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3GPP TS25.201: Physical layer “ general description.
3GPP TS25.211: Physical channels and mapping of transport channels onto physical
channels (FDD).
3GPP TS25.212: Multiplexing and channel coding (FDD).
3GPP TS25.213: Spreading and modulation (FDD).
3GPP TS25.214: Physical layer procedures (FDD).
3GPP TS25.215: Physical layer; Measurements (FDD).
3GPP TS25.301: Radio Interface Protocol Architecture.
3GPP TS25.303: Interlayer procedures in Connected Mode.
3GPP TS25.321: Medium Access Control (MAC) protocol speci¬cation.
3GPP TS25.322: Radio Link Control (RLC) protocol speci¬cation.
3GPP TS25.323: Packet Data Convergence Protocol (PDCP) speci¬cation.
3GPP TS25.324: Broadcast/Multicast Control (BMC).
3GPP TS25.331: Radio Resource Control (RRC) protocol speci¬cation.
3GPP TS25.401: UTRAN Overall Description.
3GPP TS25.402: Synchronisation in UTRAN Stage 2.
3GPP TS25.410: UTRAN Iu Interface: General Aspects and Principles.
3GPP TS25.411: UTRAN Iu interface layer 1.
3GPP TS25.412: UTRAN Iu interface signalling transport.
3GPP TS25.413: UTRAN Iu interface RANAP signalling.
3GPP TS25.414: UTRAN Iu interface data transport & transport signalling.
3GPP TS25.415: UTRAN Iu interface user plane protocols.
3GPP TS25.419: UTRAN Iu-BC interface: Service Area Broadcast Protocol (SABP).
3GPP TS25.420: UTRAN Iur Interface: General Aspects and Principles.
3GPP TS25.421: UTRAN Iur interface Layer 1.
3GPP TS25.422: UTRAN Iur interface signalling transport.
3GPP TS25.423: UTRAN Iur interface RNSAP signalling.
3GPP TS25.424: UTRAN Iur interface data transport & transport signalling for CCH data
streams.
3GPP TS25.425: UTRAN Iur interface user plane protocols for CCH data streams.
3GPP TS25.426: UTRAN Iur and Iub interface data transport & transport signalling for
DCH data streams.
3GPP TS25.427: UTRAN Iur and Iub interface user plane protocols for DCH data streams.
3GPP TS25.430: UTRAN Iub Interface: General Aspects and Principles.
3GPP TS25.431: UTRAN Iub interface Layer 1.
3GPP TS25.432: UTRAN Iub interface: signalling transport.
3GPP TS25.433: UTRAN Iub interface NBAP signalling.
3GPP TS25.434: UTRAN Iub interface data transport & transport signalling for CCH
data streams.
424 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


3GPP TS25.435: UTRAN Iub interface user plane protocols for CCH data streams.
3GPP TS25.853: Delay budget within the access stratum.
3GPP TS25.855: High Speed Downlink Packet Access (HSDPA); Overall UTRAN
description.
3GPP TS25.931: UTRAN Functions, examples on signalling procedures.
3GPP TS26.071: AMR speech Codec; General description.
3GPP TS33.102: 3G security; Security architecture.
3GPP TS33.103: 3G security; Integration guidelines.
3GPP TS33.105: Cryptographic Algorithm requirements.
3GPP TS33.120: Security Objectives and Principles.
3GPP2 C.S0001-0: Introduction to cdma2000 Spread Spectrum Systems, Release 0.
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/
7
UMTS Transmission
Networks


7.1 INTRODUCTION TO RAN TRANSMISSION
Figure 7.1, shows again the basic structure of the universal mobile telecommunications
system (UMTS) network. It is within the radio access network (RAN) that asynchronous
transfer mode (ATM) is used to transport traf¬c, speci¬cally, the Iu, Iub and Iur (not
shown, but interconnects radio network controllers; RNCs) interfaces. The Uu interface
is the air interface between the 3G terminal and the base station (BTS).
The UMTS core network consists of two domains of operation. The top domain is
the circuit switched core network (CS-CN), which is essentially an upgraded version
of the existing second-generation global system for mobile communications (GSM) core,
dealing principally with voice traf¬c. The lower domain is the general packet radio service
(GPRS) core, which introduces a packet switched backbone, based on the Internet protocol
(IP). The GSM core then connects to other circuit switched networks, such as the PSTN,
and the IP core connects to an external IP network, either the Internet or a private
intranet.
The RAN must be able to connect to both domains, through the Iu interface. ATM is
currently the only technology that can effectively connect to both, and the mechanisms
are well established in the industry.
To link to the GSM core, the Iu interface is connected through an internetworking unit
(IWU). This unit performs two functions: ¬rst, it interfaces the wideband switching and
signalling of the ATM network to the 64 kbps time division multiplexing (TDM) circuits
of the GSM ˜A™ interface; second, it performs voice transcoding functions between the
two networks.


