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PART
4
MULTIMEDIA
NETWORKS
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)
Broadband, Multimedia Networks
and the
B-ZSDN
The emergence
of
‘multimedia’ computers and software which use all sorts
of
different audio,
data, image and video signals simultaneously has heralded
a
new generation
of
computers and
computer ‘applications’ and spurred the need to develop and deploy a new universal technology
for telecommunications. The broadband-ISDN
(B-ISDN)
will be this new universal technology.
It will be
a
powerful network, capable of carrying many different types of services and
communication bitrates.
25.1
MULTIMEDIA APPLICATIONS: THE DRIVER
FOR BROADBAND NETWORKS
A
multimedia
application is one in which different types of communication signal are
transmitted simultaneously. Thus, for example,
multimedia
computers are able simul-
taneously to present spreadsheets, work processing and presentation foil ‘windows’
on
the screen while simultaneously also showing a video, projecting a ‘video telephone call’
and playing a soundtrack (Figure
25.1).
Multimedia applications greatly increase the
possibilities
of
the modern computing and entertainment industries, and they are
finding their way into
all
walks
of
modern life:
e
Teleworking
0
Telemedicine
e
Tele-education
0
Teleshopping
439
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)
440
BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN
Figure
25.1
A
modern multimedia computer workstation
(Courtesy
of
Siemens
AG)
The simplest type of
teleworking
is that allowing a company executive to log his laptop
PC
into the company computer network and electronic mail facility no matter what his
location. Thus working at home and during time away on a business trip becomes
possible. Other forms of
teleworking
include applications designed to bring together
groups of remotely located individuals inio a
workgroup (workgroup applications).
A
telemedicine
application might allow a group of specialist surgeons (in different
locations) to carry out an emergency operation together, whde sharing sight of a video
signal broadcast from the operating theatre, as well as full reference to a patient’s
medical records and historic X-rays.
A
distant surgeon could indicate directly on the
video exactly where an incision should be made,
or
maybe even conduct some of the
surgery himself.
Tele-education
offers the prospect of sharing the best possible educational facilities,
information resources and equipment between a large number of dispersed locations.
Thus books, video lectures and demonstrations can be available to students, even in
their own homes.
Multimedia applications are also flourishing in the home environment. Here, cable
television companies already widely offer ‘cable’ connections bringing simultaneous live
television, public telephone service and
video-on-demand
services, as well as
teleshop-
ping
(catalogue shopping from the television or computer screen).
VIDEO
COMMUNICATION
441
25.2 VIDEO COMMUNICATION
The advance in video coding schemes and videocommunications has been one of the
main contributing factors in the rapid emergence of multimedia applications.
A
number
of standards have laid the basis for standardized coding of video information, provid-
ing for different qualities and bitrates. At the low bitrate end of the scale, ITU-T recom-
mendation H.261 provides a standard algorithm for a videoconference
codec (coder/
decoder),
suitable for coding relatively static images for transmission across low bitrate
digital lines. Transmission at rates even as low as 64 kbit/s are possible, giving accept-
able quality for a ‘picture telephone’ conversation or a
videoconference
between seated
(and therefore relatively slow moving) participants in two or more different meeting
room locations locations.
The H.261 algorithm works in a similar manner to ADPCM (Chapter 38). In effect
the whole area of a video screen is assumed to be made up of a matrix of dots, each of a
given colour (typically one of 256 shades). Thus each dot
(orpixel, picture element)
may
be represented by an 8-bit digital value. The entire picture can be represented by
sending all the digital values corresponding to the complete picture dot matrix (or bit
map like the fax image discussed in Chapter 4). As a large number of dots make up the
picture, it takes a little while at the start to establish the initial video
frame.
However,
once the first frame has been established, it is sufficient to send the information
corresponding to the difference between ‘freeze frames’ to recreate all subsequent
frames. Provided that the movement of the image is relatively restricted, sending only
the differences in the picture enables a much lower transmission bitrate to be used.
