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Synchronous digital hierarchy (SDH) technology is widely used for data communications. It is responsible for accurately timed transmission and reception of data bit at very fast rates. SDH technology usually uses wired or wireless media, such as digital microwave, and optical media, such as optical fiber.
By using synchronous data transmission, SDH technology makes it possible to transmit and receive huge amounts of bits because the bit rate very high and the bits are accurately timed, or synchronous. The wide networks of global system for mobile communications (GSM) service have very good performance because of the synchronous nature of SDH technology. If synchronous transmissions were not employed, GSM calls would drop out too frequently.
SDH technology allows the wired or wireless receiver to synchronize on an incoming data stream. This makes it possible to propagate a timing reference down the network. In a cluster of SDH sites, a master clock will have a very accurate timing source. One good source of global bit timing is global positioning system (GPS) satellites, which are able to provide accurate timing reference for data bits.
For wireless transmission, the digital microwave equipment is very popular for hops about 5 km (3.11 miles) to about 25 km (15.43 miles). Typical digital microwave equipment may carry four or eight E1 links, a European standard 32-channel carrier. One, E1 carries 2,048 kilobits per second (kbps), so an E1 link carries 32 x 64.
In GSM networks, each cell site may need up to eight channels on an E1 carrier. There are 32 channels, so there will be about four channels allocated to each cell site. In order to share one E1, there is a need for an add/drop multiplexer (ADM). The ADM provides the channels or timeslots while also supplying the channels from the audio derived from subscribers from the site.
For the above GSM network, there will be four cell sites being supported by one E1 carrier. The first site will manage four channels and leave the rest of the channels for the other sites to add signal and drop signal into. Meanwhile, all the cell sites in hierarchical structure will need to be synchronized all the way to the mobile switching office (MSO). The term "backhaul” refers to the entire set of digital signals that need to be hauled back to the “switch”, or MSO, where these signals are either returned to the local network of cell sites or passed into another network via a switch called the gateway MSO.