Xem mẫu
- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
26
Asynchronous Transfer
M o d e ( ATM)
ATM looks set to become the first universal telecommunication technology, capable of switching
and transporting all types of telecommunication connection (e.g. voice,data video, multimedia).
It willformthebasisofthefuture broadbandintegratedservicesdigitalnetwork(B-ISDN).
Because of the anticipated importance of ATM, wediscussherethetechnicalprinciples and
terminology in depth, defining the main jargon and explaining what marks out ATM from its
predecessors. In particular, discuss the principlesof statistical multiplexing and the specifics of
we
cell switching.
26.1 A FLEXIBLE TRANSMISSIONMEDIUM
An ATM-equipped transmission line or telecommunication network is able to support
0 usage by multipleuserssimultaneously
0 differenttelecommunicationneeds(e.g.telephone, datatransmission,LANinter-
connection, videotransmission, etc.)
0 each application running at different transmission speeds (i.e. with differing band-
width needs)
However, these capabilities are also offeredby predecessor technologies, so why bother
with ATM, you might ask? What distinguishes ATM from its predecessors is that it
performsthesefunctionsmore efficiently. ATM is capable of aninstant-by-instant
adjustment in the allocation of the available network capacity between the various users
competing for its use. Rather than allocatingfixed capacity between the two communi-
cating parties for the durationof a call or session, ATM ensures that the line capacity is
optimally used on a moment-by-momentbasis, by carrying only the needed, or ‘useful’,
information.
451
- 452 ASYNCHRONOUS TRANSFER MODE
(ATM)
The dynamic allocation of bandwidth is achieved by ATM using a newly developed
technique called cell switching.
relay Understanding principles
its is
the key to
understanding ATM, its strengths and limitations. In our discussion, we shallrefer
frequently to the work of the ATM Forum, an industry-wide common interest group,
comprisingtelecommunications equipment manufacturers,
network operatorsand
users who have combined to speed the process of developing and agreeing technical
standards for ATM.
26.2 STATISTICALMULTIPLEXINGANDTHEEVOLUTION OF
CELLRELAY SWITCHING
ATM is based upon a statistical multiplexing technique called cellrelay switching.
Statistical multiplexing, we discussed in
as Chapters 9 and 18,is widely used to improve the
efficiency of data networks. As shall see, it can also used effectively to carryspeech
we be
connections, provided extra provisions are made to control the propagation delay.
The majorbenefit of statistical multiplexing that the useful carrying capacityof the
is
line is maximized by avoiding the unnecessary transmission of redundant information
(i.e. pauses in speech or idle periods on data lines). In addition, as the full capacityof
the line (i.e. its full speed) ismade available for each individual connection for carriage
of information, the transmission time (propagation time) may be reduced.
In the example of Figure 26.1, we recap the technique of statistical multiplexing.
Three separate users (represented by sources A, B and C ) are to communicate over the
same transmission line, sharing the line resources by means of statistical multiplexing.
The three separate source circuits are fed into a statistical multiplexor, which is con-
nected by a single line to the demultiplexor thereceiving end. (A similar
at arrangement,
using a second line for the receive channel, but with multiplexor on the right and
demultiplexor on the left, will also be necessary but this is not shown).
The statistical multiplexor sends whatever it receives from any of the source channels
directly onto the transmission line. Why is it called statistical multiplexing? Simply
because it relies on the statistical unlikelihood of all three channels wanting to send
information simultaneously. Actually, for a short period of time, the multiplexor is
designed to be able to cope simultaneous transmission from of the sources. This
with all
separate source transmission line arrival
re-sorted
circuits sianals
source A 'L
demultiplexor
source B
Figure 26.1 The principle of statistical multiplexing
- RELAYCELL PROBLEMS TO BE
THE BY SOLVED 453
is done by sending the most important signal directly to line and storing the lesser
important signal in a bufSer for an instant until the first signal has been transmitted. This
causes a slight variation in the time taken by signal packets to traverse the network.
a data
Variation in the time packets take to traverse networkis not important.So long
as theaverage delay is not great, computer users do notnotice whether some typed char-
acters appear imperceptibly faster or slower than others. As a result, packet-switched
networking techniques have dominated the world of data transmission. Voice networks
meanwhilehaveremainedcircuit switched networks, because they cannottolerate
variable delays, because they tend to ‘chop up’ the signal.
The strength of circuit switching is the guaranteed throughput and fast and constant
signal propagation time over the resulting connection. This critical so that acceptable
is
voice quality can be achieved (any variation in the signal propagation time manifests
itself to the listener as a rather broken up form the original signal,an effect known as
of
jitter). A telephone call connected in a circuit-switched manner is much like an empty
pipe between two telephone users. Whatever one speaker talks into the pipe comes out
at the other end in an identical format, but only one pair of callers can use the pipe
during any particular call.
And so it came to pass... that there were networks ford a t a . . . and separate networks
for telephones. The cell relay technique of statisticalmultiplexing used in ATM is
designed to contain the variation in propagation delay experienced by delay-sensitive
signals such as voice and video. It is heralded as the first technique capable of efficient
voice and data integration within a single network.
