Written by: Per Ola Andersson, Håkan Asp, Aldo Bolle, Harry Leino, Peter Seybolt and Richard Swardh

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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.

Background
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 micro-wave 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.