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basis of the UMTS network. The chapter focuses on the operation of the ¬rst release of
UMTS, Release 99, but explains the changes to this network as it evolves to an all-IP
infrastructure. The operation of signalling protocols throughout the network is described
in signi¬cant detail. A basic overview of the operation of the CDMA2000 system is also
presented for reference.
Chapter 7 explains the application of ATM technology as a transport layer within the
UMTS radio access network. At the time of development of UMTS, ATM was the only
technology that could support the different types of traf¬c on the same infrastructure, while
guaranteeing performance and meeting rigorous QoS requirements. In addition, ATM is
a proven technology at integrating with both ISDN and IP networks, which is essentially
the technologies around which the UMTS core network domains are based. A key feature
of the application of ATM in a UMTS context is the extensive use of adaptation layer
2 (AAL2) as a transport for both real-time and non-real-time applications in the radio
access network, a utilization not previously seen. Pivotal to the application of AAL2 is the
ability to dynamically establish and release AAL2 connections using the AAL2 signalling
protocol, and its operation is also explained.
Chapter 8 discusses the use of IP in UMTS as the network evolves to Release 4. In
Release 4, the traditional circuit switched core network infrastructure of GSM is replaced
with an IP-based softswitch architecture. This chapter explains the operation of new
protocols to support this architecture, where the role of the mobile switching centre
(MSC) is split into control using an MSC server and traf¬c transfer with a media gateway
(MGW). The real-time extensions to IP for support of voice transport, the real-time
transport protocol and the real-time transport control protocol (RTP/RTCP), are covered
here. The MSC server uses a protocol to control its media gateways, and the operation
of the media gateway control protocol (MEGACO), as speci¬ed for UMTS, is explained.
For call control, the bearer-independent call control (BICC) protocol is speci¬ed between
MSC servers, and the signalling transfer (sigtran) protocol stack is used for the transport
of SS7 signalling over an IP network. Both are also explained.
Chapter 9 looks to UMTS Release 5, where IP use is extended through the UTRAN to
the BTS. The various transport options for using IP in UTRAN are described. The session
initiation protocol (SIP) is explained, as it is now the protocol speci¬ed for VoIP, mobility
management and instant messaging in UMTS. This chapter also looks to other IP protocols
and their possible use within UMTS, such as multi-protocol label switching (MPLS).
A list of the current versions of the speci¬cations can be found at http://www.3gpp.org/
specs/web-table specs-with-titles-and-latest-versions.htm, and the 3GPP ftp site for the
individual speci¬cation documents is http://www.3gpp.org/ftp/Specs/latest/
2
Principles of Communications


2.1 CIRCUIT- AND PACKET SWITCHED DATA

Many practical communication systems use a network which allows for full connectivity
between devices without requiring a permanent physical link to exist between two devices.
The dominant technology for voice communications is circuit switching. As the name
implies, it creates a series of links between network nodes, with a channel on each
physical link being allocated to the speci¬c connection. In this manner a dedicated link
is established between the two devices.
Circuit switching is generally considered inef¬cient since a channel is dedicated to the
link even if no data is being transmitted. If the example of voice communications is con-
sidered, this does not come close to 100% channel ef¬ciency. In fact, research has shown
that it is somewhat less that 40%. For data which is particularly bursty this system is even
more inef¬cient. Generally before a connection is established, there is a delay; however,
once connected, the link is transparent to the user, allowing for seamless transmission at
a ¬xed data rate. In essence, it appears like a direct connection between the two stations.
Some permanent type circuits such as leased lines do not have a connection delay since
the link is con¬gured when it is initially set up. Circuit switching is used principally in
the public switched telephone network (PSTN), and private networks such as a PBX or a
private wide area network (WAN). Its fundamental driving force has been to handle voice
traf¬c, i.e. minimize delay, but more signi¬cantly permit no variation in delay. The PSTN
is not well suited to data transmission due to its inef¬ciencies; however, the disadvantages
are somewhat overcome due to link transparency and worldwide availability.
The concept of packet switching evolved in the early 1970s to overcome the limitations
of the circuit switched telecommunications network by implementing a system better
suited to handling digital traf¬c. The data to be transferred is split into small packets,
which have an upper size limit that is dependent on the particular type of network. For
example, with asynchronous transfer mode (ATM) the cell size is ¬xed at 53 bytes whereas

Convergence Technologies for 3G Networks: IP, UMTS, EGPRS and ATM J. Bannister, P. Mather and S. Coope
™ 2004 John Wiley & Sons, Ltd ISBN: 0-470-86091-X
12 PRINCIPLES OF COMMUNICATIONS


an Ethernet network carries frames that can vary in size from 64 bytes up to 1500 bytes.
A packet contains a section of the data plus some additional control information referred
to as a header. This data, which has been segmented at the transmitter into packet sizes
that the network can handle, will be rebuilt into the original data at the receiver. The
additional header information is similar in concept to the address on an envelope and
provides information on how to route the packet, and possibly where the correct ¬nal
destination is. It may also include some error checking to ensure that the data has not
been corrupted on the way. On a more complex network consisting of internetworking
devices, packets that arrive at a network node are brie¬‚y stored before being passed
on, once the next leg of the journey is available, until they arrive at their destination.
This mechanism actually consists of two processes, which are referred to as buffering
and forwarding. It allows for much greater line ef¬ciency since a link between nodes
can be shared by many packets from different users. It also allows for variable rates of
transmission since each node retransmits the information at the correct rate for that link. In
addition, priorities can be introduced where packets with a higher priority are transmitted
¬rst. The packet switched system is analogous to the postal system. There are two general
approaches for transmission of packets on the network: datagrams and virtual circuits.



