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 ModelFive 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 ProtocolsIu: 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 ArchitectureIub: 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

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 ArchitectureTransport 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 ArchitectureOriginally 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:

The Layer-3 signaling and control data can also be handled by an enhanced IP stack using a tunneling function (see Figure 10.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.

First Generation (1G): Analog Cellular
The introduction of cellular systems in the late 1970s and early 1980s represented a quantum leap in mobile communication (especially in capacity and mobility). Semiconductor technology and microprocessors made smaller, lighter weight, and more sophisticated mobile systems a practical reality for many more users. These 1G cellular systems still transmit only analog voice information. The most prominent 1G systems are Advanced Mobile Phone System (AMPS), Nordic Mobile Telephone (NMT), and Total Access Communication System (TACS). With the 1G introduction, the mobile market showed annual growth rates of 30 to 50 percent, rising to nearly 20 million subscribers by 1990.

Second Generation (2G): Multiple Digital Systems
The development of 2G cellular systems was driven by the need to improve transmission quality, system capacity, and coverage. Further advances in semiconductor technology and microwave devices brought digital transmission to mobile communications. Speech transmission still dominates the airways, but the demands for fax, short message, and data transmissions are growing rapidly. Supplementary services such as fraud prevention and encrypting of user data have become standard features that are comparable to those in fixed networks. 2G cellular systems include GSM, Digital AMPS (D-AMPS), code division multiple access (CDMA), and Personal Digital Communication (PDC). Today, multiple 1G and 2G standards are used in worldwide mobile communications. Different standards serve different applications with different levels of mobility, capability, and service area (paging systems, cordless telephone, wireless local loop, private mobile radio, cellular systems, and mobile satellite systems). Many standards are used only in one country or region, and most are incompatible. GSM is the most successful family of cellular standards (GSM900, GSM–railway [GSM–R], GSM1800, GSM1900, and GSM400), supporting some 250 million of the world’s 450 million cellular subscribers with international roaming in approximately 140 countries and 400 networks.

2G to 3G: GSM Evolution
Phase 1 of the standardization of GSM900 was completed by the European Telecommunications Standards Institute (ETSI) in 1990 and included all necessary definitions for the GSM network operations. Several tele-services and bearer services have been defined (including data transmission up to 9.6 kbps), but only some very basic supplementary services were offered. As a result, GSM standards were enhanced in Phase 2 (1995) to incorporate a large variety of supplementary services that were comparable to digital fixed network integrated services digital network (ISDN) standards. In 1996, ETSI decided to further enhance GSM in annual Phase 2+ releases that incorporate 3G capabilities.

GSM Phase 2+ releases have introduced important 3G features such as intelligent network (IN) services with customized application for mobile enhanced logic (CAMEL), enhanced speech compression/decompression (CODEC), enhanced full rate (EFR), and adaptive multirate (AMR), high–data rate services and new transmission principles with high-speed circuit-switched data (HSCSD), general packet radio service (GPRS), and enhanced data rates for GSM evolution (EDGE). UMTS is a 3G GSM successor standard that is downward-compatible with GSM, using the GSM Phase 2+ enhanced core network.

IMT–2000
The main characteristics of 3G systems, known collectively as IMT–2000, are a single family of compatible standards that have the following characteristics:

• Used worldwide
• Used for all mobile applications
• Support both packet-switched (PS) and circuit-switched (CS) data transmission
• Offer high data rates up to 2 Mbps (depending on mobility/velocity)
• Offer high spectrum efficiency

Figure 1. Multiple Standards for Different Applications and CountriesIMT–2000 is a set of requirements defined by the International Telecommunications Union (ITU). As previously mentioned, IMT stands for International Mobile Telecommunications, and “2000” represents both the scheduled year for initial trial systems and the frequency range of 2000 MHz (WARC’92: 1885–2025 MHz and 2110–2200 MHz). All 3G standards have been developed by regional standards developing organizations (SDOs). In total, proposals for 17 different IMT–2000 standards were submitted by regional SDOs to ITU in 1998—11 proposals for terrestrial systems and 6 for mobile satellite systems (MSSs). Evaluation of the proposals was completed at the end of 1998, and negotiations to build a consensus among differing views were completed in mid 1999. All 17 proposals have been accepted by ITU as IMT–2000 standards. The specification for the Radio Transmission Technology (RTT) was released at the end of 1999.