At 384 kbit/s, fairly high quality video images can be reproduced using the H.261
algorithm, and even public television signals are easily recognizable. The quality, how-
ever, does not match that of the original signal. For this reason, a number of other
coding techniques are also used, where very high quality video images are to be stored
and transmitted in conjunction with modern computer storage devices and telecom-
munications networks. These are the standards of the
Motion Pictures Experts Group
(MPEG),
an industry common-interest group of major companies, cooperating to
promote standards in this area.
25.3 THE EMERGENCE
OF
THE B-ISDN
The demands of new graphical computer software, the emergence of videotelephony
and cable television, and the political pressure for the development of the
Information
Highway
have all combined in their various ways to stimulate the development of
switched
broadband
networks. The result has been the development of
broadband ISDN
(broadband integrated services digital network,
or
B-ISDN).
We saw in Chapter 10 how the
integrated services digital network (ISDN)
was one
step in the development chain of a universal
integrated network,
and how it may replace
the conventional telephone and data networks in the next few years. However,
narrow-
band ISDN (N-ISDN)
in its current form falls a long way short of the ultimate needs of a
multi-service network, because of the relatively restricted bit rate available
on
individual
channels and because of the lack
of
service flexibility arising from the fact that it evolved
442
BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN
Television
@
e
%
?k!i\al
audio
Multiservice
Figure
25.2
The concept
of
the
Broadband Integrated Services Digital Network (B-ISDN)
out
of
telephone network principles. The maximum rate currently possible on ISDN is
n
X
64
kbit/s, up to
2
Mbit/s. Such rates are not enough for very high speed file transfer
between mainframe computers, or for interactive switching of broad bandwidth signals
such as high definition television (HDTV) and video.
Increasing the unit bit rate
(64
kbit/s) of ISDN would increase bit rates and make
broadband
and
multimedia
services possible, but it would be at the expense of gross
inefficiency in network resources, particularly when carrying bursty data signals.
So
B-ISDN is not merely an extension of N-ISDN, but instead also capitalizes on the latest
data switching techniques.
Current world technical standards for B-ISDN describe the general functions which
are
to
be performed by a B-ISDN, and define the network interfaces to be used, but the
technological details are not yet fully defined.
25.4
THE SERVICES TO
BE
OFFERED
BY
B-ISDN
The services offered by B-ISDN networks split into two categories:
interactive services
and
distribution services
(Figure
25.3
shows).
Interactive services
are normal communications between just two parties. There are
three subtypes: messaging, conversational and retrieval services.
An
example of a
conversational service
is a telephone conversation or a point-to-point
data connection, where the two end-points of the communication are in
real-time
connected with one another and thus
converse.
A
message service
is a telecommunication service similar to the postal service.
A
message is submitted to the network (as a letter is posted). Sometime later, the message
is delivered to the given address. Message services are usually not
guaranteed.
The
network is not able to check whether the recipient address is valid, and may return no
confirmation to the sender of receipt. Thus ‘no reply’ may result either because the
recipient never got the message or because he chose not to respond.
A
retrieval
service is one in which a caller accesses a central server, database or
storage archive, requesting the delivery of certain specified information. He might, for
example, receive data about holiday or request news video clips, etc.
THE EMERGENCE
OF
THE ATM SWITCHING TECHNIQUE AS THE HEART
OF
ATM 443
B-ISDN
services
1
1
I
interactive services
I
I
distribution services
I
III
I
[
service;
I
1
conversational
I
services
I
I
services
I
messaging retrieval
individual
individual
presentation
presentation
control control
Figure
25.3
The service types offered by B-ISDN
Distribution services
are services in which the information from a single source is
distributed to many recipients at the same time. Distribution services are subdivided
into those with or without individual
user presentation control.
An example
of
a
distribution service without individual
user presentation control
is the broadcasting of
national television. All the recipients receive the same signal at the same time. An
example of a distribution service with individual user presentation control is
video-on-
demand.
Here only those users who wish to pay and receive a given film do
so.
25.5
THE EMERGENCE
OF
THE ATM SWITCHING TECHNIQUE
AS THE HEART
OF
ATM
Much of the initial development work which led to B-ISDN started in the data
networking field, as engineers tried to develop
fast packet switching
techniques suitable
for the carriage of emerging video and ‘image’
applications.
