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UMTS Networks : Architecture, Mobility and Services

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306 <strong>UMTS</strong> <strong>Networks</strong><br />

Table 10.2 ATM adaptation layers used in UTRAN interfaces<br />

AAL5 Iu: CS C-plane, PS C/U-plane, internal C-plane of the transport network<br />

Iur: C-plane, internal C-plane of the transport network<br />

Iub: C-plane, internal C-plane of the transport network<br />

AAL2 Iu: CS U-plane<br />

Iur: U-plane<br />

Iub: U-plane<br />

C-plane ¼ Control plane; U-plane ¼ User plane.<br />

is used as the common carrier for user data in all interfaces, except the PS domain user<br />

data at the Iu interface.<br />

The sublayers of the AAL protocol stack in different UTRAN interfaces, as shown in<br />

Figure 10.8, are not described in detail here. The key observation in the selection of<br />

these convergence protocols is that, in all Release 99 control-plane protocols for the PS<br />

domain, convergence could already be accomplished in two alternative ways: either the<br />

convergence protocols specified by ITU-T for signalling transport over AAL5 were<br />

selected or IP-over-AAL5 was used. In addition, user-plane convergence for the PS<br />

domain at the Iu interface was already part of Release 99 <strong>and</strong> based on the User<br />

Datagram Protocol (UDP) over IP. In 3GPP Release 5 this led to even wider utilisation<br />

of IP-based transport in all UTRAN <strong>and</strong> CN interfaces, as described next.<br />

10.3.3.2 IP Transport<br />

IP-based transport started to gain more momentum towards the end of the 3GPP R99<br />

specification campaign. In order to create a basis for full-scale utilisation of IP internetworking<br />

technology as the common transport for the future evolution of <strong>UMTS</strong><br />

networks, it became necessary to study the IP option for both signalling <strong>and</strong> user data<br />

transport in all UTRAN interfaces.<br />

Ever since 3GPP R5, user data can be transported across all UTRAN interfaces by<br />

using the IP protocol stack as well (see Figure 10.8). The common part of the IP stack<br />

used for user data transport is UDP/IP: on the Iub <strong>and</strong> Iur interfaces, radio frames are<br />

transported as plain UDP datagrams; on the Iu-PS interface, the tunnelling protocol<br />

(GTP-U) is used to carry user data packets; <strong>and</strong> on the Iu-CS interface, the Real Time<br />

Protocol (RTP) is used on top of UDP/IP to carry speech frames.<br />

3GPP specifications for the UTRAN transport network require all UTRAN nodes to<br />

support IPv6. Support for IPv4 is optional <strong>and</strong> the specifications do not preclude<br />

implementation of just IPv4. However, in order to support the transition from IPv4<br />

to IPv6, it is recommended to use dual-stack (IPv4 <strong>and</strong> IPv6) implementations in<br />

UTRAN nodes.<br />

Although 3GPP specifications do not m<strong>and</strong>ate any single link layer protocol to be<br />

used in the IP transport network, there is a minimum requirement to support the Pointto-Point<br />

Protocol with High-level Data Link Control (HDLC) framing according to the<br />

Internet Engineering Task Force’s (IETF) Requests for Comments (RFCs) 1661 <strong>and</strong><br />

1662. Other data link protocols that fulfil the same requirements can also be used. One

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