The most important IMT–2000 proposals are the UMTS (W-CDMA) as the successor to GSM, CDMA2000 as the interim standard ’95 (IS–95) successor, and time division–synchronous CDMA (TD–SCDMA) (universal wireless communication–136 [UWC–136]/EDGE) as TDMA–based enhancements to D–AMPS/GSM—all of which are leading previous standards toward the ultimate goal of IMT–2000.

UMTS allows many more applications to be introduced to a worldwide base of users and provides a vital link between today’s multiple GSM systems and IMT–2000. The new network also addresses the growing demand of mobile and Internet applications for new capacity in the overcrowded mobile communications sky. UMTS increases transmission speed to 2 Mbps per mobile user and establishes a global roaming standard.

UMTS is being developed by Third-Generation Partnership Project (3GPP), a joint venture of several SDOs—ETSI (Europe), Association of Radio Industries and Business/Telecommunication Technology Committee (ARIB/TTC) (Japan), American National Standards Institute (ANSI) T-1 (USA), telecommunications technology association (TTA) (South Korea), and Chinese Wireless Telecommunication Standard (CWTS) (China). To reach global acceptance, 3GPP is introducing UMTS in phases and annual releases. The first release (UMTS Rel. ’99), introduced in December of 1999, defines enhancements and transitions for existing GSM networks. For the second phase (UMTS Rel. ’00), similar transitions are being proposed as enhancements for IS–95 (with CDMA2000) and TDMA (with TD–CDMA and EDGE).

The most significant change in Rel. ’99 is the new UMTS terrestrial radio access (UTRA), a W–CDMA radio interface for land-based communications. UTRA supports time division duplex (TDD) and frequency division duplex (FDD). The TDD mode is optimized for public micro and pico cells and unlicensed cordless applications. The FDD mode is optimized for wide-area coverage, i.e., public macro and micro cells. Both modes offer flexible and dynamic data rates up to 2 Mbps. Another newly defined UTRA mode, multicarrier (MC), is expected to establish compatibility between UMTS and CDMA2000.

Figure 2. Evolutionary Concept 

UMTS (Rel. ’99) incorporates enhanced GSM Phase 2+ Core Networks with GPRS and CAMEL. This enables network operators to enjoy the improved cost-efficiency of UMTS while protecting their 2G investments and reducing the risks of implementation.In UMTS release 1 (Rel. ’99), a new radio access network UMTS terrestrial radio access network (UTRAN) is introduced. UTRAN, the UMTS radio access network (RAN), is connected via the Iu to the GSM Phase 2+ core network (CN). The Iu is the UTRAN interface between the radio network controller (RNC) and CN; the UTRAN interface between RNC and the packet-switched domain of the CN (Iu–PS) is used for PS data and the UTRAN interface between RNC and the circuit-switched domain of the CN (Iu–CS) is used for CS data.

“GSM–only” mobile stations (MSs) will be connected to the network via the GSM air (radio) interface (Um). UMTS/GSM dual-mode user equipment (UE) will be connected to the network via UMTS air (radio) interface (Uu) at very high data rates (up to almost 2 Mbps). Outside the UMTS service area, UMTS/GSM UE will be connected to the network at reduced data rates via the Um.

Maximum data rates are 115 kbps for CS data by HSCSD, 171 kbps for PS data by GPRS, and 553 kbps by EDGE. Handover between UMTS and GSM is supported, and handover between UMTS and other 3G systems (e.g., multicarrier CDMA [MC–CDMA]) will be supported to achieve true worldwide access.

Figure 3. Transmission RateThe public land mobile network (PLMN) described in UMTS Rel. ’99 incorporates three major categories of network elements:

• GSM Phase 1/2 core network elements: mobile services switching center (MSC), visitor location register (VLR), home location register (HLR), authentication center (AC), and equipment identity register (EIR)
• GSM Phase 2+ enhancements: GPRS (serving GPRS support node [SGSN] and gateway GPRS support node [GGSN]) and CAMEL (CAMEL service environment [CSE])
• UMTS specific modifications and enhancements, particularly UTRAN

Network Elements from GSM Phase 1/2
The GSM Phase 1/2 PLMN consists of three subsystems: the base station subsystem (BSS), the network and switching subsystem (NSS), and the operations support system (OSS). The BSS consists of the functional units: base station controller (BSC), base transceiver station (BTS) and transcoder and rate adapter unit (TRAU). The NSS consists of the functional units: MSC, VLR, HLR, EIR, and the AC. The MSC provides functions such as switching, signaling, paging, and inter–MSC handover. The OSS consists of operation and maintenance centers (OMCs), which are used for remote and centralized operation, administration, and maintenance (OAM) tasks.