26.3 THEPROBLEMS TO BE SOLVED BYCELLRELAY
Returning to the statistical multiplexing example of Figure 26.1, a typical data packet
contains between 1 and 256 characters (i.e. between 8 and 2048 bits), and the linespeed
is typically 9600 bit/s. Therefore the delay at a time when twosourcestry to send
simultaneously and one of the packets hasbe temporarily stored in a buffer will be up
to
to 200ms (2048/9600 S) longer than when there is no simultaneous transmission. In
other words,theremay be up to 200ms of jitter. This is unacceptablefor speech
transmission, but this is not the only difficulty to be overcome. Another problem is in
minimizing the loss of line bandwidthtothemanagement overhead of statistical
multiplexing, as we discuss next.
To allow a statistical demultiplexor (Figure 26.1) to sort out the various packets
belonging to the different logical
connections, andforward them to correct
the
destinations (A to A , B to B, C to C, etc.) there needs to be a header attached to each
packetto say which logical connection (i.e.telephoneconversation ordata com-
munications session) itbelongs to. The header (Figure 26.2) is crucial, but hasthe
disadvantage that it adds to the information which must be carried between multi-
plexor and demultiplexor. At the demultiplexor, the header is removed so it does not
disturb the receiver, but meanwhile it has generated an overhead load for the trans-
mission line. It is thus impossible using statistical multiplexing techniques to achieve
100% loading with rawuser information. Some of the capacity needs to be given up to
carry the overhead.
- 454 ~ ASYNCHRONOUS TRANSFER MODE (ATM)
seDarate source transmissiorr
line re-sorted arrival
circuits sianals
source A .. . .
source B
source C
7
Figure 2 3 Statistical multiplexing headers and overhead
6
The major challengesfor ATM developers are to minimize thejitter experienced by
speech, video and other delay-sensitive applications, while simultaneously optimizing
line efficiency by minimizing networkoverhead. As we shall see, these demands contend
with one another.
26.4 THE TECHNIQUE OF
CELL RELAY
Cell relay is a form of statistical multiplexing that is similar in many ways to packet
switching, except that the packets are instead calledcells. Each of the' cells is of a fixed
rather than a variable size. The fixed cell size defined by ATM standards is 48 octets
(bytes) plus a 5 octet header (i.e. octets in all, see Figure 26.3). The transmission line
53
speeds currently foreseen used are either 2
to-be Mbit/s, 12Mbit/s, 25 Mbit/s, 34 Mbit/s,
45 Mbit/s, 52Mbit/s, 155Mbit/s or 622Mbit/s. We may thus conclude that
0 the overhead is at least 5 bytes in 53 bytes, i.e. >9%
0 the duration of a cell is at most (i.e. at 2 Mbit/s line rate) 53 X 8 bit42 Mbits-'
=0.2ms (12ps at 34Mbits-')
As the cell duration is relatively short, prhvided that a priority scheme is applied to
allow cells from delay-sensitive signal sources (e.g. speech, video, etc.) to have access
to the next cell slot, then the jitter (variation. in signal
propagation delay) can be kept
very low (i.e. of orderof 0.2ms); not zero.asis possible with circuit switching, at
the but
least low enough to give a subjectively acceptable quality for telephone listeners or
video watchers. Jitter-insensitive r ~ sources (e.g. datacommunication channels) can
t c
be made to wait for the allocation of the next low priority slot.
The jitter could be reduced still further by reducing the cell size, but this would
increase the proportion of the line capacity neededto carry the cell headers, and thus
reduce the line efficiency.
48 octet (byte) infomfion & or cell payload
I
d
5s
Figure 2 . ATM 53 byte cell format
63
- THE ATM 455
~. ......................
free slot 1 cell cell
Figure 26.4 The cells and slots of cell relay
26.5 THE ATM CELLHEADER
The cell header carries informationsufficient to allow the ATM network to determine to
which connection (and thus to which destination port and end user) eachshould be cell
delivered. We could draw a comparison with a postal service and imagine each of the
cells to be a letter of 48 alphanumeric characters contained in an envelope on which a
five-digit postcode appears. You simply drop your letters (cells) in in the right order and
they come out in the same order at the other end, though maybejittered in time.
slightly
Just like a postal service has numerous vans, lorries and personnel to carry different
lettersoverdifferentstretchesandsorting offices todirectthelettersalongtheir
individual paths, so an ATM network can comprise a mesh of transmission links and
switches to direct individual cells by inspecting the address contained in the header
(Figure 26.5).
The ATM ‘switch’ acts inmuch the sameway as a postal sorter. On its incomingside
is a FIFO (first in-first out) buffer, like a pile of letters. At the front of the buffer (like
the top letter in the pile) is the cell which has been waiting longest to be switched. New
cells arriving are added to the back of the buffer. The switchingprocessinvolves
looking at each in turn, and determining from the address in the header which
cell held
outgoing line should be taken. The is then added to the
cell FIFO outputbuffer which is
on
queueing cells waiting to be transmitted this line. The cell then proceeds to thenext
exchange.
