519-06191-0

[Come98] D. Comer, Computernetzwerke und Internets, Prentice Hall, 1998, ISBN3-8272-9552-1,

2001: ISBN 3-8273-7023[Coul02] Couloris et.al.; „Verteilte Systeme“, Addison- Wesley, 2002, ISBN 3-8273-7022-1[Hals96] F.Halshall, „Data Communications, Computer Networks and Open Systems“, 4thedition,

Edison-Wesley, 1996, ISBN 0-201-42293-X[Kann] Kanbach, Körber, ISDN - Die Technik, Hüthig-Verlag[Reim95] Reimers, et.al.; Digitale Fernsehtechnik, Springer Verlag, 1995, ISBN 3-540-58993-7[Sieg99] Gerd Siegmund,“Technik der Netze“, 4.Auflage, Hüthig Verlag, Heidelberg, 1999,

ISBN 3-7785-2637-5[Spra91] J.D.Spragins,et.all, Telecommunications Protocols and Design, Addison WesleyPublishing Company, 1991, ISBN 0-201-09290-5[Stall90] William Stallings, Local and Metropolitan Area Networks, 1990; MacMillen Publishing Company,

ISBN 0-02-415465-2[WAL02(1)] Walke, B., „Mobilkommunikation, Band1“, Teubner Verlag, 2002


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Goals

understand the need for MAC (media access control)understand the most important MAC strategies for ISDN, data networks and mobile networksclassify different MAC-strategiesevaluate the performance of different MAC-strategies

The medium access control (MAC) is mandatory in case of shared usage of a media. A media could be a twisted pair, an optical fibre, the spectrum in the air (air interface), or in general any media which is able to transport communication content. Only in case one user is using one medium in one direction only, the MAC is not necessary. It could be implemented, but it need not. The control of the usage of the channel (media) could be left to the subscriber.Already in case of bidirectional usage a scheduler has to control the access from both ends of the communication. This scheduler can be simple, because one station could be declared as master. The master is responsible for the scheduling. This is a type of central control. Imagine the Master has not only to control one station but several. The master is a little bit more busy, but with sufficient memory and computing power he will be able to handle the situation. There is no principle difficulty in such centralized control task.It gets much more complicated in case there is no knowledge at the master on the states of the slaves. A protocol handling such scenario needs to operate on guesses on the behavior of the other stations competing for the usage of the medium. There will be situations of conflict, which means more than one station is accessing the medium at the same time. The result will be a corruption of messages. This situation is called decentralized and uncoordinated. The protocol must be able to detect such corruptions and to react on that, in order to allow for a proper communication.With all these issues the MAC has to deal. In this chapter we will study the principal MAC properties for different media.


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need for MAC?

Competition for the usage of a transmission medium

NTS0-bus4 wireISDN Sobus:

limited to the signalling channel

802.3, coaxial cable, max 10Mbit/s (5Mbit/s) LAN

Local Area Network(LAN) with sharedmedia, e.g. Ethernet

Why is in the caseof ISDN only the access to the signalling channelin competition?

There are networks for which many users share a common channel using a multi access scheme. We find the examples on

the signaling channel for the ISDN S0 bus (in this case not for the communication channel, which is controlled by by the switch and the switch knows the status of the users and channels)on Local Area Networks (LAN) (note: latest developments in LAN´s go back to dedicated channels, using switching technologies)mobile networkspacket radio networksAdhoc-networkssatellite networks


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need for MAC?