Figure 4. UMTS Phase 1 NetworkNetwork Elements from GSM Phase 2+

GPRS
The most important evolutionary step of GSM toward UMTS is GPRS. GPRS introduces PS into the GSM CN and allows direct access to packet data networks (PDNs). This enables high–data rate PS transmission well beyond the 64 kbps limit of ISDN through the GSM CN, a necessity for UMTS data transmission rates of up to 2 Mbps. GPRS prepares and optimizes the CN for high–data rate PS transmission, as does UMTS with UTRAN over the RAN. Thus, GPRS is a prerequisite for the UMTS introduction.

Two functional units extend the GSM NSS architecture for GPRS PS services: the GGSN and the SGSN. The GGSN has functions comparable to a gateway MSC (GMSC). The SGSN resides at the same hierarchical level as a visited MSC (VMSC)/VLR and therefore performs comparable functions such as routing and mobility management.

CAMEL
CAMEL enables worldwide access to operator-specific IN applications such as prepaid, call screening, and supervision. CAMEL is the primary GSM Phase 2+ enhancement for the introduction of the UMTS virtual home environment (VHE) concept. VHE is a platform for flexible service definition (collection of service creation tools) that enables the operator to modify or enhance existing services and/or define new services. Furthermore, VHE enables worldwide access to these operator-specific services in every GSM and UMTS PLMN and introduces location-based services (by interaction with GSM/UMTS mobility management). A CSE and a new common control signaling system 7 (SS7) (CCS7) protocol, the CAMEL application part (CAP), are required on the CN to introduce CAMEL.

Network Elements from UMTS Phase 1
As mentioned above, UMTS differs from GSM Phase 2+ mostly in the new principles for air interface transmission (W–CDMA instead of time division multiple access [TDMA]/frequency division multiple access [FDMA]). Therefore, a new RAN called UTRAN must be introduced with UMTS. Only minor modifications, such as allocation of the transcoder (TC) function for speech compression to the CN, are needed in the CN to accommodate the change. The TC function is used together with an interworking function (IWF) for protocol conversion between the A and the Iu–CS interfaces.

UTRAN
The UMTS standard can be seen as an extension of existing networks. Two new network elements are introduced in UTRAN, RNC, and Node B. UTRAN is subdivided into individual radio network systems (RNSs), where each RNS is controlled by an RNC. The RNC is connected to a set of Node B elements, each of which can serve one or several cells.

Figure 5. UMTS Phase 1: UTRANExisting network elements, such as MSC, SGSN, and HLR, can be extended to adopt the UMTS requirements, but RNC, Node B, and the handsets must be completely new designs. RNC will become the replacement for BSC, and Node B fulfills nearly the same functionality as BTS. GSM and GPRS networks will be extended, and new services will be integrated into an overall network that contains both existing interfaces such as A, Gb, and Abis, and new interfaces that include Iu, UTRAN interface between Node B and RNC (Iub), and UTRAN interface between two RNCs (Iur). UMTS defines four new open interfaces:

• Uu: UE to Node B (UTRA, the UMTS W–CDMA air interface
• Iu: RNC to GSM Phase 2+ CN interface (MSC/VLR or SGSN)
  • Iu-CS for circuit-switched data
  • Iu-PS for packet-switched data
• Iub: RNC to Node B interface
• Iur: RNC to RNC interface, not comparable to any interface in GSM

The Iu, Iub, and Iur interfaces are based on ATM transmission principles.

The RNC enables autonomous radio resource management (RRM) by UTRAN. It performs the same functions as the GSM BSC, providing central control for the RNS elements (RNC and Node Bs).

The RNC handles protocol exchanges between Iu, Iur, and Iub interfaces and is responsible for centralized operation and maintenance (O&M) of the entire RNS with access to the OSS. Because the interfaces are ATM–based, the RNC switches ATM cells between them. The user’s circuit-switched and packet-switched data coming from Iu–CS and Iu–PS interfaces are multiplexed together for multimedia transmission via Iur, Iub, and Uu interfaces to and from the UE.