You may think that afive-digit postcode is rather inadequate as a means address- of
ing all the likely users of an ATM network, and it might be, were it not for several
provisions of the ATM specifications. First, the ‘digits’ are whole octets (base 256)
rather than decimal digits (base 10). This means that the header has the range for over
10l2combinations (40 bits), though only a maximum 28 bits (2.7 X 108combinations)
of
are ever used for addressing. Second, the addresses (correctly called identiJers) are only
allocated to active connections.
101
- 456 (ATM) MODE TRANSFER
ASYNCHRONOUS
ATMconnectionsareallocatedan identiJier during call set-up,andthis is re-
allocated to another connection when the connection is cleared. In this way the number
of different identifiers need not directly reflect the number of users connected to the
network(whichmay be manymillions),onlythenumberofsimultaneouslyactive
connections. In addition, various subregionsof the network may use different identifier
schemes, thus multiplying the available capacity, but then demanding the ability of
network nodes to translate (i.e. amend) identifiers in the five-octet header.
By highly efficient usage of the information carried in the header, the length of the
header can be kept to a minimum. As a result, the network overhead is minimized.
26.6 THECOMPONENTS OF ANATMNETWORK
There are four basic types of equipmentwhich go to make up an ATM network.
These are
e customer equipment (CEQ),also called B-TE (broadband-ISDN terminal equipment)
e ATM switches
m ATM crossconnects
e ATM multiplexors
TheseelementscombinetogethertomakeanetworkasshowninFigure 26.6. A
number of standard interfaces are also defined by the ATM specifications as the basis
for the connections between the various components. The most important interfaces are
e the User-Network Interface (UNZ)
e the Network-Node Interface (NNZ)
m the Inter-Network Interface (ZNI)
These are also shown in Figure 26.6.
customer I
equipment 2 ; ATM
I cross-
customer j multiplexor ; connect
equipment 7
W Q ) ;
ATM j ATM
;
customer switch switch
equipment ;
W Q )
inter-network
UN/ NNI
user
network
interface
network interface
node
second ATM
network
Figure 26.6 The components of an ATM network (ITU-T network reference model for ATM)
- THE COMPONENTS OF AN ATM NETWORK 457
ATM customer equipment ( C E Q ) is any item of equipment capable of communicat-
ing across an ATM network. Oneof the most popular of today’s visions is the concept
of multimedia applications, devices capable of enabling their users simultaneously to
transmit synchronized video, electronic mail, data applications and telephone messages
over the same line at the same time.
TheATM user network interface (UNZ) is thestandard technical specification
allowing ATM customer equipment (CEQ) from various different manufacturers to
communicate over a network provided by yet another manufacturer. It is the interface
employedbetween ATMcustomerequipmentand either ATM multiplexor, ATM
crossconnect or ATM switch. It consists of a set of layered protocols aswe shall discuss
later.
Customer equipments communicate with one another across an ATM network by
meansofa virtualchannel ( V C ) . The VC mayeither be set-up and cleared down
on a call-by-call basis similar to a telephone network, in which case the connection is
a switched virtual circuit ( S V C ) or it may be a permanently dedicated connection (like a
leaseline or private wire), in which case it is a permanent virtual circuit ( P V C ) .
An ATM multiplexor allows different virtual channels from different ATM UN1 ports
to be bundled for carriage over the same physical transmission line. Thus two or three
customers outlying from the main exchange (Figure 26.6) could share a common line.
Returning to our analogy with the postal system, the ATM multiplexor performs a
similar function to a postal sack; it makes easier the task of carrying a number of
different messages to the sorting station (ATM switch)by bundling a number of virtual
channels into a single container, a virtual p a t h ( V P ) . More about virtual channels and
virtual paths later in the chapter (Figures 26.10-26.13).
An ATM crossconnect is a slightly more complicated device than the ATM multi-
plexor. It is analogous to a postaldepot, where the various vanloadsof mail are unloaded,
the various sacks sorted and adjusted into different van loads. At the postal depot, the
individualsacksremainunopened, and at the ATM crossconnect,the virtual path
contents, the individual virtual channels remain undisturbed. The ATM crossconnect
appears again in Figure 26.12.
A full ATM switch is the most complex and powerful of the elements making up an
ATM network. It is capable not only of cross connecting virtual paths, but also of
sortingand switchingtheircontents,the channels (Figure26.13). It is the
virtual
equivalent of a full postal sorting office, where sacks can eitherpass through unopened,
or can be emptied and each letter individually re-sorted. It is the only type of ATM
node device capable of interpreting and reacting upon user or network signalling for the
establishment of new connections or the clearing of existing connections.
The ATM network node interface (NNZ) is the interface used between nodes within
the network or between different sub-networks. A standardized NNI gives the scope to
build an ATM network from individual nodes or sub-networks supplied by different
manufacturers.
The inter-network interface (ZNZ) allows not only for intercommunication, but also
for clean operational and administrative boundaries between interconnected networks.
