The wireless industry has become such a large user of satellite bandwidth and Earth stations that the cellular backhaul market consistently is rated as one of the best growth markets for satellite. While the business continues to evolve, it remains focused on the basic act of moving traffic from one cell site to another, but the technology behind an operator’s system can have an impact on the operations and the revenue the network produces.
Although the central theme of cellular backhaul is the transmittal of voice traffic, traffic patterns and volumes differ greatly from one cell site to another, which begs the question: Which satellite technology is best suited for the cellular backhaul market?
Architectural Overview of GSM Network
Wireless service providers around the globe primarily use GSM and CDMA architectures to deliver wireless services to subscribers. The majority of wireless carriers in the United States use CDMA technology, and their combined networks represent almost the entire global market share for this wireless technology. GSM, on the other hand, enjoys a far larger market penetration with approximately 85 percent global market share. Since there is ready access to fiber and microwave connectivity in the United States for backhauling traffic from cell sites, the majority of which are CDMA, the largest demand for satellite connectivity is from GSM carriers
The architecture of a GSM wireless system is hierarchal in nature, with the lowest level being a base station subsystem, the part of the network responsible for handling traffic and signaling between a mobile phone and the network switching subsystem that covers a geographic area known as a cell. Each base station subsystem has one or more base station controllers which communicates to one or more base transceiver stations (BTS) that typically control six to 10 transceivers, the hardware that controls the radio links between the subscribers handsets and the radio towers. The base station controllers aggregates the traffic for each cell and then communicates with the mobile switching center. In addition to routing calls to other wireless handsets and to the PSTN, the mobile service switching center maintains several databases, which keep track of subscriber’s phone numbers and which cell each handset in the network is communicating through.
An Abis interface is the connection between a BTS and a base station controller and is designed around the T1/E1 interface. One of the challenges to this interface is that it must transport an entire E1 frame (2.048 Mbps) from the BTS to the base station controller, even if the system supports only a small handful of voice channels. IP optimizers converts the E1 Abis interface to IP and then optimizes traffic by removing silence and unused channels from the traffic that must be sent, thereby reducing the amount of satellite bandwidth that is needed to support an individual BTS.
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