The RNC uses the Iur interface, which has no equivalent in GSM BSS, to autonomously handle 100 percent of the RRM, eliminating that burden from the CN. Serving control functions such as admission, RRC connection to the UE, congestion and handover/macro diversity are managed entirely by a single serving RNC (SRNC).

If another RNC is involved in the active connection through an inter–RNC soft handover, it is declared a drift RNC (DRNC). The DRNC is only responsible for the allocation of code resources. A reallocation of the SRNC functionality to the former DRNC is possible (serving radio network subsystem [SRNS] relocation). The term controlling RNC (CRNC) is used to define the RNC that controls the logical resources of its UTRAN access points.

Figure 6. RNC FunctionsNode B
Node B is the physical unit for radio transmission/reception with cells. Depending on sectoring (omni/sector cells), one or more cells may be served by a Node B. A single Node B can support both FDD and TDD modes, and it can be co-located with a GSM BTS to reduce implementation costs. Node B connects with the UE via the W–CDMA Uu radio interface and with the RNC via the Iub asynchronous transfer mode (ATM)–based interface. Node B is the ATM termination point.

The main task of Node B is the conversion of data to and from the Uu radio interface, including forward error correction (FEC), rate adaptation, W–CDMA spreading/despreading, and quadrature phase shift keying (QPSK) modulation on the air interface. It measures quality and strength of the connection and determines the frame error rate (FER), transmitting these data to the RNC as a measurement report for handover and macro diversity combining. The Node B is also responsible for the FDD softer handover. This micro diversity combining is carried out independently, eliminating the need for additional transmission capacity in the Iub.

The Node B also participates in power control, as it enables the UE to adjust its power using downlink (DL) transmission power control (TPC) commands via the inner-loop power control on the basis of uplink (UL) TPC information. The predefined values for inner-loop power control are derived from the RNC via outer-loop power control.

Figure 7. Node B OverviewUMTS UE
The UMTS UE is based on the same principles as the GSM MS—the separation between mobile equipment (ME) and the UMTS subscriber identity module (SIM) card (USIM). Figure 8 shows the user equipment functions. The UE is the counterpart to the various network elements in many functions and procedures.

Figure 8. UE Functions

The infrastructure of a wireless network interconnects wireless users and end systems. The infrastructure might consist of 

Base Stations

The base station is a common infrastructure component that interfaces the wireless communications signals traveling through the air medium to a wired network—often referred to as a distribution system. Therefore, a base station enables users to access a wide range of network services, such as web browsing, e-mail access, and database applications. A base station often contains a wireless NIC that implements the same technology in operation by the user’s wireless NIC.

Base stations go by different names, depending on their purpose. Anaccess point, for instance, represents a generic base station for a wireless LAN. A collection of access points within a wireless LAN, for example, supports roaming throughout a facility. The NIC within a user’s computer device connects with the nearest access point, which provides an interface with systems within the infrastructure and users associated with other access points. As the user moves to a part of the facility that’s closer to another access point, the NIC automatically reconnects with the closest access point to maintain reliable communications.

Residential gateways and routers are more advanced forms of base stations that enable additional network functions. The gateway might have functions, such as access control and application connectivity, that better serve distributed, public networks. On the other hand, arouter 

As show in Figure 2-4, a base station might support point-to-point or point-to-multipoint communications. Point-to-point systems enable communications signals to flow from one particular base station or computer device directly to another one. This is a common infrastructure for supporting long-range wireless communications links. For example, a wireless Internet service provider (WISP) can use this system to transport communications signals from a base station at a remote site— such as a home or office— to a base station near a communications facility.

base stations, access controllers, application connectivity software, and a distribution system. These components enhance wireless communications and fulfill important functions necessary for specific applications.would enable operation of multiple computers on a single broadband connection.

Figure 2-4 Base Stations Support Different Configurations

As the name implies, point-to-multipoint functionality enables a base station to communicate with more than one wireless computer device or base station. An access point within a wireless LAN implements this form of communications. The access point represents a single point whereby many computer devices connect to and communicate with each other and systems within the wireless infrastructure.