It is based on the NNI but includes more fetaures for ensuring security, control and
proper administration of inter-carrier connections (i.e. where networks of two different
operators are interconnected). ATM Forum calls this interfaceB-ZCZ (broadband inter-
carrier interface).
- 458 (ATM) MODE TRANSFER
ASYNCHRONOUS
26.7 THE ATM ADAPTATION LAYER (AAL)
An extra functionality added to basic ATM network (correctly
is a called theA TMLayer)
to accommodate the carriage of various different types of connection-oriented and
connectionless network services (Chapter Figure 26.6). This functionalityis contained
25,
in theA T M adaptation layer.The ATMadaptation layer ( A A L ) lays out a of ruleson
set
how the 48 byte cell payload can be used, and how it should be coded. These special
codings enable the end devices which are communicating across the A T M Layer to
communicate with one another using any of the possible connection-oriented or con-
nectionless service as desired.
The servicesoffered by the ATM adaptationlayer ( A A L ) are classified into four
classes or types (the standards use both terminologies). The distinguishing parameters
of the various classes are as illustrated in Table 26.1.
AnexampleofaClassAservice is circuitemulation (i.e. aconnection service
providing for ‘clear channel’ connections like hard-wired digital circuits). In the ATM
specifications such services are referred to as constant bit rate ( C B R ) or circuit emula-
tion services (CES). Thus a constant bit rate video or speech signal would be an AAL
Class A service and would use AAL1.
Variable bit rate ( V B R ) video and audio is an example of a class B service. Thus an
audio speech signal which sends information duringsilent periodsis an exampleof a
no
Class B VBR service and would use AAL2.
Class C and Class D cover the connection-oriented and connectionless data transfer
services. Thus anX.25 packet switching service would supported by a Class C service,
be
and connectionless data services like electronic mail and certain types of LAN router
service would be Class D. Both classes C and D use AAL types AAL3/4 or AAL5.
26.8 ATM VIRTUAL CHANNELSAND VIRTUAL PATHS
A virtual channelextended all the way across an ATMnetwork (ATM Layer) actually
is
a virtual channel connection ( V C C ) . Thisconnection is composed of anumber of
shorter length virtual channel links, which when laid end-to-end make up the VCC.
Table 26.1 Service classification of the ATM adaption layer (AAL)
Transmission
characteristic Class A Class B Class C Class D
AAL Type AAL Type 1 AAL Type 2 AAL Type 314 AAL Type 314
(AALl) (AALZ) (AAL3/4), (AAL3/4),
AAL Type 5 AAL Type 5
(AAL5) (AAL5)
Timing relation between Required Not required
source and destination I
Bit rate Constant Variable
Connection mode Connection-oriented I Connectionless
- ONTROL USER, AND MANAGEMENT PLANES 459
virtual channel connection (VCC)
virtual channel link
ATM multiplexor
or switch function
I R.
customer .': connection
path
virtual ...
equipment B
GEQ) crass- ATM ...
connectfunction '...
"\* *
physical transmission path
Figure 26.7 The relationship between virtual channels, virtual paths and physical transmission
paths
A virtual channel link is a part of the overallVCC, and shares the same endpoints as a
virtualpath connection ( V P C )(Figure 26.7). The ideaof a virtualpath ( V P )is valuable in
the overall design and operationATM networks.As we have already discovered the
of in
earlier part of a chapter, a virtual path has a function rather like a postal sack. In the
same way that a postal sack helpsto ease the handling letters which all share a similar
of
destination, so a virtual pathhelps to ease the workload of the ATM network nodes by
enabling them to handle bundled groups of virtual channels. Thus a virtual path ( V P )
carries a numberof different virtual channel links, which in theirown separate ways may
be concatenated with other virtual channel links to make VCCs.
Just like virtual channels, virtual paths can be classified into virtual path connections
(VPCs) and virtual path links, where a VPC is made up by the concatenation of one
ormorevirtualpathlinks. A virtual path link is deriveddirectlyfrom a physical
transmission path.
26.9 USER, CONTROL AND MANAGEMENT PLANES
Before two customer equipments ( C E Q ) may communicate with one another (i.e. trans-
fer information) across the plane (U-plane) of an ATM network, a connection must
user
first be established. The connection is established by means of a control or a manage-
ment communication between theCEQ and the network. This communication may take
one of five forms (Figure 26.8)
0 control plane communication (access)
0 control plane communication (network)
0 management plane communication type l
0 management plane communication type 2
0 management plane communication type 3
- 460 ASYNCHRONOUS TRANSFER MODE (ATM)
management plane
communication
.................. NMC (network management centre)
customer
equipment
management planet management planet
type-
:
2 communication type3
;
VP or VC VP or VC
crossconnect crossconnect
1
control plane
communication
(network)
.............................
control plane communication (access)
.....................................................................
information transfer across user plane
the
...............................................................................