The switching technique
Table
25.1
Potential
applications
of
the
various B-ISDN service categories
Category Service type Potential applications
Interactive
services Conversational
services
Voice
telephony
Messaging
services
Internet electronic
mail
Retrieval services On-line database service
Distribution services Without user control Broadcast
TV
With user control Pay-on-view
TV
or video-on-demand
444
BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN
developed as the basis for B-ISDN is an adaption of the packet switching technique
used in datacommunications networks. It is called
ATM
(asynchronous transfer mode).
A common failing
of
telecommunication switching techniques previous to ATM was
their restriction to low bit rate data conveyance or fixed bandwidth circuit switching.
Thus the major difficulty which had to be overcome in developing ATM was the need to
optimize B-ISDN to carry simultaneously both low and very high bit rate services, and
to meet simultaneously the exacting demands and rapid signal conveyance necessary for
speech and equivalent signals.
In classical data packet switching networks the emphasis
is
placed on assured and
error-free delivery of data. This rules out such networks for the carriage of live speech
and video signals due to the prolonged and uneven delays experienced during propaga-
tion and delivery of the user’s information.
Figure
25.4
illustrates the degradation of a packetized speech channel, sharing a
packet network with other speech and data users. The network is subject to congestion,
so
that occasional packets are held up in the buffer. The result is a rather disjointed
signal, with ‘staccato’, ‘snowy’ and ‘clipping’ effects.
Another limitation of pre-ATM data networks, when used to carry the bitrates
typically used for voice and video signals, was their relative inefficiency. When sending
single packets of a small number of bits (eight for a single speech
sample),
the network is
kept heavily loaded just carrying and processing packet headers! Furthermore the error
detection and correction procedures which ensure that each packet value is received
correctly are largely irrelevant for speech and video signals, because the ear or eye tends
to ‘fill-in’ any slight inconsistencies in the signal anyway.
Despite these problems, statistical multiplexing (as used in data networks) is con-
sidered to be the ideal basis for broadband networks, as large numbers
of
connections
can be accommodated of different and varying bandwidths. A large number of packets
per second give a high bandwidth for one user, whereas a smaller number of packets per
second yield lower bit rates for others.
Delay
buffer
Undelayed
(normally spaced)
[normally spaced)
Figure
25.4
Degradation
of
speech on normal
packet
networks
THE EMERGENCE
OF
THE ATM SWITCHING TECHNIQUE AS THE HEART
OF
ATM
445
In 1988, a statistical multiplexing technique called ATM was provisionally accepted
by CCITT as the switching basis for future B-ISDNs, and a number of detailed
CCITT (now ITU-T)
recommendations
were issued to define the basic technique.
These provide for
0
simultaneous carriage of mixed telecommunication service
0
a connection establishment
contract,
in which the end-user device may specify
during connection set-up the required connection bitrate and connection quality
parameters, including maximum permissible delay and delay variation; this is then
to be
guaranteed
by the network for the duration of the call
0
network overload control, preventing new connections being established, and
allocating available capacity to higher priority or delay sensitive (e.g. voice and
video) connections first
0
a limit to the delay experienced by video, voice and other delay-sensitive signals
0
relatively high efficiency in the use of bandwidth for all types of applications,
achieved by removing error correction methods in cases (e.g. voice and video
applications) where these are superfluous
The ‘packets’ conveyed by ATM are called
cells.
Like the
packets
of
X.25,
each
cell
of
ATM consists of an information field (in which the user data is held and conveyed)
and a
header
which ‘tells’ the network where to deliver the packet, and provides a
sequence number
so
that cells can be re-assembled in the correct order at the receiving
end). But, unlike the packets of
X.25,
the cells in ATM are all
of
a fixed length
(48 byte
payload
plus a
5
byte header). The fixed length gives the scope for overcoming
the limitations of earlier packet networks.