Access Controllers

In the absence of adequate security, quality of service (QoS), and roaming mechanisms in wireless network standards, companies offer access-control solutions to strengthen wireless systems. The key component to these solutions is an access controller, which is typically hardware that resides on the wired portion of the network between the access points and the protected side of the network. Access controllers provide centralized intelligence behind the access points to regulate traffic between the open wireless network and important resources. In some cases, the access point contains the access control function.

Access controllers apply to a wide range of applications. In a public wireless LAN, for example, an access controller regulates access to the Internet by authenticating and authorizing users based on a subscription plan. Similarly, a corporation can implement an access controller to help a hacker sitting in the company’s parking lot from getting entry to sensitive data and applications.

The use of an access controller reduces the need for smart access points, which are relatively expensive and include many non-802.11 features. Generally, vendors refer to these smarter access points as being enterprise-grade components. Proponents of access controllers, however, argue that 802.11 access points should focus on RF excellence and low cost. Proponents also argue that access points should centralize access control functions in an access controller that serves all access points. These thin access points primarily implement the basic wireless network standard (such as IEEE 802.11), and not much more.

The users of access controllers realize the following benefits when deployed with thin access points:

• Lower Costs—Access points with limited functionality cost less, which generally results in lower overall system costs. This is especially true for networks requiring a larger number of access points, such as an enterprise system. The use of thin access points results in cost savings of approximately $400 per access point. In larger networks, this savings far outweighs the additional cost of an access controller, which costs $5000 on the average.

• Open Connectivity—Smart access points offer enhancements related to security and performance to the basic wireless connectivity that wireless network standards offer. The problem in many cases is that these enhancements are only possible if the user devices implement a wireless NIC made by the same vendor as the access point. This significantly reduces the openness of the system and limits the selection of vendors. On the other hand, thin access points can easily communicate using the basic wireless network protocol with wireless NICs made by multiple vendors, while the access controller transparently provides enhancements.

• Centralized Support—An advantage of placing the smarts of the network in an access controller is that the system is easier to support, primarily because fewer touch points are in the network. If all of the intelligence of the network is within the access points, support personnel must interface with many points when configuring, monitoring, and troubleshooting the network. An access controller enables the access points to have fewer functions, reducing the need to interface with the access points when performing support tasks.

  • Authentication—Most access controllers have a built-in database for authenticating users; however, some offer external interfaces to authentication servers such asRemote Authentication Dial-In User Service (RADIUS) and Lightweight Directory Access Protocol (LDAP). For smaller, private networks, an internal database might suffice. For enterprise solutions, however, external and centralized authentication servers provide better results.

  • Encryption—Some access controllers provide encryption of data from the client to the server and back, using such common methods such as IPSec. This provides added protection beyond what the native wireless network standard provides. Some of these features, however, are also part of web browsers.

  • Subnet Roaming—In order to support roaming from one network to another, access controllers provide roaming across subnets without needing to re-authenticate with the system. As a result, users can continue utilizing their network applications without interruption as they roam about a facility. This feature is especially useful for larger installations where access to the network for specific users will span multiple subnets.

  • Bandwidth Management—Because users share bandwidth in a wireless network, it’s important to have a mechanism to ensure specific users don’t hog the bandwidth. Access controllers provide this form of bandwidth management through the assignment of user profiles based on required QoS levels. A profile specifies the types of services, such as web browsing, e-mail, and video streaming, as well as performance limits. For example, an unsubscribed visitor attempting to utilize a public wireless LAN could classify as fitting a “visitor” profile, which might only allow access to information related to the local hotspot. A subscriber, however, could have a different role that allows him to have a broadband Internet connection.

Access controllers often provide port-based access control, allowing administrators to configure access to specific applications on a per-user basis. The port, which is actually a number (such as 80 for http), corresponds to a specific type of application. For example, an access controller can block access to port 80, which forces a user to log in before being able to browse web pages. After users enter their username and password, the access controller will validate their identity through an authentication server. The network application could, as an alternative, use digital certificates for authentication purposes. This function regulates the user access to the protected network.

Access controllers generally employ the following features:

Application Connectivity Software

Web surfing and e-mail generally perform well over wireless networks. All it takes is a browser and e-mail software on the client device. Users might lose a wireless connection from time to time, but the protocols in use for these relatively simple applications are resilient under most conditions.