Figure 26.8 User, control and management planes of an ATM network (ITU-T recommenda-
tion 1.3 1 1)
A control plane communication (access) is a one conducted between CEQ (customer
equipment) andanATM switch. During suchacommunication,which uses UNI
signalling, a connection is established or released (in the case of SVCs, switched virtual
circuits) much like dialling a telephone number in a telephone network. Control plane
communications (network) follow, as theATM switch communicates (using network
will
signalling) withother nodes in the network to establish the complete network connection.
Once the connection is established, the user transfers information (i.e. communicates)
across the user plane.
The connection could alsohave been established manually by the service technicians
at the network management centre (a PVC, permanent virtual circuit). In this case, the
user uses a management plane communication type I from his CEQ to the NMC to
request the establishment of a permanent connection. This could be carried by UN1
signalling or could simply be a telephone call. The various switches and other network
elements are then configured from the NMC by means of messages sent by management
plane communication type 2.
Management plane communication type3 is initiated by ATM switches which require
to refer to the NMC for information, authority or other assistance in the process of
connection set-up. (It may be, for example, that certain high bandwidth connections
require authorization from the NMC to prevent network congestion at peak times).
26.10 HOW IS A VIRTUAL CHANNEL CONNECTION
(VCC) SET UP?
A UN1 signallingvirtualchannel (SVC, but not to be confused with SVC, switched
virtual connection) is a virtual channel or virtual path connection at a UN1 dedicated
specifically to UNI signalling. Signallingvirtualchannelsmayalsoexist at an N N I
interface.
- SIGNALLING CHANNELS
VIRTUAL AND META-SIGNALLING CHANNELS
VIRTUAL 461
A signallingmessagesentoverthe SVC (signalling V C ) might be‘set up virtual
connection number one between user A anduser B’. Another message might be ‘clear the
connection between A and ATM uses a dedicated channel for signalling information
B’.
(common channel signalling as we discussed in Chapter 7). Drawing a parallel between
ATM signalling and narrowband ISDN signalling,the UN1 signallinginterface is
equivalent to the narrowband ISDN signallingdefined by ITU-Trecommendation
Q.931 (and indeed is based on it and specified in Q.2931), and ATM NNI signalling is
based on signalling system 7 ( S S 7 ) as used in narrowband ISDN.
At the time when a CEQ requests to set up a new SVC (switched virtual circuit)
connection across an ATM network, it must first negotiate with the network over the
UNI signalling VC, declaring the required peak cell rate, quality of service ( Q O S ) class
and other parameters needed. The connection admission control ( C A C ) function at the
ATM switch then decides whether sufficient resources are available to allow immediate
connection. If so, the connection is set up. If not, the connection request is rejected to
protect the quality of existing connections. (The analogyis the telephone user’s receipt
of busy tone when no more lines are available). During the negotiation, virtual paths
and connectionsbetween the various nodes and other equipments are allocated, and the
referencenumbers of theseconnections,thecombination of virtual path identifiers
( VPIs) andvirtual channel identifiers ( VCIs),are confirmed over the signalling channel.
These values (VPI and VCI) then appear in the header of any cells sent, to identify all
those cells which relate to this Connection.
26.11 SIGNALLING VIRTUAL CHANNELSAND META-SIGNALLING
VIRTUAL CHANNELS
Both management and control communication in an ATM network take place via
signalling virtual channels (SVCs). At NNI interfaces, SVCs are usually permanently
configured between the various servers (i.e. control processors) controlling a particular
B-ISDN service (e.g. video on demand, picture telephone service, etc.). However, unlike
narrowband ISDN, signallingvirtualchannels ( S V C ) are not normally permanently
available at UNI.Instead, they are established on demand means of ameta-signalling
by
virtual channel ( M S V C ) . This is a permanently allocated UN1 signalling channel of a
fixed bandwidth. It is found in the virtual path VPI 0 and has aVC1 value standard to
=
the particular network.
By meansthe of meta-signalling
virtual
channel, the
end device (CEQ) can
establish an SVC (signalling VC) to the ATM switch (c-plane) or to the network man-
agementcentre(m-plane) for signallingcommunication.A serviceprojileidentifier
( S P I D ) carried in the meta-signalling determines which service the user requires, and
enablesasignallingvirtualchannel to the appropriate signalling point server to be
established.
The functionality or device which existsat the end of a signalling virtual channeland
conducts the act of signalling is called a signalling point ( S P ) . Such functionality exists
in customer equipment (CEQ) and in ATM switches. A signalling transfer point ( S W )
is a switching point for the information carried in signalling virtual channels. Using a
single signalling VC via an STP, an SPmaycommunicatesignalling messages to
- 462 (ATM) MODE TRANSFER
ASYNCHRONOUS
STP
SP SP
UN1
I
A C NNI D B
CEQ CEQ
(customer
equipment)
Figure 26.9 Signalling points (SPs) and signalling transfer points (STPs)
numerous other SPs using eitherassociated-mode or quasi-associated mode signalling, as
we discussed in Chapter 12. STPs improve the efficiency and reliability of the signal-
ling network (Figure 26.9).