Competition for the usage of a transmission medium during registration phases of mobile stations (MS)

mobile technologies:DECT, GSM, IS95, HSCSD, GPRS,UMTS

In case the MS (mobile station) is switched of, there is no knowledge in the Basestation or the Basestation controller on this subscriber. Thus there can be no dedicated channel for this subscriber. If the MS is switched on, it needs first to access a common channel and apply for a dedicated channel. The common channel is used in competition to other subscribers in the same state. Consequently we need a media access control.Abbreviations:DECT: digital enhanced cordless telephony (ETSI and ECMA standard) (see [WAL02(2)])GSM: global system mobile (ETSI standard) (see [WAL02(1)])IS95: Interim Standard 1995 of the Telecommunications Industry Association (TIA) USA for the first CDMA

mobile system (see [WAL02(1)]) , compare ZVEI in Germany (see introduction NUP, standards)HSCSD: High Speed Circuit Switched Data (channel trunking in GSM to increase the channel capacity in SM for

Data) (see [WAL02(1)])GPRS: General Packet Radio System (packet switched enhancement of GSM) (see [WAL02(1)])UMTS: Universal Mobile Telecommunication System (expected successor of GSM) (see [WAL02(1)])


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Station 2

Station 5

Station 3

Station 4

Station 1

need for MAC?

Basic Service Set (IEEE 802.11)

W-LAN, AdHoc-Network

The common media is the air interface, all membersof the AdHoc network areCompeting at least duringthe registration process

In many cases, a network is installed using an infrastructure. This infrastructure takes over some central tasks and serves as an access to the wired network. In HIPERLAN/2, the Access Points (AP) take over this task. They have a wired as well as a wireless interfaces. By the way, there are a number of different technologies and protocols which could serve as wireless LAN:

HIPERLAN (High PErformance Radio Local Area Network) of ETSI [WAL02(2)])WLAN (Wireless LAN) of IEEE (802.11x) [WAL02(2)])Bluetooth: short range data link or network

and even others which partly show functionalities of LAN´s (high speed data transport in the access area):

UMTS (Universal Mobile Telephone System) [WAL02(1)])DAB (Digital Audio Broadcast)DVB (Digital Video Broadcasting) [Reim95]

During this lecture we are going to concentrate on HIPERLAN due to its enhanced functionality compared to wired LAN´s. This stands also for the WLAN, but the WLAN is an American standard with some drawbacks in its functionality which is compensated by its better position in the market.The coverage area of an AP and its associated terminals is called radio cell in general. In the case of IEEE 802.11 (see below), it is called a basic service set, in Bluetooth a scatternet (to scatter=umherstreuen; the terminals are scattered in the area of coverage served by an access point).


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Station A1

Station A2

Station B1

Station B2

Station B3

Station A1

Access Point A Access Point B

need for MAC?

Radio Cell (HIPERLAN)

Basic Service Set (IEEE 802.11)

The common media is the airinterface, in addition we have

Influences between the cells!

Wireless LAN´s can be grouped to cells or basic service sets, which are comparable to the cells of cellular telephony networks. In the HIPERLAN-System the MAC is of the scheduled type. The Access Point serves as scheduler. This scheme allows a better performance for high traffic loads. The W-LAN system in contrast operates in a random mode like the Ethernet, which is sufficient for low traffic loads. In principle it can be expected that data terminals will at least from time to time carry high traffic load. This forced the Europeans (ETSI, HIPERLAN) to deviate from the american (IEEE802.11, W-LAN) approach.The traffic channels in the HIPERLAN-scheme are dedicated to connections and thus terminals. They do not need MAC. Bat the traffic channels are assigned after request. During the request phase the data terminals compete for transmission capacity, therefore this phase is handled by a MAC protocol.


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when is the MAC function not needed?


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influences on the MAC

distance of stations to each other (power and delay)visibility of stations (SNR, Signal to Noise Ratio))throughput of the network, e.g. in Kbit/sfairness of the usage of the transmission media by competing terminalstransmission speed of channelsburstiness of trafficpacket length

The distance between stations influences the ability to detect collisions, e.g. the IEEE802.3 is limited to 2500m