Beyond these simple applications, however, special application connectivity software is necessary as an interface between a user’s computer device and the end system hosting the application’s software or database. Applications could be warehouse management software running on an IBM AS/400, a modeling application located on a UNIX box, or a time-management system residing on an old mainframe system. The databases are part of a client/server system where part, or all of the application software, resides on the client device and interfaces with a database such as Oracle or Sybase. In these cases, application connectivity software is important in addition to access points and controllers to enable communications between the user’s computer device and the application software or databases located on a centralized server.

The following are various types of application connectivity software:

• Terminal Emulation—Terminal emulation software runs on a computer device, making the device operate as a terminal that provides a relatively simple user interface to application software running on another computer. The terminal merely presents screens to the user and accepts input rendered by the applications software. For example, VT220 terminal emulation communicates with applications running on a UNIX host, 5250 terminal emulation works with IBM AS/400-based systems, and 3270 terminal emulation interfaces with IBM mainframes.

The advantage of using terminal emulation is its low initial cost and changes made to the application automatically take affect when the user logs in. Wireless systems using terminal emulation, however, might not be able to maintain continuous connections with legacy applications, which have timeouts set for more reliable wired networks. Timeouts will automatically disconnect a session if they don’t sense activity within a given time period. As a result, IT groups might spend a lot of time responding to end-user complaints of dropped connections and incomplete data transactions. Therefore, implementing terminal emulation can have a disastrous effect on long-term support costs.

• Direct Database Connectivity—Direct database connectivity, sometimes referred to as client/server, encompasses application software running on the user’s computer device. With this configuration, the software on the end-user device provides all application functionality and generally interfaces to a database located on a central server. This enables flexibility when developing applications because the programmer has complete control over what functions are implemented—and is not constrained by a legacy application located on a central computer. Direct database connections are often the best approach when needing flexibility in writing the application software. A problem, however, is that the direct database approach depends on the use of Transmission Control Protocol/Internet Protocol (TCP/IP), which is not well-suited for communications across a wireless network.

• Wireless Middleware—Wireless middleware software provides intermediate communications between user computer devices and the application software or database located on a server. (See Figure 2-5.) The middleware—which runs on a dedicated computer (middleware gateway) attached to the wired network—processes the packets that pass between the user computer devices and the servers. The middleware software primarily offers efficient and reliable communications over the wireless network while maintaining appropriate connections to application software and databases on the server through the more reliable wired network. Sometimes this is referred to as session persistence.

  • Optimization techniques—Many middleware products include data compression to help reduce the number of packets sent over the wireless link. Some implementations of middleware use proprietary communications protocols, which have little overhead as compared to traditional protocols, such as TCP/IP.

  • Intelligent restarts—With wireless networks, a transmission can be unexpectedly cut at midstream. Intelligent restart is a recovery mechanism that detects the premature end of a transmission. When the connection is reestablished, the middleware resumes transmission from the break point instead of at the beginning. This avoids errors from occurring in applications that utilize databases.

  • Data bundling—Some middleware is capable of combining smaller data packets into a single large packet for transmission over the wireless network, which can help lower transmission service costs of WANs. Since some wireless data services charge users by the packet, data bundling results in a lower aggregate cost.

  • Screen scraping and reshaping—The development environment of some middleware products allows developers to use visual tools to shape and reshape portions of existing application screens to more effectively fit data on the smaller display of some non-PC wireless devices, such as PDAs and bar code scanners.

  • End system support—Wireless middleware interfaces with a variety of end system applications and databases. If clients need access to tomultiple types of applications and databases, wireless middleware acts as a concentrator. For example, a user can use the middleware connection to interface with applications on an AS/400 and UNIX box simultaneously without needing to be concerned about running the correct terminal emulation software.

Figure 2-5 Wireless Middleware Efficiently Interconnects Computer Device Applications to Hosts and Servers

Look for the following features in middleware products:

Distribution System

A wireless network is seldom entirely free of wires. The distribution system, which often includes wiring, is generally necessary to tie together the access points, access controllers, and servers. In most cases, the common Ethernet comprises the distribution system.

The IEEE 802.3 standard is the basis for Ethernet and specifies the use of the carrier sense multiple access (CSMA) protocol to provide access to a shared medium, such as twisted-pair wiring, coaxial cable, and optical fiber. CSMA is the predominant medium access standard in use today by both wired and wireless networks.