26.12 VIRTUAL CHANNEL IDENTIFIERS (VCIs) AND
VIRTUAL PATH IDENTIFIERS (VPIs)
As we saw in Figure 26.7, virtual channel connections comprise concatenated virtual
path connections. Each is identijied by reference numbers carried by the cell headers of
active connections called virtual channel identlJiers ( VCZs) and virtual path identifiers
( VPZS).
Figure 26.10 illustrates how a physical connection may be subdivided into a number
of different virtual paths, each with a unique VPI. Each VPI inturn may be subdivided
into several virtual channels, each with a separate VCI. The combination of VPI and
VC1 values is unique to each UN1 or NNI and is sufficient to identify any active connec-
tion at the interface (i.e. on the same physical connection).
An ATM multiplexor, as we discussed earlier, allows a number of virtual channels
from separate virtual paths to be combined over a single virtual path. Thus the virtual
channels carried by physical links 1,2 and3 of Figure 26.11 are combined together into
a single virtual path carried by physical link 4. In this way three separate end user
devices use separate virtual channels to share a single physical connection line from
ATM multiplexor to the ATM network.
An ATM crossconnect allows rearrangement of virtual paths without disturbance of
the virtual channels which they contain. Thus in Figure 26.12, the contents of incoming
VCI,
physical VClb
connection
4
Figure 26.10 Virtual path and virtual channel identifiers
- VIRTUAL
CHANNEL IDENTIFIERS (VCIS) AND VIRTUAL
PATH IDENTIFIERS (VPIS) 463
physical link 1, VPI=l. VCI=l
0
physical link 2,VPI=2, VCI=l
C 3
physical link 4,
VPI=4. VCls 1 , 2 8 3
physical link 3, VPI=3, VCI=l
C ,
Figure 26.11ATM multiplexor
VPI=l, VCls 1 & 2 VPI=4, VCIS 3 & 4
7 r
VPI=2, VCls 3 & 4 VPI=5, VCls 5 & 6
3
JC >L
VPI=3,
VCls 5&6 1 VCls
VPI=6, &2
7 2
J '
L
Figure 26.12ATM crossconnect
virtual path VPI = 1 are crossconnected to outgoing virtual path VPI = 6, the VCIs
remaining unchanged. An ATM crossconnect thus a simple form of ATM switch, but
is
one which need only process (and translate (i.e. amend)) VPI values.
A full ATM switch (Figure 26.13) must have the capability not only to crossconnect
virtual paths, but also to switch virtual channels between different virtual paths. This
requires the additional ability to process and translate VCIs held in cell headers. It is
thus a relatively complex device. Good performance depends onvery fast processing of
both VPIs and VCIs in the cell headers. A full ATM switch is consequently a costly
device.
V 1 24
C
V1 2
C 1
V1 2
C 2
VC1 23
I
1 V1
C 21
I
I VC1 22
T VP crossconnect I
Figure 26.13 Full ATM switch
- 464 ASYNCHRONOUS TRANSFER MODE IATM)
26.13 INFORMATIONCONTENTAND FORMAT OF
THE ATMCELL HEADER
The main function of the ATM cell header is to carry the VPI and VC1 information
whichallowstheactivenetworkelements to switch the cells of activeconnections
through the network. The exact format of the cell header is as shown in Figure 26.14.
The cell header comprises 40 bits, of which 24 (UNI) or 28 (NNI) are used for the
virtual path and virtual channel identifiers. Together, the VPIjVCI fields are called the
routingfield. There are four otherfieldswhich occupy the remainder of the header. The
PT (payload type)field is occupied by the payload type identijier (PTZ). This identifies
the contentsof the cell (the informationfield or payload) as either a user data cell, a cell
containing network management information, or a resource management cell. The cell
loss priority ( C L P )bit (when set to value 1 is used to identify less important cells which
may be discarded first at a time of network or link congestion. The genericPOW control
(GFC) field is used to control the cell transmission between the customer equipment
(CEQ) and the network (Figure 26.15). Finally, the header error control ( H E C ) field
serves to detect errors in the cell header caused during cell transmission.
When there is no trunk congestion (i.e. there is no appreciable accumulation of cells
waiting in the multiplexor buffer to be transmitted over the trunk) then the GFC field is
set to the uncontrolled transmission mode. However, if there is a sudden surge of cells
from all of the CEQ devices and the multiplexor experiences congestion (i.e. the filling
of its cell buffers to a critical threshold level) then the GFC field is used to subject the
8 7 6 5 4 3 2 1 octet
GFC (at UNI). VPI (at NNI) VPI
VPI VC1
VC1
VC1 PT I CLP 4
HEC 5
GFC = Generic Flow Control PT = PayloadType
VPI = Virtual
Path
Identifier CLP = Cell Loss Priority
VC1 = Virtual
Channel
Identifier HEC = Header Error
Control
Note: the GFC is used only at the
UNI, at the NNI bits 5-8of octet 1 are used as VPI
:3%,
Figure 26.14 Structure of the ATM cell header
device A
device B multiplexor
deviceC I CEQ
Figure 26.15 Generic flow control regulates trunk congestion at a multiplexor
- ATM 465
Table 26.2 The functional layers ofATM
Layer name Sublayer name Further sublayer
Higher layers
ATM adaptation layer (AAL) Convergence
sublayer
(CS) Service
specific (SS)
Common part (CP)
Segmentation and reassembly
(SAR) sublayer
ATM layer VC level
VP level
Physical layer Transmisssion convergence
(TC) sublayer
Physical medium (PM)
0 Service access point (SAP) ~ an imaginary point between functional layers.
cell flow from the various CEQ devices to controlled transmission. This limits the rate
at which the CEQ devices may continue to send cells of one or more different types to
the network.
