[11] Nu P 07 1

Protocolls of the OSI-layer 2
Link Layer
MAC (Medium Access Control)
Kapitel 7.1

Netze und Protokolle
Dr.-Ing. J. Steuer

Institut für Kommunikationstechnik
www.ikt.uni-hannover.de

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“, 4th
edition,
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 Wesley
Publishing 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

© UNI Hannover, Institut für Allgemeine Nachrichtentechnik

Goals

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

(2)

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.


need for MAC?

Competition for the usage of a transmission medium

limited to the
S0-bus
ISDN Sobus: signalling channel
NT
4 wire

Why is in the case
of ISDN only the
access to the
signalling channel
Local Area Network
in competition?
802.3, coaxial cable,
(LAN) with shared max 10Mbit/s (5Mbit/s) LAN
media, e.g. Ethernet

(3)

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 networks
packet radio networks
Adhoc-networks
satellite networks


need for MAC?

during registration phases of mobile stations (MS)

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

(4)

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)])


need for MAC?

Station 1

Station 3

Station 5

Station 2
Station 4

The common media is the
air interface, all members
Basic Service Set (IEEE 802.11)
of the AdHoc network are
W-LAN, AdHoc-Network
Competing at least during
the registration process

(5)

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).


need for MAC?

Access Point A Access Point B

Station A1

Station B1
Station A1

Station B3
Station A2
Station B2

The common media is the air
Radio Cell (HIPERLAN) interface, in addition we have
Basic Service Set (IEEE 802.11) Influences between the cells!

(6)

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.


when is the MAC function not needed?

(7)


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/s
fairness of the usage of the transmission media by
competing terminals
transmission speed of channels
burstiness of traffic
packet length

(8)

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


MAC principles

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

Discuss
differences
with respect
to efficiency
Random
Scheduler
Access
(9)


MAC principles

Scheduling random access

without with sensing
fixed demand without with sensing
fixed demand sensing(ALOHA) (CS)
assignment assignment sensing(ALOHA) (CS)
assignment assignment

central distributed
central distributed before before & during
pure slotted
control control before before & during
pure slotted
control control ALOHA ALOHA transmission transmission
ALOHA ALOHA transmission transmission
(CSMA) (CSMA/CD)
(CSMA) (CSMA/CD)

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

(10)


MAC principles (Examples)


fixed demand without with sensing
fixed demand sensing(ALOHA) (CS)
assignments assignment sensing(ALOHA) (CS)
assignments assignment

central distributed
pure slotted
pure slotted
(CSMA) (CSMA/CD)
(CSMA) (CSMA/CD)

• TDMA/reservation • slotted ALOHA:
• Polling GSM, Random Access Channel (RACH)
• Token Passing • CSMA/CD: Ethernet

(11)


TDMA (Time Division Multiple Access)

frame i frame i+1 frame i+2
time 1
guard time

station 1 station 2 station m-2 station m-1 station m
control data data data data data
time 2

no packet
ready?
control information?
yes
guard time?
wait for
advantages and disadvantages?
assigned slot

transmit
packet

(12)


Reservation in TDMA

time 1
guard time
n<m
slot 1 slot 2 slot n-2 slot n-1 slot n
time 2

no packet
ready?
yes
• Number of stations is not limited sharply
reservation and waiting
• traffic behaviour as pure TDMA
forassigned slot

• reservation has to be performed by signalling
transmit
(not shown here)
packet

Application: HIPERLAN

(13)


Polling scheduling with central control

station 11 station k station R
station station k station R
message closed
message closed
by go ahead from
by go ahead from
station k
station 1 poll
poll message closed
poll station m
station k by go ahead from
station 1
central controller station m
central controller

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

(14)


Token passing (scheduling with distributed
control)
station 11
station
station rr station 22
station station

station kk
station

• 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

(15)


Random Access with no sensing (ALOHA
and slotted ALOHA)
shared transmission medium:
station 11
station
station station

or

station kk
station

yes
wait two way
no
yes delay to start of propagation time
transmit acknoledgement?
packet to transmit?
next slot quantized to
slot times

no

delay k packets compute random
transmission time backoff integer k

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

(16)

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


Random Access with sensing
Carrier Sense with Multiple Access (CSMA)
station 11
station
station station

station kk
station

yes
Medium wait two way
no yes carrier is free delay to start of propagation time
transmit acknowledgement?
packet to transmit? sense
quantized to
next slot
strategy
slot times

no
Medium is occupied
delay k packets compute random
transmission time backoff integer k

(17)


