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Release 99 typically will be the first release deployed by a WSP in their migration to UMTS. R99 specifies the addition of the UMTS Radio Access Network (UTRAN), which is typically added to circuit-switched voice infrastructure and GPRS "Internet access".
Taking a few steps back, a GSM network would logically look like Figure 3.
Figure 3. GSM Network
It is interesting to note that in the early days of GSM, such a network was perceived as complex!
Addition of a GPRS overlay network would logically look like Figure 4.
Figure 4. GSM/GPRS Network
This overlay network effectively increased the bandwidth of the core network to allow high-speed data transfer with an "always-on" connection. The restricting factor for end-to-end high-speed data transfers became the Radio Access Network.
The addition of UMTS R99 effectively adds a new "front-end" high-speed network to the voice and data networks, removing the last major "speed bump". After the addition of UMTS R99 a wireless network would logically look like Figure 5.
Figure 5. UMTS Release 99 Network
The main reason why the UTRAN can offer a high-speed connection compared to the GSM/GPRS RAN is in the air interface mechanisms. In GSM/GPRS networks, modulation schemes known as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are used. In UMTS networks, the modulation scheme is known as Wide-band Code Division Multiple Access (WCDMA), which has two basic modes of operation: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). These UMTS modulation schemes are inherently more efficient than their GSM/GPRS counterparts, which in turn enable faster connections.
The new network elements introduced as part of UMTS R99 are:
- The Radio Network Controller (RNC)
- The Node B
Radio Network Controller
The RNC is responsible for control of the radio resources in its area. One RNC will control multiple Node Bs.
The RNC in UMTS networks provides functions equivalent to the Base Station Controller (BSC) functions in GSM/GPRS networks. The major difference is that RNCs have more intelligence built-in than their GSM/GPRS counterparts. For example, RNCs can autonomously manage handovers without involving MSCs and SGSNs. This was something not possible using standard BSCs in GSM/GPRS networks.
Node B
The Node B is responsible for air interface processing and some Radio Resource Management functions.
The Node B in UMTS networks provides functions equivalent to the Base Transceiver Station (BTS) in GSM/GPRS networks. Node Bs are typically physically co-located with existing GSM base transceiver station (BTS) to reduce the cost of UMTS implementation and minimize planning consent restrictions. This is likely to have a detrimental effect on UMTS quality as the UMTS Node Bs are being placed in non-optimal locations. UMTS operates at higher frequencies than GSM/GPRS and therefore the signal range is less.
New Interfaces
The new interfaces added as part of R99 are:
- Iu-CS
- This is the circuit-switched connection for carrying (typically) voice traffic and signaling between the UTRAN and the core voice network. The main signaling protocol used is RANAP.
- The equivalent interface in GSM/GPRS networks is the A-interface.
- Iu-PS
- This is the packet-switched connection for carrying (typically) data traffic and signaling between the UTRAN and the core data GPRS network. The main signaling protocol used is RANAP.
- The equivalent interface in GSM/GPRS networks is the Gb interface.
- Iur
- The primary purpose of Iur is to support inter-MSC mobility. When a mobile subscriber moves between areas served by different RNCs, the mobile subscriber's data is now transferred to the new RNC via Iur. The original RNC is known as the Serving RNC and the new RNC is known as the Drift RNC. The main signaling protocol used is RNSAP.
- There is no equivalent interface in GSM/GPRS networks.
- Iub
- This is the interface used by an RNC to control multiple Node B's. The main signaling protocol used is NBAP.
- The equivalent interface in GSM/GPRS networks is the Abis interface. The Iub interface is in the main standardized and open, unlike the Abis interface in GSM/GPRS.
- Uu
- This is the interface between the User Equipment and the network. That is, it is the UMTS air interface.
- The equivalent interface in GSM/GPRS networks is the Um interface.