An ATMcell is transmitted in the orderof octets (i.e. octet 1 first, followed by octet 2,
then octet 3, etc.). The most signiJicant bit ( M S B ) of each octet (i.e. bit 8) is transmitted
first. Thus first the header and then the payload is transmitted, MSB first.
26.14 ATM PROTOCOL LAYERS
Table 26.2 illustrates the protocol layers of ATM. We use this in the discussion which
follows to define the terminology of ATM and to explaintherelationships of the
various layers to one another.
26.15 THE ATM TRANSPORT NETWORK
The foundation of the various protocol layers (the protocol stack, the set of functions
which together make information transfer possible) is the physical medium used for the
carriage of electrical or optical signals. The physicallayer is a specification which
defines what electrical or optical signals and voltages, etc., should be used. In addition,
it sets out a procedure for transferring data information across the line, providing for
clocking of the bits sent and the monitoring of the equipment. The physical layer of
ATM is similar function
in tothe physical layer (layer 1) of the open systems
inferconnection ( O S I ) protocol stack (Chapter 9).
The physical layer is divided into two sublayers. These are the physical medium sub-
layer and the transmission convergence ( K ) sublayer. The physical medium sublayer
defines the exact electrical and optical interface, the line code and the bit timing. The
TC sublayer provides for framing of cells, for cell delineation, for cell rate adaption to
- 466 ASYNCHRONOUS TRANSFER
MODE (ATM)
the information carriage capacity of the line, and for operational monitoring of the
various line components (regenerator section ( R S ) , digital section ( D S ) or transmission
p a t h ( T P ) ) . Preferred physical media defined for use with ATM include optical fibre
and coaxial cable. There is also some scopeto use twisted pair cable.
It is the ATM layer which controls the transport of cells across an ATM network,
setting up virtual channel connections and controlling the submission rate (generic flow
control) of cells from user equipment. The service provided to the ATM layer by the
physical layer is the physical transport of a valid flow of cells. This is 'delivered' at a
conceptual 'point' called the physical layerservice access point ( P L - S A P ) . The flow of
cells is correctly called a service data unit ( S D U ) , in fact the PL-SDU (physical layer
service data unit).
Figure 26.16 Maintenance test tool for ATM (Courtesy of Siemens A G ) . The operation and maintenance
(OAM) cells of ATM provide for advanced measurement of network performance without affecting live
connections.
- CAPABILITY OF THE ATM ADAPTION LAYER
(AAL) 467
The ATM layer controls the service provided to it by the physical layer by means of
service primitive commands. These are standardized requests and commands exchanged
between the control function within the ATM layer (in thejargon called the A T M layer
entity ( A T M - L E ) ) and the physical layer entity ( P L - L E ) . They allow, for example, a
particular ATM-LE torequest transfer of a flow of cells (service data unit). Conversely,
the physical layer may wish temporarily to halt the transfer of cells to it by the ATM
layer because of a problem with the physical medium.
The transmissionconvergencesublayer receives informationintheform of cells
provided to it by the ATM layer. This is the PL-SDU, or more specifically, the TC-
SDU. These cells are supplemented by further information, including PL-cells (physical
layer cells) and OAM cells (operations and maintenance cells). The extra information,
an example of protocol control information (PCZ) turns the TC-SDU into a TC-PDU
(protocol data unit). It ensuresthecorrecttransmission of informationacrossthe
physical medium. The TC-PDU is passed to the physical medium sublayer, where it is
called the PM-SDU (physical medium service data unit).
Finally, the PM-SDU is converted to thePM-PDU by addition of furtherPCZ and is
passed tothe mediumitself.Theform of thePM-PDU(andthusthe conversion
performed by the physical medium sublayer) is dependent upon the type of medium
used (e.g. electrical, optical, etc.). To accommodate a change of the physical medium,
onlythe physicalmedium sublayer need be swapped.Otherhardware and software
components (e.g. corresponding to the ATM layer) can be re-used.
Together the ATM layer, the physical layer and the physical medium are called itself
an A T M transport network. An A T M transport network is capable of conveying
information in the form of cells between network end-points. However, so that the
information content carried by an ATM transport network can be correctly interpreted
by the receiver, there are further higher layer protocols defined. The most important of
these is the A T M adaptation layer ( A A L ) .