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
426 UMTS TRANSMISSION NETWORKS


Radio Access Network Core Network


Circuit
Uu Iub Iu Switched


Packet
Switched
Mobile
Base Station RNC
Station


Figure 7.1 UMTS network structure

In addition, there are three major differences between the GSM and 3G cellular networks.

1. GSM is designed around a TDM structure, principally the 64 kbps connection. This
is not the case with 3G, where wideband services up to a maximum of 2 Mbps are
being offered.
2. Since GSM is intended for voice, all traf¬c is symmetric, that is, utilizing the same
resources in the uplink and downlink. However, 3G applications such as video
streaming or web browsing are typically asymmetric in nature and this should be
re¬‚ected in the transport technology to maximize resource usage.
3. Voice traf¬c is always allocated a constant, small amount of resources in the GSM
network, the time slot. However, both video and data traf¬c require support of
variable bit rates, which can be provisioned for in ATM.

Another more practical consideration is that, as will be seen, ATM is independent of
the underlying media or protocol, but generally is seen being transported over another
technology, such as the synchronous digital hierarchy (SDH) or E1/T1. Since a cellular
operator considering an upgrade to 3G will already have a network infrastructure, ATM
provides the ¬‚exibility of working with that existing infrastructure.



7.2 INTRODUCTION TO ATM
The demands placed on modern networks have increased dramatically in recent years
in terms of types of service and speed of operation. The following are some of the
requirements:

• handling of different types of traf¬c on the same network (voice, video, data);
• provision of economically priced access to users;
• a reliable and ¬‚exible communications link.

ATM is now a widely used technology, which may best address all of these requirements.
The different types of traf¬c pose vastly differing demands on a network (Table 7.1).
7.2 INTRODUCTION TO ATM 427


Table 7.1 Network demands of different traf¬c types
Voice Video Data
Real-time Non-real-time
Bandwidth Small, constant Variable Variable Variable
Error tolerance High Low Low Very low
Symmetry Symmetric Asymmetric Asymmetric Asymmetric
Example Phone call Video phone Video on demand Email



5 byte
48 byte payload
header
53 byte
cell

Figure 7.2 The ATM cell structure

Like UMTS, ATM is designed to support voice, video and data applications. To interface
the wideband code division multiple access (WCDMA) air interface to this in the ¬xed-line
network, ATM is the choice as the most appropriate technology to provide the backbone.
ATM is similar in many respects to Frame Relay. Just like Frame Relay, it assumes
that the medium of transfer is of high quality and therefore does not protect the data from
error. However, Frame Relay frames have a variable length, which introduces a variable
delay. Hence, it is not well suited to sending voice and video. On the other hand, ATM
uses a small, ¬xed packet size, called a cell “ note that ATM is also referred to as ˜cell
relay™. An ATM cell is 53 bytes in size, consisting of a 5-byte header and 48 bytes of user
data, as shown in Figure 7.2. Although ATM does not protect the data, it does include
error checking in the form of a cyclic redundancy check (CRC), to look for single bit
errors in the header, since even for a very low error medium such as ¬bre, most errors that
do occur have been determined to be single bit. The data can be anything: voice or video
packets, for example. But also, the data can be from some other protocol, split up into
cells. LAN emulation, where Ethernet is sent over ATM, is a popular choice. In this case,
the Ethernet frame can be up to 1518 bytes long and will be broken up into a number of
cells for transportation. The different types of payload need to be handled differently and
this is done by an ATM adaptation layer (AAL). The cells are transferred over a virtual
circuit, which is prebuilt. This virtual circuit can be either switched, where it is built at
connection time and torn down once ¬nished, or permanent, where it is speci¬ed and
built at subscription time.
The rationale of using a ¬xed size cell is that for real-time traf¬c, variation in delay is
the most damaging. Therefore if the cells are ¬xed in length, this removes irregularities in
the time cells are transmitted. However, the network must also keep the delay variation to
a minimum, and this is guaranteed through negotiated quality of service (QoS), discussed
later. Also, with a ¬xed length cell, intermediate devices, such as switches, need not
examine the header to determine the length, as is the case with IP packets.
Being small, it takes a shorter time to ¬ll the cells and transmit, minimizing the delay in
˜packing™ a cell, referred to as packetization delay. Consider Figure 7.3, where a 16 kbps
428 UMTS TRANSMISSION NETWORKS


