UMTS Interfaces

Many new protocols have been developed for the four new interfaces specified in UMTS: Uu, Iub, Iur, and Iu. This tutorial is organized by the protocols and shows their usage in the interfaces. That means protocols will be described individually. Only the references to the interfaces are indicated. Interface specific explanations of the protocols are, however, not included. Before we review the individual interface protocols, we introduce the UMTS general protocol model.

General Protocol Model [3G TS 25.401]
UTRAN interface consists of a set of horizontal and vertical layers (see Figure 9). The UTRAN requirements are addressed in the horizontal radio network layer across different types of control and user planes. Control planes are used to control a link or a connection; user planes are used to transparently transmit user data from the higher layers. Standard transmission issues, which are independent of UTRAN requirements, are applied in the horizontal transport network layer.

Figure 9. UTRAN Interface—General Protocol Model

Five major protocol blocks are shown in Figure 9:

• Signaling bearers are used to transmit higher layers’ signaling and control information. They are set up by O&M activities.
• Data bearers are the frame protocols used to transport user data (data streams). The transport network–control plane (TN–CP) sets them up.
• Application protocols are used to provide UMTS–specific signaling and control within UTRAN, such as to set up bearers in the radio network layer.
• Data streams contain the user data that is transparently transmitted between the network elements. User data is comprised of the subscriber’s personal data and mobility management information that are exchanged between the peer entities MSC and UE.
• Access link control application part (ALCAP) protocol layers are provided in the TN–CP. They react to the radio network layer’s demands to set up, maintain, and release data bearers. The primary objective of introducing the TN–CP was to totally separate the selection of the data bearer technology from the control plane (where the UTRAN–specific application protocols are located). The TN–CP is present in the Iu–CS, Iur, and Iub interfaces. In the remaining interfaces where there is no ALCAP signaling, preconfigured data bearers are activated.

Application Protocols
Application protocols are Layer-3 protocols that are defined to perform UTRAN–specific signaling and control. A complete UTRAN and UE control plane protocol architecture is illustrated in Figure 10. UTRAN–specific control protocols exist in each of the four interfaces.

Figure 10. Iu RANAP Protocol Architecture

Figure 11. Application Protocols

Iu: Radio Access Network Application Part (RANAP) [3G TS 25.413]

This protocol layer provides UTRAN–specific signaling and control over the Iu (see Figure 11). The following is a subset of the RANAP functions:

• Overall radio access bearer (RAB) management, which includes the RAB’s setup, maintenance, and release
• Management of Iu connections
• Transport of nonaccess stratum (NAS) information between the UE and the CN; for example, NAS contains the mobility management signaling and broadcast information.
• Exchanging UE location information between the RNC and CN
• Paging requests from the CN to the UE
• Overload and general error situation handling

Iur: Radio Network Sublayer Application Part (RNSAP) [3G TS 25.423]

UTRAN–specific signaling and control over this interface contains the following:

• Management of radio links, physical links, and common transport channel resources
• Paging
• SRNC relocation
• Measurements of dedicated resources

Figure 12. Iur RNSAP Protocol Architecture

Iub: Node B Application Part (NBAP) [3G TS 25.433]

UTRAN specific signaling and control in the Iub includes the following (see Figure 13):

• Management of common channels, common resources, and radio links
• Configuration management, such as cell configuration management
• Measurement handling and control
• Synchronization (TDD)
• Reporting of error situations

Uu: Radio Resource Control (RRC) [3G TS 25.331]

This layer handles the control plane signaling over the Uu between the UE and the UTRAN (see also Figure 13). Some of the functions offered by the RRC include the following:

• Broadcasting information
• Management of connections between the UE and the UTRAN, which include their establishment, maintenance, and release
• Management of the radio bearers, which include their establishment, maintenance, release, and the corresponding connection mobility
• Ciphering control
• Outer loop power control
• Message integrity protection
• Timing advance in the TDD mode
• UE measurement report evaluation
• Paging and notifying

(Note: The RRCs also perform local inter-layer control services, which are not discussed in this document.)

Two modes of operation are defined for the UE—the idle mode and the dedicated mode. In the idle mode the peer entity of the UE’s RRC is at the Node B, while in the dedicated mode it is at the SRNC. The dedicated mode is shown in Figure 10.

Higher-layer protocols to perform signaling and control tasks are found on top of the RRC. The mobility management (MM) and call control (CC) are defined in the existing GSM specifications. Even though MM and CC occur between the UE and the CN and are therefore not part of UTRAN specific signaling (see Figure 15), they demand basic support from the transfer service, which is offered by duplication avoidance (see 3G TS 23.110). This layer is responsible for in-sequence transfer and priority handling of messages. It belongs to UTRAN, even though its peer entities are located in the UE and CN.