(collision domain)Systems with radio connections need to „see“ each other in order to be able to detect collisions The required throughput of the network influences the complexity of the MAC. If the throughput is high compared to the capacity we need to establish MAC protocols with high efficiency; if the throughput is low in contrast to the capacity we can allow for a less efficient MAC. This consideration led to the application of the slotted ALOHA for the access to the RACH (Random Access Channel) of the GSM system. This channel is used only for the request from the mobile Station to the network in order to apply for dedicated channels. The volume of the signaling is low. Thus we can allow for a low efficiency. Similar thoughts let us find that a bulk data transfer on a Local Area Network (LAN) needs a MAC which minimizes the collisions. If we stick to the random MAC than CSMA/CD is the choice.Fairness is often an opposite requirement to throughput. A fair MAC allows for each traffic source on

average the same transmission capacity. Even if we implement no priorities fairness is difficult to establish. The MAC for the Dual Queueing Distributed Bus - a Metropolitan Area Network (IEEE 802.6) - tried to establish fairness. The DQDB is a very good subject for study purposes, but it did not gain an important position in the market.The transmission speed of the channels dictate the time available in the protocol stack. The higher the transmission speed, the more critical is the time spent in the stack. Take the ATM Protocoll with a targeted speed of 2.5Gbit/s. There is not much time left to perform complicated operations in the stack. Thus the MAC has to be extremly time efficient.A high burstiness of traffic generates a high peak load in the stack. This effects the stack in the

same mannor than a high transnmission speed.Long packets put less burdon on the stack compared to short packets


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MAC principles

Scheduling vs. Random access:scheduling means stations ready to send are waiting until it is their turn to operateUnder the random access scheme a station tries to access the transmission media as soon as it has to send something (immediately)

Scheduler RandomAccess

Discussdifferenceswith respect to efficiency


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MAC principles

SchedulingScheduling

fixed assignmentfixed assignment

demand assignmentdemand assignment

central control

central control

distributed control

distributed control

random accessrandom access

without sensing(ALOHA)without sensing(ALOHA)

with sensing (CS)with sensing (CS)

before transmission(CSMA)


before & during transmission(CSMA/CD)


pure ALOHA

pure ALOHA slotted

ALOHAslotted ALOHA

How does the channel selection and the channel assignment of aPDH-, SDH-, ISDN- and Ethernet-LAN-system fit into the above scheme?


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MAC principles (Examples)


fixed assignmentsfixed assignments


central control

central control

distributed control

distributed control








pure ALOHA

pure ALOHA slotted

ALOHAslotted ALOHA

• TDMA/reservation• Polling• Token Passing

• slotted ALOHA:GSM, Random Access Channel (RACH)

• CSMA/CD: Ethernet


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TDMA (Time Division Multiple Access)

time 1

frame i frame i+1 frame i+2

time 2control data datadatadata data

station 1 station 2 station m-2 station m-1 station m

packetready?

wait forassigned slot

transmitpacket

no

yes

guard time

control information?guard time?advantages and disadvantages?


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Reservation in TDMA

time 1



slot 1 slot 2 slot n-2 slot n-1 slot n

packetready?

reservation and waitingforassigned slot

transmitpacket

no

yes

guard timen<m

advantages and disadvantages?• Number of stations is not limited sharply• traffic behaviour as pure TDMA• reservation has to be performed by signalling

(not shown here)

Application: HIPERLAN


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Polling scheduling with central control

central controllercentral controller

station 1station 1 station Rstation Rstation kstation k

pollstation 1

message closedby go ahead fromstation 1 poll

station k

message closedby go ahead fromstation k poll

station m message closedby go ahead fromstation m

• efficient with high load from all stations• inefficient if stations have low or zero load• half duplex links need resynch, full duplex links canstay synchronized


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Token passing (scheduling with distributed control)

station 1station 1station 2station 2

station kstation k

station rstation r

• transmission medium of ring (IEEE 802.5) or bus (IEEE 802.4) type [logically the bus behaves like a ring

• a token (packet with permission to send) is handed from station to station• all stations read all packets• all packets are circulating on the ring and have to be removed after one turn• the packet control requires receiving and sending of all packets in all stations,

which requires highly reliable stations and adds delay in each station• the token is a distinctive bit pattern, how can the transmission be data transparent?• if no station has to transmit, a free token is circulating. What happens if a bit error

occurs?• priority handling is possible


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Random Access with no sensing (ALOHA and slotted ALOHA)


station kstation k

station rstation r

or

shared transmission medium:

packet to transmit? delay to start ofnext slot

yesno

transmitwait two way

propagation timequantized to

slot times

acknoledgement?

yes

compute randombackoff integer k

no

delay k packetstransmission time

Discuss high load scenario!Do you see the advantage of the slotted ALOHA?