The relatively short length of the cell (as compared with a normal data
packet
or
frame)
means that user data messages must be broken up into a large number of indi-
vidual cells for transmission. This is inefficient, but on the positive side the smaller cells
allow a transmission priority scheme to be established. High priority cells (e.g. deriving
from a voice or video connection) can now be inserted between the cells which make up
a single data frame, rather than having to wait until the complete frame has been
transmitted. They thus experience significantly lower delay than they would have done
in the case of a classical packet-switched data network.
Unfortunately, from a voice and video perspective the 48 byte information content of
an ATM cell is very large compared to the single byte samples normally transmitted.
The inefficiency associated with processing cell headers militates against sending very
small (e.g. one byte) cells, corresponding to individual speech samples. Instead we
must make do with conveying a number of consecutive speech samples in a single cell.
A packet size of 48 bytes allows 48 separate samples to be sent simultaneously. This
corresponds to
6
milliseconds worth of conversation (at 64 kbit/s). Thus a
6
ms propaga-
tion delay is inflicted on the signal as we wait for the cell to
fill
up before we can send it.
Nonetheless, this is a small (and maybe unnoticed) price to pay for high network
efficiency and the scope of B-ISDN.
446
BROADBAND, MULTIMEDIA NETWORKS AND THE B-ISDN
CEQ
used and paid
for
on-demand
CEQ
=
Customer Equipment
U
Figure
25.5
The basic switching capabilities
of
B-ISDN
25.6 CONNECTION TYPES SUPPORTED BY B-ISDN
Over and above the switching of straightforward point-to-point connections, B-ISDN
(through ATM) distinguishes itself from its predecessing networks not only in its ability
to switch high bitrate connections, but also in its ability to set up broadcast (i.e.
mufti-
cast)
distributions of high bandwidth signals (e.g. video or television programmes). The
broadcast capability, meanwhile, will enable cable TV companies to develop eco-
nomically efficient metropolitan fibre cable networks for the distribution of pay-TV or
video-on-demand
(Figure
25.5).
Figure
25.6
illustrates the main types of connections and services which B-ISDN and
ATM networks will offer. The diagram also shows how ATM relates to B-ISDN.
25.7 USER DEVICE CONNECTION TO B-ISDN
The
user-network interface (UNI)
illustrated in Figure
25.6
is the standardized interface
for connecting users end devices to a B-ISDN network for the purpose of communication.
Various
reference points
(defined by ITU-T recommendation
1.41
3)
are slightly different
variants of the UN1 interface, according to how the device is connected to the network
These are similar to the narrowband ISDN reference points, as Figure
25.7
shows.
The
TB interface
is the basic UN1 interface by which user equipments are connected
to a B-ISDN at the B-NT1. The
B-NTl (broadband ZSDN network termination type
l
)
provides line terminating functions for the public network operator (power feeding to
line, management test functions, etc.). Where the user device is connected directly to the
USER
DEVICE CONNECTION
TO
B-ISDN
441
constant bit rate
(CBR)
service
frame relay service
X.25
service
ATM
FDDI
SMDS
(DQDB)
switched voice service
sound retrieval service
video on-demand
network
Term
Meaning
CBR constant bit rate (a point-to-point connection across an ATM network
providing service similar to a private wire or leaseline
DQDB dual queue dual bus (the technology behind SMDS)
FDDI fibre distributed data interface (a campus technology for interconnecting
NNI network-node interface (an ATM network interface)
SMDS switched multimegabit data service (an existing broadband network type)
UN1 user-network interface (an ATM network interface)
LAN routers)
Figure
25.6
ATM and B-ISDN: the connection types available
private UN1
public UN1
private
local
interface
public network interface
SB
TB
UB
B-TE1
B-NT1
B-NT2
'
to network
R
I
["m+'
B-TEZ
Figure 25.7 B-ISDN reference configurations at the UN1
line the
U,
interface
is used (e.g. the case where the optical version of the
UN1
is
employed).
Where an electrical interface is used at the UNI,
a B-NT1 may be required
to
perform
electrical to optical conversion of the signal for connection to
a fibre network connec-
tion. Alternatively, the B-ISDN network operator may choose
to
install
a
device
to
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