Since the inception, 2G system has evolved with new functionalities during the years and standardization releases. So for discussion on further detail of the system capabilities, one needs to consider the 3GPP release to which the system complies with. Initially in 2G, the focus was on providing mobile voice service. One technological difference compared to the previous ‘1G’ systems is that in 2G voice and all services are digital, not analogue. For further reading on 2G see [12] and [13].
Enhancements in the 2G network include AMR (Adaptive Multi-Rate) speech, high speed circuit switched data (HSCSD), GPRS (General packet radio service), and EDGE (Enhanced data rates for global evolution). Term GERAN (GSM/EDGE Radio access network) is used to refer to the 2G radio access network with these enhanced capabilities. GERAN also includes an Iu-interface option, which is inherited from the 3G system. Mobile stations may then operate either in A/Gb mode or in Iu mode, depending on how the 2G radio access network interfaces the core network.
The system architecture of the GERAN network is shown in Figure 3.4. The base station subsystem (BSS) internal interface between the BTS and the BSC is Abis.
Figure 3.4 GERAN structure [12].
3.2.1 Circuit Switched Traffic
Figure 3.5 shows the GERAN user plane with both A-mode and Iu mode interfaces. The initial 2G (A-mode) protocols are indicated with a grey colour. The CS user plane is optimized for the transport of voice.
Figure 3.5 2G circuit switched user plane [12].
For the A mode, the protocol model for the CS domain user plane is simple: there are only physical and logical channels that are mapped into timeslots. Encryption is supported by the BTS, controlled by the BSC and the core network.
The initial GSM voice codec is RPE-LTP (Regular pulse excited long term prediction), which results in 13 kbit/s bit rate (full-rate). Before connecting the mobile voice traffic to the traditional PSTN, transcoding is needed via a transcoding and rate adaptation function (TRAU). Voice is carried between the MS (Mobile Station) and the TRAU or MSC. GERAN is carrying the voice frames between the air interface Um and the A interface. In the Iu mode, there are further options.
In practice TRAU is often located close to the core network (MSC), since by this means transport timeslots can be saved. One GSM-coded voice channel occupies a single 16 kbit/s subtimeslot compared to the 64 kbit/s voice rate (A-law or u-law encoded) in the traditional PSTN.
With half-rate, 5.6 kbit/s is required for voice. Enhanced full rate (EFR) is 12.2 kbit/s, increasing speech quality. AMR (Adaptive Multi-Rate) also improves quality for both half-rate and full-rate channels, by adapting its bit-rate to channel conditions. Part of the capacity of the AMR is used for channel coding.
The A interface is in the initial 2G standard a TDM-based interface. In 3GPP Rel-7, A over IP was introduced for the control plane [24]. The user plane protocol stack for A over IP, specified in 3GPP Rel-8, consists of RTP/UDP/IP [25]. For a BSS/MGW pair, two consecutive UDP ports are used, one reserved for RTP (Real-time Transport Protocol), and the other one for RTCP (RTP Control Protocol). The use of RTCP is optional. The same UDP port number is used in both receive and transmit directions. For the layers below IP, basically any L2 protocol is allowed, however 3GPP mandates at least Ethernet.
RTP multiplexing can optionally be supported, if negotiated successfully with RTCP. With RTP multiplexing, several RTP user connections can be carried within the UDP/IP packet. For RTP multiplexing an additional multiplexing header is included as defined in [25], [26]. This saves bandwidth, as many RTP packets may share a single UDP/IP packet. RTP header can also be compressed.
The control plane of 2G circuit switched domain is shown in Figure 3.6.
Figure 3.6 2G circuit switched control plane [12].
Figure 3.6 shows the GERAN circuit switched control plane with both A-mode and Iu mode interfaces. The initial 2G (A-mode) protocols are indicated with a grey colour.
Non-access stratum signalling (MM/UMM/CC/SS) is transmitted transparently over the GERAN. Over the A interface they are carried using Direct Transfer application part (DTAP).
BSC is responsible for the radio resource management, RR (Radio resource) in the figure. BSC communicates with the mobile station, but also with the BTS with these RR messages. Part of the messaging from MS to the BSC is transparent to the BTS. BTS then maps these messages to the RR′ messages that are carried over the air interface.
In the A mode in the CS domain control plane, LAPD is the control plane protocol over the Abis. LAPDm marks the LAPD protocol over the air interface, while LAPD is used between the BTS and the BSC over the Abis. RR messages are carried over the LAPD protocol.
LAPD is based on the ISDN specifications. GSM specifications refer to EN 300 125 in addition to the CCITT Recommendation Q.921. For a BTS, there may be multiple LAPD links over the Abis, which carry signalling, operation and maintenance, and layer-2 management procedures. LAPD terminal endpoint identifiers (TEIs) are used for addressing.
For the A interface, IP based control plane is supported, with the SIGTRAN protocol stack, BSSAP/SCCP/M3UA/SCTP/IP. BSSAP consists of BSSMAP (BSS Management Application Part) and of DTAP (Direct Transfer Part). BSSMAP is the messaging between the BSS and the core network. SCCP is a Signalling Connection Control Part of the CCITT signalling system No.7, and M3UA (Message Transfer Part 3 User Adaptation Layer) is an adaptation to the SCTP.
3.2.2 Packet Switched Traffic
For the 2G packet switched domain in the user plane, Gb interface supports packet switched data between a Packet Control unit (PCU) and the SGSN. PCU can be considered as an addition to the BSS and implemented e.g. within a BSC.
The Gb interface uses network service virtual links (NS-VL) and network service virtual connections (NS-VC) as abstractions. The underlying transmission is defined as a subnetwork, which initially is standardized to be Frame Relay.