Figure 13. Uu and Iub RRC Protocol Architecture

Transport Network Layer: Specific Layer-3 Signaling and Control Protocols
Two types of layer-3 signaling protocols are found in the transport network layer:

1. Iu, Iur: Signaling Connection Control Part (SCCP) [ITU-T Q.711–Q. 716] This provides connectionless and connection-oriented services. On a connection-oriented link, it separates each mobile unit and is responsible for the establishment of a connection-oriented link for each and every one of them.
2. Iu–CS, Iur, Iub: ALCAP [ITU–T Q.2630.1, Q.2150.1, and Q.2150.2]. Layer-3 signaling is needed to set up the bearers to transmit data via the user plane. This function is the responsibility of the ALCAP, which is applied to dynamically establish, maintain, release, and control ATM adaptation layer (AAL)–2 connections. ALCAP also has the ability to link the connection control to another higher layer control protocol. This and additional capabilities were specified in ITU–T Q.2630.1. Because of the protocol layer specified in Q.2630.1, a converter is needed to correspond with underlying sublayers of the protocol stack. These converters are called (generically) signaling transport converter (STC). Two converters are defined and applied in UTRAN:
   • Iu–CS, Iur: AAL–2 STC on message transfer part (MTP) level 3 (broadband) for Q.2140 (MTP3b) [Q.2150.1]
   • Iub: AAL–2 STC on service-specific connection-oriented protocol (SSCOP) [Q.2150.2]

Transport Network Layer Specific Transmission Technologies
Now that we have a circuit-switched and packet-switched domain in the CN and a growing market for packet-switched network solutions, a new RAN must be open to both types of traffic in the long run. That network must also transmit the Layer-3 signaling and control information. ATM was selected as the Layer-2 technology, but higher-layer protocols used in the transport network layer demonstrate the UMTS openness to a pure IP solution.

Iu, Iur, Iub: ATM [ITU-T I.361]
Broadband communication will play an important role with UMTS. Not only voice but also multimedia applications such as videoconferencing, exploring the Internet, and document sharing are anticipated. We need a data link technology that can handle both circuit-switched and packet-switched traffic as well as isochronous and asynchronous traffic. In UMTS (Release ’99), ATM was selected to perform this task.

An ATM network is composed of ATM nodes and links. The user data is organized and transmitted in each link with a stream of ATM cells. AALs are defined to enable different types of services with corresponding traffic behavior. Two of these are applied in UTRAN:

1. Iu–CS, Iur, Iub: AAL–2 [ITU-T I.363.2]—With AAL–2, isochronous connections with variable bit rate and minimal delay in a connection-oriented mode are supported. This layer was designed to provide real-time service with variable data rates, such as video. Except for the Iu–PS interface, AAL–2 is always used to carry the user data streams.
2. Iu–PS, Iur, Iub: AAL–5 [ITU-T I.363.5]—With AAL–5, isochronous connections with variable bit rate in a connection-oriented mode are supported. This layer is used for Internet protocol (IP) local-area network (LAN) emulation, and signaling. In UTRAN, AAL–5 is used to carry the packet-switched user traffic in the Iu–PS-interface and the signaling and control data throughout.

In order to carry signaling and control data, the AAL–5 has to be enhanced. Here, UTRAN offers both a classical ATM solution and an IP–based approach:

1. Signaling AAL and MTP3b—To make signaling AAL (SAAL) available in place of the AAL–5 service-specific convergence sublayer (SSCS), the SSCOP, which provides a reliable data transfer service, and the service-specific coordination function (SSCF), which acts as coordination unit, are defined.
2. Iu, Iur, Iub: SSCOP [ITU–T Q.2110]—The SSCOP is located on top of the AAL. It is a common connection-oriented protocol that provides a reliable data transfer between peer entities. Its capabilities include the transfer of higher-layer data with sequence integrity, flow control, connection maintenance in case of a longer data transfer break, error correction by protocol control information, error correction by retransmission, error reporting to layer management, status report, and more.

Two versions of the SSCF are defined: one for signaling at the user-to-network interface (UNI), and one for signaling at the network to node interface (NNI):

1. Iub: SSCF for at the UNI (SSCF) [ITU–T Q.2130]—The SSCF–UNI receives Layer-3 signaling and maps it to the SSCOP and visa versa. The SSCF–UNI performs coordination between the higher and lower layers. Within UTRAN, it is applied in Iub with the NBAP and ALCAP on top of the SSCF–UNI.
2. Iu, Iur: SSCF at the NNI (SSCF-NNI) [ITU–T Q.2140]—The SSCF-NNI receives the SS7 signaling of a Layer 3 and maps it to the SSCOP, and visa versa. The SSCF-NNI performs coordination between the higher and the lower layers. Within UTRAN, MTP3b has the higher Layer 3, which requires service from the SSCOP-NNI.