The slotted ALOHA- mechanismn is implemented in the registratition phase of the GSM system.


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Random Access with sensing Carrier Sense with Multiple Access (CSMA)


station kstation k

station rstation r


packet to transmit? delay to start ofnext slot

yesnotransmit

wait two way propagation time

quantized to slot times

acknowledgement?

yes


no


carriersense

strategy

Medium is occupied

Medium is free


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Random Access with sensing before and during transmission /Collision Detection (CSMA/CD)


station kstation k

station rstation r


packet to transmit?yesno

transmit Collision detected/jamming received?

no

abort transmission

yes

transmit yammingSignal*

carriersense

strategy

Implemented in IEEE 802.3

Medium is occupied



* Only in case of collision detected


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MAC of the ISDN S0-Bus

Transmission in frames on S0Activation procedure in the physical layerFrame synchronization in the MAC layerFrame recognition by violation of code rulesDistributed MAC for the d-channel onlyMAC for the user channel by framing



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Leitungscodierung S0

+-

+-

AMI-Code

+-

+-

inv. AMI-Code0 0001 1 1 1

0 0001 1 1 1

S0-Frame (2)

AMI - Alternate Mark Inversion: Eine Markierung (1) wird abwechselnd mit positivem bzw. negativem Impuls dargestellt.Pseudoternärer Code: zwei logische Zustände (Null, Eins) werden auf 3 physikalische Zustände abgebildet (pos. Impuls +, kein Impuls 0, neg. Impuls -)Am ISDN-Basisanschluß eingesetzt wird ein invertierter AMI-Code: nicht die (1), sondern die (0) wird abwechselnd mit positivem bzw. negativem Impuls dargestellt.


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S0-Frame (3)

A AktivierungsbitB1 Bit des 1. B-KanalsB2 Bit des 2. B-KanalsD Bit des D-KanalsE Bit des EchokanalsF Rahmenbit

FA HilfsrahmenbitL DC-AusgleichbitM Multiframingbit = 0N = FAS Spare = 0

. an diesen Stellen ist derCode gleichanteilsfrei

D L. F L. B1B1B1B1B1B1B1B1 E D A F N B2B2B2A B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L.

D L. F L. B1B1B1B1B1B1B1B1 L. D L. FA L. B2B2B2B2B2B2B2B2 L. D L. B1B1B1B1B1B1B1B1 L. D L. B2B2B2B2B2B2B2B2 L. D L. F L.

0

1

0

t

2 bits offset

TE NT

NT TE

48 bits in 250 microseconds

2 bit offset between frame from NT TE and NT TE in order to allow the terminals to read the e-bit before they write the next d-bit

How is a collisiondetected?

Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls).2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung)Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs)48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück.Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NTCoderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt:

1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-).

A-Bit (Bus aktiviert)

Literatur: [Kann]


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S0-Frame (4)

Why is there no e-bit for the other direction in the Frame from TE NT?






0

1

0

t

2 bits offset

TE NT

NT TE





Literatur: [Kann]


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S0-Frame - frame detectionCase 1: d-bit from the NT at the end of the frame will be „0“ and negative,

the L-bit will be „0“ and positive in order to compensate. The F-bitwill be „0“ and positive in order to be in conflict with the coding rule.






0

1

0

t

2 bits offset

TE NT

NT TE





Literatur: [Kann]


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S0-Frame - frame detectionCase 2: d-bit from the NT at the end of the frame will be „0“ and positive,

the L-bit will be „1“in order to compensate. The F-bitwill be „0“ and positive in order to be in conflict with the coding rule.