In 3GPP Rel-4, the interface specification introduces IP as an alternative to Frame Relay for the implementation of the subnetwork. The NS-VL is mapped into an IP endpoint, and, in the case of IP transport, the NS-VCs consists of connectivity between source and destination IP addresses and UDP ports [31]. The abstraction intends to hide the realization of the network service from the upper layer (BSS GPRS Protocol).
Figure 3.7 shows the user plane protocol stack carried over the air interface and the GERAN to the SGSN. In the SGSN the user plane traffic is further relayed to the GGSN (Gateway GPRS Support Node) and to the Gi interface.
Figure 3.7 2G packet switched user plane [12].
Figure 3.7 shows GERAN with both A-mode and Iu mode interfaces. The initial 2G/GPRS (Gb-mode) protocols are indicated with a grey colour.
Between the MS and the SGSN, SNDCP (Subnetwork Dependent Convergence Protocol) with Logical Link Control (LLC) protocol is used. LLC is based on LAPD and HDLC (High Level Data Link Control) concepts, supporting acknowledged and unacknowledged mode transfers. Service access point identifiers (SAPIs) identify the LLC layer service access points for the higher layer protocols. LLC includes ciphering, so the user data is encrypted between the MS and the SGSN.
RLC (Radio Link Control) layer provides services like segmentation and reassembly of the LLC layer PDUs and supports both an acknowledged and an unacknowledged mode—these are specified in GSM 04.60. For the acknowledged mode delivery, selective retransmission of unsuccessfully delivered RLC data blocks is supported.
MAC (Medium Access Control) layer supports the transmission of logical channels over the physical channels. Physical channels may be dedicated or shared. MAC layer configures the mapping of logical channels into the physical channels.
Between the GERAN and the core network, signalling protocol used is BSSGP, based on the SS7 signalling stack. BSSGP provides buffering and mapping of parameters between the RLC/MAC layers and the BSSGP layer as a relay function. A network service (IP or Frame Relay) transports the BSSGP layer PDUs. Another option is to interface the core network in the Iu mode.
For the packet switched domain control plane, the protocol stack is shown in Figure 3.8.
Figure 3.8 2G packet switched domain control plane [12].
Figure 3.8 shows the GERAN with both Gb-mode and Iu mode interfaces. The initial 2G/GPRS (Gb-mode) protocols are indicated with a grey colour.
Between the GERAN and the core network, signalling protocol used is BSSGP, based on the SS7 signalling stack. BSSGP uses network service, which can be realized by Frame relay or by IP based stack. Another option is to interface to the core network in the Iu mode.
3.2.3 Abis
Abis interface consists of traffic channels (TCH) for transceivers (TRX) and signalling channels (LAPD channels). Traffic channels are used for voice and data (e.g. GPRS/EDGE). Logical channels include traffic channels (TCHs), intended for encoded speech of full rate (TCH/F) or half rate (TCH/H), or CS data. TCHs are bi-directional. Packet Data Traffic channels (PDTCHs) carry user data. Additionally there are channels for control traffic.
The capacity of one traffic channel depends on the voice codec used (Full-rate, half-rate) and on the modulation and coding scheme (MCS) used for the data traffic. Traffic channel capacities are 8, 16 or 64 kbit/s. Multiple traffic channels may be combined to realize higher data rates.
Signalling channels on the Abis interface include both BSC-BTS signalling, and BSC-MS signalling. Signalling channel capacities are 16, 32 or 64 kbit/s.
Transport on the Abis is TDM between the BTS and the BSC, referring to ITU-T Blue Book definitions of G.703 for physical and electrical characteristics. On the Abis, traffic is mapped into timeslots (64 kbit/s) or sub-timeslots (8, 16, 32 kbit/s). An E1 interface consists of 32 timeslots, a T1/JT1 interface of 24 timeslots, each of 64 kbit/s capacity. One 64 kbit/s timeslot is further divided into four subtimeslots of 16 kbit/s. In the TDM based Abis network, traffic can be optimized by multiplexing traffic, even down to a 8 kbit/s level using TDM multiplexers. With GPRS and EDGE, the Abis definition remains the same. There is no evolution of Abis in the 3GPP for an IP based transport.
Native TDM circuits may be used, or the whole E1/T1 frame may be carried over a packet network using circuit emulation. Vendor specific IP Abis solutions exist, where Abis traffic content is carried over the IP protocol (without emulating the Abis). IP based Abis is not specified in 3GPP. Existing TDM based Abis may be emulated over a packet network e.g. by using a cell site gateway.
Especially with GPRS/EDGE data traffic, TDM based Abis is inefficient, since the timeslots are reserved constantly for each user. To improve the utilization and Abis capacity, timeslots may be allocated dynamically from a pool so that data users share these pooled timeslot resources. In this type of implementation, timeslots are allocated to a user only during a period of time, after which another user gets served. This improves the efficiency of the Abis, as otherwise resources would be reserved also during the idle times of a data session. Solutions for the pooling and flexible resource allocation differ between implementations, as this is not covered in 3GPP.
Inefficiency exists also with voice traffic over the TDM based Abis. Variable rate voice codecs do not transmit at a constant rate. However at Abis a constant rate (a subtimeslot) is consumed. With IP based implementation of Abis this can be addressed.
From a QoS viewpoint the system is simple with a native TDM network. All the traffic channels and traffic types are treated identically. All timeslots pass through and there is no congestion after the traffic has been admitted to the system. Blocking occurs if there are no free timeslots available in the Abis, which is not common, since all traffic channels of the air interface (radio timeslots) are directly mapped to Abis timeslots.
With dynamic or flexible Abis optimization the mapping changes dynamically and the Abis usage becomes more flexible. Still there is no need to differentiate traffic either as all timeslots are transmitted without loss or excessive delay.