GSM transport evolution
The success of GSM to date has been nothing short of sensational. What is more, its future continues to hold great promise. However, two important challenges must yet be resolved to make GSM the mobile communications system for the “next billion users” and to guarantee the commercial success of its mobile data services: focus on low total cost of ownership (TCO), and deployment of enhanced GPRS (EGPRS) in every network. In addressing these challenges, GSM transport or backhaul constitutes one very interesting and dynamic area of development.
Transmission to radio base station (RBS) sites accounts for a large part of the cost of operating radio access networks. Where operators have built their own transport networks, transmission also represents substantial investment (Figure 1).
Traditional transmission from RBSs to a base station controller (BSC) is realized using static time-division multiplexing (TDM) circuits.
This technology has not been optimized for efficient transmission of bursty data traffic, however. Nor is it easily adapted to the low-cost transport technologies, such as IP and Ethernet, which are being rolled out over copper, fiber, and microwave networks.
The good news is that recent developments in GSM transport and switching technologies promise to overcome these issues, making GSM suitable to provide services both to low-revenue users and to users of high-speed data services.
Technical overview, Packet Abis
Packet Abis, Ericsson’s collective solution for these developments (Figure 2), includes a new protocol for transferring data over the Abis link. Simply put, from the air interface the solution sends all traffic (voice, data and signaling) in packets over Abis with a minimum of repacking and reformatting. The solution can be used over TDM transport networks as well as with IP or Ethernet transport services. Ericsson’s Packet Abis solution is built around two main features:
– Abis Optimization; and
– Abis over IP.
Low TCO using TDM transport networks
– Abis Optimization, which is Ericsson’s solution for delivering Packet Abis over TDM transport networks, saves bandwidth by solely transferring bits that contain data. All other bits (for example, those that were previously used to maintain a constant bit rate to fit the PDH channel format) are no longer inserted.
Ericsson has integrated the Abis Optimization solution into its base station subsystem (BSS). The Abis Optimization feature is based on a software upgrade of the RBS, and the introduction of a packet gateway (PGW) in the BSC, to terminate the Packet Abis protocol. To save even more bandwidth, one may add a site integration unit (SIU) at RBS sites to exploit statistical multiplexing gains between RBSs.
Dimensioning example, Abis Optimization
The number of E1/T1 links required per site with Abis Optimization depends on site configurations and traffic mix. In T1 markets, one RBS 2106/2206 with up to 12 TRXs can cover three sectors with only one T1. With “classic” Abis, two T1s would be required for this configuration.
In E1 markets, one E1 can support two RBSs with up to 18 TRXs. For standard base station configurations, this could translate into a savings of up to 50%.
Low TCO with Packet Abis using IP or Ethernet transport services
The Abis over IP solution enables operators to use IP and Ethernet transport networks to connect RBSs to the BSC and thereby benefit from the lower costs of IP – and Ethernet-based transport services.