26.16 CAPABILITY OF THE ATM ADAPTATION LAYER(AAL)
As the name suggests, the A T M adaptation layer ( A A L ) provides for the conversion of
the higher layer information into a format suitable for transport by an ATM transport
network. The higherlayers are information, devices or functions of unspecific type
whichrequiretocommunicateacrosstheATMnetwork. Higherlayerinformation
carried by the ATM network may be either
0 user information (user plane)of one of a number of different forms (e.g. voice, data,
video, etc.) as categorized by the AAL service classes (Table 26.1)
0 control information (control plane) for setting up or clearing connections
0 network management information (management plane) for monitoring and config-
uring network elementsor for sending requests between network management staff.
Like the other layers,the AAL accepts AAL-SDUs from the higher layers and passes an
AAL-PDU to the layer below it (the ATM layer), where it is known as an ATM-SDU.
However, unlike the ATM and physical layers a number of different alternative
services
- 468 (ATM) MODE TRANSFER
ASYNCHRONOUS
can be madeavailabletothe higherlayers abovetheAAL,thusallowing differ-
ent types of information to be adapted for carriage across a common ATM transport
network. It is the ATM adaptationlayer which gives ATM networks their capability to
transfer all sorts ofdifferentinformationtypes.It is splitintotwosublayers:the
convergence sublayer, CS (where the alignment of the various information types into a
common format takes place and division into cells occurs); and the segmentation and
reassemblysublayer, S A R (where the cells are
numbered sequentiallyto allow
reconstruction in the right order at the receiving end).
26.17 PROTOCOL STACK WHEN COMMUNICATING VIA AN
ATM TRANSPORT NETWORK
Figure 26.17 illustrates the peer-to-peer communications which take place when two
user end devices communicate with one another by means of an ATM transport switch.
The ATM switch supports only the lowest three protocol layers, and ‘speaks’ peer-to-
peer with each of the ends, translating protocol information such as VPIs andVCIs as
necessary andrelaying user information. Meanwhile, at the ATM adaptationlayer
( A A L ) and the higher layers, the two end devices communicate peer-to-peer directly
over the connection established by the lower three layers. This information remains
uninterpreted and passes ‘transparently’ through the network.
user
actual communications path
_ _ _ _ _ _W imaginary peer-to-peercommunication
Figure 26.17 Protocol layer representation of two end devices communicating via ATM layer
switch
- OTOCOL ATM 469
If we were to monitor the wire between either of the end devices and the ATMswitch
of Figure 26.17 then we would observe communication at each of the layers. What we
actually observe are cells, but structured a little like a Russian doll. The smallest doll
(right inside) is the information that we want to carry between the users (the higher
layer information). All the other dolls are the protocol information (PCI), one doll for
each of the lower layers, each providing a function critical to the reliable carriage and
correct interpretation of the message.
26.18 ATM PROTOCOL REFERENCE MODEL(PRM)
Strictly, Figure 26.17 illustrates only the protocol stack for the user plane (i.e. for the
transfer of information between end devices once the connection has been established.
The AAL protocolsused on the control and management planes will usually differ from
the AAL protocols used on the user plane of the same connection, though identical
protocols will be used at the ATM transport layers. This is illustrated schematically in
the ATM protocol reference model ( P R M ) as shown in Figure 26.18.
In the case of the control and management planes, the network itself must interpret
the higher layer information and react to it. The control plane AALs (for the user-
network signalling in setting up a switched virtual circuit, SVC) will typically need to
be suited for data information transfer. Similar protocols will also be necessary for
the management plane. In contrast, in the case of communication across the user plane,
the higher layer information may take any number of different forms (speech, data,
etc.), but the individual network elements themselves (e.g. switches) may be incapable
of recognizing and interpreting these various forms.
In real switches and ATM end user devices,common or duplicate hardware and soft-
ware may thus be used for management, control and user planes at ATM and physical
/l
/
/. ................ management plane....................
2
Figure 26.18 The B-ISDN protocol reference model (Courtesy o L W )
f
- 470 ASYNCHRONOUS TRANSFER MODE (ATM)
layers, but distinct hardware and software will be necessary for signalling and user
information transfer at the AAL and higher layers of these planes.
The different types of user, control and management services are carried by the AAL
by means of service-spec@ convergence services. Examples of specific user plane services
offered by the ATM adaptation layer ( A A L ) are
0 frame relay SSCS (service speciJic convergence sublayer) service
SMDS (switched multimegabit data service) SSCS service
0 reliable data delivery SSCS service (a packet-network like data network service)
0 LAN emulation SSCS service
0 desktopquality video SSCS service
0 entertainmentquality video SSCS service
0 further services still in the stage of development by ATM forum
UN1 NNI
*----, communication or signalling
DSS2 digital subscriber signalling system2
B-ISUP broadband integrated services user part
MTP3 message transfer protocol layer3
SSCS service specific convergence sublayer
SSCF service specific coordination function
SSCOP service specific connection-oriented protocol
CP common part (convergence sublayer)
SAR segmentation and reassembly sublayer
AAL5 AAL service type 5
UN1 user--network interface
NNI network-node interface
Figure 26.19 UN1 and NNI protocols used on the control plane
nguon tai.lieu . vn