2.1.1 Datagram approach
With the datagram approach, each packet is treated independently, i.e. once on the net-
work, a packet has no relation to any others. A network node makes a routing decision
and picks the best path on which to send the packet, so different packets for the same
destination do not necessarily follow the same route and may therefore arrive out of
sequence, as illustrated in Figure 2.1. The headers in the ¬gure for each of the packets
will have some common information, such as the address of the receiver, and some infor-
mation which is different, such as a sequence number. Reasons for packets arriving out
of sequence may be that a route has become congested or has failed. Because packets can
arrive out of order the destination node needs to reorder the packets before reassembly.
Another possibility with datagrams is that a packet may be lost if there is a problem at a
node; depending on the mechanism used the packet may be resent or just discarded. The
Internet is an example of a datagram network; however, when a user dials in to an ISP via
the PSTN (or ISDN), that link will be a serial link, most probably using the PPP protocol
(see Chapter 5). This access link is a circuit switched connection in that the bandwidth is
dedicated to the user.


2.1.2 Virtual circuits
Since the packets are treated independently across the network, datagram networks tend
to have a high amount of overhead because the packet needs to carry the full address of
the ¬nal destination. This overhead on an IP network, for example, will be a minimum of
20 bytes. This may not be of signi¬cance when transferring large data ¬les of 1500 bytes
or so but if voice over IP (VoIP) is transferred the data may be 32 bytes or less and here
2.1 CIRCUIT- AND PACKET SWITCHED DATA 13


data


H packet1 H packet2 H packet3 H packet4

Datagram Network
H
pac
ket
4 H packet1
Source




H
pa
cke
t3
H packet2



Destination

Figure 2.1 Datagram packet switched network


it is apparent that the overhead is signi¬cant. This approach establishes a virtual circuit
through the nodes prior to sending packets and the same route is used for each packet.
The system may not guarantee delivery but if packets are delivered they will be in the
correct order. The information on the established virtual circuit is contained in the header
of each packet, and the nodes are not required to make any routing decisions but forward
the packets according to the information when the virtual circuit was established. This
scheme differs from a circuit switched system as packets are still queued and retransmitted
at each node and they do have a header which includes addressing information to identify
the next leg of the journey. The header here may be much reduced since only localized
addressing is required, such as ˜send me out on virtual circuit 5™ rather than a 4-byte
address for the IP datagram system. There are two types of virtual circuit, permanent
and switched:

• A permanent virtual circuit is comparable to a leased line and is set up once and then
may last for years.
• A switched virtual circuit is set up as and when required in a similar fashion to a
telephone call. This type of circuit introduces a setup phase each and every time prior
to data transfer.

Figure 2.2 shows a network containing a virtual circuit. Packets traverse the virtual
circuit in order and a single physical link, e.g. an STM-1 line, can have a number of
virtual circuits associated with it.
The term connectionless data transfer is used on a packet switched network to describe
communication where each packet header has suf¬cient information for it to reach its
destination independently, such as a destination address. On the other hand, the term
connection-oriented is used to denote that there is a logical connection established between
two communicating hosts. These terms, connection-oriented and connectionless, are often
incorrectly used as meaning the same as virtual circuit and datagram. Connection-oriented
14 PRINCIPLES OF COMMUNICATIONS


data


H packet1 H packet2 H packet3 H packet4

Virtual Circuit Network
H
pac
ket
4 H packet3
Source




H
pa
cke
t2
H packet1



Destination

Figure 2.2 Virtual circuit

and connectionless are services offered by a network, whereas virtual circuits and datagrams
are part of the underlying structure, thus a connection-oriented service may be offered on
a datagram network, for example, TCP over IP.


2.2 ANALOGUE AND DIGITAL COMMUNICATIONS
In an analogue phone system, the original voice signal is directly transmitted on the
physical medium. Any interference to this signal results in distortion of the original
signal, which is particularly dif¬cult to remove since it is awkward to distinguish between
the signal and noise as the signal can be any value within the prescribed range. When
the signals travel long distances and have to be ampli¬ed the ampli¬ers introduce yet
further noise. Also, it is extremely easy to intercept and listen in to the transmitted
signal. With digital transmission, the original analogue signal is now represented by a
binary signal. Since the value of this signal can only be a 0 or a 1, it is much less
susceptible to noise interference and when the signal travels long distances repeaters can
be used to regenerate and thus clean the signal. A noise margin can be set in the centre
of the signal, and any value above this is considered to be of value 1, and below of
value 0, as illustrated in Figure 2.3. The carrier does not generally transport as much
information in a given time when compared to an analogue system, but this disadvantage
is far outweighed by its performance in the face of noise as well as the capability of
compressing the data. Furthermore, an encryption scheme can be added on top of the data
to prevent easy interception. For this reason, all modern cellular communication systems
use digital encoding.


2.2.1 Representing analogue signals in digital format
Since the telephone exchange now works on a digital system in many countries, this
necessitates the transmission of analogue signals in digital format. For example, consider
2.3 VOICE AND VIDEO TRANSMISSION 15



0 1 1 0 1 0 0 1
Ideal Signal


noise
margin


Signal with Interference

Figure 2.3 Digital transmission

transmitting mobile device mobile network

low pass A/D
coder
filter converter

analog microphone digital


receiving mobile device

low pass D/A
decoder
filter converter

analog speaker

Figure 2.4 Digital transmission of analogue signal

transmitting voice across the mobile telephone network. Figure 2.4 shows such a system.

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