0

1

0

t

2 bits offset

TE NT

NT TE





Literatur: [Kann]


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S0-Frame - frame detection

Case 3: d-bit from the NT at the end of the frame will be „1“, the L-bit will be „0“and positive if the last “0” before the L-bit was negative in order to compensate or the L-bit will be “1” if the last “0” before the L-bit was “0” and positive(compensationnot necessary). The F-bit will be „0“ and pos. to be in conflict with the coding rule.






0

1

0

t

2 bits offset

TE NT

NT TE





Literatur: [Kann]


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The end


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need for MAC?

Competition for the usage of a transmission medium


limited to the signalling channel

Why is in the case of ISDN only the access to the signalling channel in competition ?

The user channel, the B-channel with 64Kbit/s, is needed permanently during a communication session, otherwise the Shannon sample rate of 125µs can not be guaranteed. Therefore the B-channel will be a dedicated channel which is assigned by the switch during call set up phase and released at the end of the connection.The traffic on the signalling channel is highly bursty, it can not be foreseen when it is needed. Therefore the D-channel is a packet channel, for which the users have to compete.The usage will be assigned for individual packets orsequences of packets.

There are networks for which many users share a common channel using a multi access scheme. We find the examples on

the signaling channel for the ISDN S0 bus (in this case not for the communication channel, this is dedicated by the switch)on Local Area Networks (LAN) (note: latest developments in LAN´s go back to dedicated channels, using switching technologies)mobile networkspacket radio networksAdhoc-networkssatellite networks


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when is the MAC function not needed?

E.g. to communicate between two points, a communication media need to be used. If this medium is used unidirectional there is no need for MAC (media access control)!

no MAC necessary, but might be implemented


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MAC principles

Scheduling vs. Random access:scheduling means stations ready to send are waiting until it is their turn (efficient channel control under high load! But,high overhead under low load conditions)Under the random access scheme a station tries to access the transmission media as soon as it has to send something (immediately) (collisions under high load! But, low overhead under low load conditions)

Scheduler RandomAccess

Discussdifferenceswith respect to efficiency


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MAC principles


fixed PDH-, SDHassignmentfixed PDH-, SDHassignment


central control

central control

distributed control

distributed control








pure ALOHA

pure ALOHA slotted

ALOHAslotted ALOHA

How does the channel selection and the channel assignment of aPDH-, SDH-, ISDN- and Ethernet-LAN-system fit into the above scheme?

ISDN ISDNEthernet-LAN


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TDMA (Time Division Multiple Access)

time 1



station 1 station 2 station m-2 station m-1 station m

packetready?

wait forassigned slot

transmitpacket

no

yes

guard time

control information?Frame length and Frame start (Frame delimiter)guard time?Data packets from stations suffer from different latency times which creates the danger of overlapping (collision)advantages and disadvantages?Efficient for high traffic load, inappropriate for low traffic load

View on a shared media:

View on a single terminal:


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S0-Frame (3)

Are there other cases?

Let the d-bit be a logical „0“, the e-bit will be a “0” in any case, because the logical “0” is dominant! In this case the “blue” terminal will not detect a collision and will continue!!!



0

1

0

t

2 bits offset

TE NT

NT TE


The “green” terminal which sent a “1” will get back a “0” and will stop transmitting!


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S0-Frame (3a)

Let the d-bit be a logical „0“, and let another terminal send a “0”as well. In this case a collision is not detected!!!

A collision is detected only, if one TE is transmitting a logical „1“and the other a „0“. The „0“ will continue and the „1“ will stop!



0

1

0

t

2 bits offset

TE NT

NT TE



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S0-Frame (4)

Why is there no e-bit for the other direction in the Frame from TE NT?Because the NT is not in competition with other terminals!






0

1

0

t

2 bits offset

TE NT

NT TE





Literatur: [Kann]

[11] nu p 07 1

Technology

channel media

mac media access control

medium access control

communication channel

data networks

computer networks

channel capacity

acommon channel