16kbps



header payload


64kbytes

Figure 7.3 Packetization delay

voice channel is feeding into a large packet, such as an IP packet, of length 64 kbytes. It
takes 32 seconds to ¬ll this packet:

Fill time = (64 — 103 — 8)/16 — 103 = 32 s

This is an extreme example, since in practice IP packets are generally of the order of
1500 bytes, but is exaggerated to illustrate a point. Even at 1500 bytes, the packetization
delay will be 750 ms. However, in comparison, an ATM cell will only take 24 ms to ¬ll
with voice data:
Fill time = (48 — 8)/16 — 103 = 24 ms

The drawback is, of course, the relative size of the header when compared to the
payload. With an IP packet, the basic header (IP + TCP) without extensions is 40 bytes,
which becomes signi¬cant if the payload size were to be reduced to provide a minimal
delay. ATM does not completely solve this problem, and will add nearly 10% of overhead
to the traf¬c it carries. This is affectionately referred to as ˜cell tax™.
ATM is a technology de¬nition that is independent of physical medium; however, most
ATM is carried over an optical ¬bre system, most notably SDH/SONET at 155 Mbps and
622 Mbps. One key advantage of ATM is that it is scalable in that it is easy to multiplex
circuits together to provide faster circuits. ATM is termed ˜asynchronous™, since traf¬c
can arrive at any time, and is not required to align to any framing or boundaries.



7.3 HISTORY AND STANDARDS
ATM refers to both a network technology and a set of standards. The standards are
proposed by two standardizing bodies: the International Telecommunications Union “
telecommunications standardization sector (ITU-T) and the ATM Forum. The latter orga-
nization is made up of many industry players and was established to hasten the rather
lengthy standardization processes of the ITU-T and develop practical standards for ATM
much quicker, particularly those concerning its application to smaller scale networks.
The ATM Forum is more geared to the provision of interoperability standards among
manufacturers of ATM products.
Its development essentially came out of the challenges facing the telephone companies
to support a broad range of traf¬c types using multiple network types across a spectrum
of circuit- and packet-switching technologies. The best solution seen was to come up with
a single new network that would replace the telephone system and the existing networks
7.3 HISTORY AND STANDARDS 429


with an integrated one supporting all types of information transfer, as mentioned above.
In addition, the network should be able to offer a range of rates and be scalable up to high
data rates. This new public network service conceptually led on from ISDN and hence
was called broadband ISDN (B-ISDN), with ATM the underlying technology of B-ISDN.
However, ATM is now also widely used as a backbone technology in private networks.
As a technology, ATM has developed from the operating principles of X.25 and Frame
Relay in that it is based on the virtual circuit concept, where connections are established
in advance of any data transfer, either by network management or through the operation
of a signalling protocol. These technologies provide a statistically multiplexed system on
top of a telephone infrastructure. Statistical multiplexing is more suitable to the transfer
of data applications, since their behaviour and usage of bandwidth is unpredictable.
Initially the concept was that the introduction of this network would enable the phone
companies to go head-on with the cable TV operators in the provision of such high
bandwidth services as video on demand. However, the implementation of such a network
is no mean feat. ATM is essentially designed to travel over ¬bre, but is capable of using
category 5 and coaxial cable for short distances. The present telephone network uses
¬bre in the exchange backbone, but it is copper-based in the local loop. To implement
ATM, all this would need to be replaced and upgraded with suitable physical media. In
addition, the existing circuit switched equipment would also need to be replaced with
ATM technology equipment.
A second major issue was that the telephone companies have many decades of expe-
rience with circuit switched technology, and relatively little with packet switched. Since
ATM is based on the latter, all this accumulated experience would be thrown out, and
a revolutionary shift made to a new technology. The compromise really has been that
ATM has been relegated to the backbone, sitting on top of existing backbone telecoms
systems (notably SDH/SONET), to provide higher bandwidth switched systems to support
high-speed traf¬c transport, a lot of which is IP traf¬c. It is currently estimated that 40%
of IP traf¬c travels over ATM. The initial idea of this ubiquitous ATM network has never

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