CSMA enables sharing of a common medium by allowing only one NIC to transmit information at any particular time. This is similar to a meeting environment where people (like NICs) speak only when no one else is talking. This gives each person responsibility in a way that distributes speaking decisions to each person. If more than one person talks at the same time, a collision occurs, and each person needs to take turns repeating what he said.

All computer devices on the network must take turns using the medium with Ethernet hubs. An Ethernet switch, however, enables multiple collision domains that can allow simultaneous transmission among users to improve performance. For larger networks beyond the size of a home or small office application, be sure to use switches for optimum performance.

Ethernet employs twisted-pair wiring, coaxial cable, and optical fiber for interconnecting network devices, such as access points and other distribution equipment. The use of coaxial cables in older wired LANs was common 10 years ago, but today most companies use twisted-pair wiring and optical fiber. The Electronic Industries Association (EIA) and Telecommunications Industry Association (TIA) specifies Category 5 (referred to as Cat 5) twisted-pair wiring, the most popular of all twisted-pair cables in use today with Ethernet.

Cat 5 consists of four unshielded twisted pairs of 24-gauge wires that support Ethernet signals over 100 meters (m)— about 300 feet— of cabling. Ethernet repeaters increase this range if necessary, which is one method of reaching a wireless network base station that’s beyond 100 m from a communications closet.

There are also other variations of twisted-pair wiring. Enhanced Cat 5 (referred to as Cat5e) makes use of all four pairs of wires to support short-range Gigabit Ethernet (1000 Mbps) connectivity. It is also backward compatible with regular Cat 5. Cat 6 and Cat 7 cable are now available, bringing more bandwidth and range to copper-based Gigabit Ethernet networks. Cat 7 cable features individually shielded twisted pairs (STP) of wires, making it ideal for installation in locations where there is a high potential for electromagnetic interference.

The following are specific types of twisted-pair options for Ethernet common to wireless LAN distribution systems:

• 10BASE-T—10BASE-T is one of the 802.3 physical layers and specifies data rates of 10 Mbps. A typical 10BASE-T cable uses two of the four pairs within a Cat 5 cable for sending and receiving data. Each end of the cable includes RJ-45 connectors that are a little larger than the common RJ-11 telephone connector used within North America.

The advantage of having extra pairs of wires open is support for other uses, such as Power-over-Ethernet (PoE). This is a mechanism in which a module injects DC current into the Cat 5 cable, enabling you to supply power to the access point from the communications closet. PoE often eliminates the need for having an electrician install new electrical outlets at every access point. For larger networks, definitely consider the use of PoE.

• 100BASE-T—Another 802.3 physical layer, 100BASE-T supports data rates of 100 Mbps. Similar to 10BASE-T Ethernet, 100-Base-T uses twisted-pair wiring, with the following options:

  • 100BASE-TX uses two pairs of Cat 5 twisted-pair wires.

  • 100BASE-T4 uses four pairs of older, lower-quality (Cat 3) twisted-pair wires.

Most implementations today use 100BASE-TX cabling. As with 10BASE-T, PoE can make use of unused pairs of wires. 100-Base-T4 was popular when needing to support 100-Mbps data rates over the older Cat 3 cabling, which was prominent during the early 1990s.

• Optical Fiber—Optical fiber is more expensive than twisted pair, but fiber can be cost effective because it supports gigabit speeds and has a range of up to two kilometers. Instead of using the traditional electrical-signal-over-copper-wire approach, optical fiber cable uses pulses of light over tiny strips of glass or plastic. This makes optical fiber cable resistant to electromagnetic interference, making it valuable in situations where electronic emissions are a concern. In addition, it’s nearly impossible to passively monitor the transmission of data through optical fiber cable, making it more secure than twisted-pair wiring.

With wireless LANs, optical fiber is a possible solution for reaching an access point located beyond a 100 m from a communications closet. This requires the use of an expensive pair of transceivers, however, which transforms electrical signals into light (and vice versa). One issue when dealing with optical fiber cable is the difficulties in splicing cables. You must work with glass or plastic materials that require precise alignment. You need special tools and training to make effective optical fiber cables. You should purchase precut fiber cables to avoid problems that are difficult to troubleshoot.