Figure 14. Iu–PS Protocol Architecture

Originally the SS7 protocol layer, SCCP relies on the services offered by MTP, so the Layer-3 part of the MTP must face the SCCP layer:

Iu, Iur: MTP3b [ITU–T Q.2210]—Signaling links must be controlled in level 3 for: message routing, discrimination and distribution (for point-to-point link only), signaling link management, load sharing, etc. The specific functions and messages for these are defined by the MTP3b, which requires the SSCF–NNI to provide its service.

The Layer-3 signaling and control data can also be handled by an enhanced IP stack using a tunneling function (see Figure 12). Tunneling is also applied for packet-switched user data over the Iu–PS interface (see Figure 14).

• IP over ATM
  • lu-PS, Iur: IP [IETF RFC 791, 2460, 1483, 2225], user datagram protocol (UDP) [IETF RFC 768] The IP can be encapsulated and then transmitted via an ATM connection, a process which is described in the RFC 1483 and RFC 2225. Both IP version 4 (IPv4) and IP version 6 (IPv6) are supported. IP is actually a Layer-3 protocol. UDP is applied on top of the unreliable Layer-4 protocol. The objective is to open this signaling link to future pure IP network solutions.

In order to tunnel SCCP or ALCAP signaling information, two protocols are applied:

• Iu–PS and Iur: Simple Control Transmission Protocol (SCTP) [IETF SCTP]—This protocol layer allows the transmission of signaling protocols over IP networks. Its tasks are comparable with MTP3b. On Iu–CS, SS7 must be tunneled between the CN and the RNC. The plan is that this is to be done with the Iu–PS and Iur [IETF M3UA].

The following does the tunneling of packet-switched user data:

• Iu–PS: GPRS tunneling protocol (GTP) [3G TS 29.060]—The GTP provides signaling through GTP–control (GTP–C) and data transfer through GTP–user (GTP–U) procedures. Only the latter is applied in the Iu–PS interface because the control function is handled by the RANAP protocol. The GTP–U is used to tunnel user data between the SGSN and the RNC.

Figure 15. UMTS Air Interface Uu

Iu, Iur, Iub: The Physical Layers [3G TS 25.411]
The physical layer defines the access to the transmission media, the physical and electrical properties, and how to activate and deactivate a connection. It offers to the higher-layer physical service access points to support the transmission of a uniform bit stream. A huge set of physical-layer solutions is allowed in UTRAN, including ETSI synchronous transport module (STM)–1 (155 Mbps) and STM–4 (622 Mbps); synchronous optical network (SONET) synchronous transport signal (STS)–3c (155 Mbps) and STS–12c (622 Mbps); ITU STS–1 (51 Mbps) and STM–0 (51 Mbps); E-1 (2 Mbps), E-2 (8 Mbps), and E-3 (34 Mbps); T-1 (1.5 Mbps) and T-3 (45 Mbps); and J-1 (1.5 Mbps) and J-2 (6.3 Mbps).

With the above protocol layers, the interfaces Iu, Iur, and Iur are fully described. There is only the air interface left for a more detailed analysis:

The Air Interface Uu [3G TS 25.301]
The air interface solution is usually a major cause for dispute when specifying a new RAN. Figure 15 shows the realization of the lower parts of the protocol stack in the UE. As can be seen, a physical layer, data link layer, and network layer (the part for the RRC) have been specified.

The physical layer is responsible for the transmission of data over the air interface. The FDD and TDD W–CDMA solutions have been specified in UMTS Rel. ’99. The data link layer contains four sublayers:

• Medium Access Control (MAC) [3G TS 25.321]—The MAC layer is located on top of the physical layer. Logical channels are used for communication with the higher layers. A set of logical channels is defined to transmit each specific type of information. Therefore, a logical channel determines the kind of information it uses. The exchange of information with the physical layer is realized with transport channels. They describe how data is to be transmitted over the air interface and with what characteristics. The MAC layer is responsible for more than mapping the logical channels into the physical ones. It is also used for priority handling of UEs and the data flows of a UE, traffic monitoring, ciphering, multiplexing, and more.
• Radio Link Control (RLC) [3G TS 25.322]—This is responsible for acknowledged or unacknowledged data transfer, establishment of RLC connections, transparent data transfer, quality of service (QoS) settings, unrecoverable error notification, ciphering, etc. There is one RLC connection per radio bearer.

The two remaining Layer-2 protocols are used only in the user plane:

• Packet Data Convergence Protocol (PDCP) [3G TS 25.323]—This is responsible for the transmission and reception of radio network layer protocol data units (PDUs). Within UMTS, several different network layer protocols are supported to transparently transmit protocols. At the moment, IPv4 and IPv6 are supported, but UMTS must be open to other protocols without forcing the modification of UTRAN protocols. This transparent transmission is one task of PDCP; another is to increase channel efficiency (by protocol header compression, for example).
• Broadcast/Multicast Control (BMC) [3G TS 25.324]—This offers broadcast/multicast services in the user plane. For instance, it stores SMS CB messages and transmits them to the UE.