Random Access with sensing before and during
transmission /Collision Detection (CSMA/CD)
station 11
station
station station
Implemented in
IEEE 802.3

station kk
station

no

yes
no
Collision detected/
carrier transmit
packet to transmit?
jamming received?
sense
strategy
yes
Medium is occupied

delay k packets transmit yamming
compute random
abort transmission
transmission time Signal*
backoff integer k

* Only in case of collision detected

(18)


MAC of the ISDN S0-Bus

Transmission in frames on S0
Activation procedure in the physical layer
Frame synchronization in the MAC layer
Frame recognition by violation of code rules
Distributed MAC for the d-channel only
MAC for the user channel by framing

S0-bus
ISDN Sobus: NT
4 wire

(19)


S0-Frame (2)

Leitungscodierung S0

AMI-Code
01011100
+ +
- -

inv. AMI-Code
01011100
+ +
- -

(20)

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.


S0-Frame (3)
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
48 bits in 250 microseconds

NT TE
D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L.
0
1
0

2 bits offs et

TE NT
D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L.

t

A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der
B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei
How is a collision
B2 Bit des 2. B-Kanals M Multiframingbit = 0
D Bit des D-Kanals N = FA
detected?
E Bit des Echokanals S Spare = 0
F Rahmenbit
(21)

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 S0
Echokanal 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 NT
Coderegelverletzung 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]


S0-Frame (4)
Why is there no e-bit for the other direction in the Frame from TE NT?


NT TE
0
1
0

2 bits offs et

TE NT

t

F Rahmenbit
(22)


Literatur: [Kann]


S0-Frame - frame detection
Case 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-bit
will be „0“ and positive in in 250 microseconds in conflict with the coding rule.
48 bits order to be

NT TE
0
1
0

2 bits offs et

TE NT
D L. F L. B1B1B1B1 B1 B1 B1 B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2 B2 L. D L. B1 B1 B1 B1 B1B1B1 B1 L. D L. B2 B2 B2 B2 B2 B2B2 B2 L. D L. F L.

t

F Rahmenbit
(23)


Literatur: [Kann]


Case 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-bit
will be „0“ and positive in in 250 microseconds in conflict with the coding rule.
48 bits order to be

NT TE
0
1
0

2 bits offs et

TE NT

t

F Rahmenbit
(24)


Literatur: [Kann]


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(compensation
not necessary). The F-bit will be 48 bits and microsecondsbe in conflict with the coding rule.
„0“ in 250 pos. to

NT TE
0
1
0

2 bits offs et

TE NT

t

F Rahmenbit (25)


Literatur: [Kann]


The end

(26)


need for MAC?


limited to the
S0-bus
ISDN Sobus: signalling channel
NT
4 wire

Why is in the case
The user channel, the B-channel with 64Kbit/s, is needed of ISDN only the
permanently during a communication session, otherwise the access to the
Shannon sample rate of 125µs can not be guaranteed. signalling channel
Therefore the B-channel will be a dedicated channel which is in competition ?
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 or
sequences of packets.

(27)

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 networks
packet radio networks
Adhoc-networks
satellite networks


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

(28)


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)

Discuss
differences
with respect
Random to efficiency
Scheduler
Access
(29)


MAC principles


fixed PDH-, SDH demand without with sensing
fixed PDH-, SDH demand sensing(ALOHA) (CS)
assignment assignment sensing(ALOHA) (CS)
assignment assignment

central distributed
pure slotted
pure slotted
ISDN ISDN (CSMA) (CSMA/CD)
(CSMA) (CSMA/CD)
Ethernet-LAN
How does the channel selection and the channel assignment of a
PDH-, SDH-, ISDN- and Ethernet-LAN-system fit into the above
scheme?

(30)


TDMA (Time Division Multiple Access)

View on a shared media:

time 1
guard time

station 1 station 2 station m-2 station m-1 station m
time 2
View on a single terminal:
control information?
no packet
Frame length and Frame start (Frame delimiter)
ready?
guard time?
yes
Data packets from stations suffer from
wait for
assigned slot
different latency times which creates the
danger of overlapping (collision)
transmit
packet
Efficient for high traffic load, inappropriate for
low traffic load
(31)


S0-Frame (3)


NT TE
0
1
0

2 bits offs et

TE NT

t

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!!!
The “green” terminal which sent a “1” will get back a
“0” and will stop transmitting!
Are there other cases?
(32)


S0-Frame (3a)


NT TE
0
1
0

2 bits offs et

TE NT

t

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!

(33)


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!

NT TE
0
1
0

2 bits offs et

TE NT

t

F Rahmenbit (34)


Literatur: [Kann]


[11] Nu P 07 1

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[11] Nu P 07 1