Introduction
The dual trends of increasing data rate requirements of backhaul connections for wireless BTS (base station transceivers) and the decreasing cost of available Gigabit Ethernet connections available make IP/Ethernet backhauling a prime choice for new installments.
However, there are hundreds of thousands BTS deployed with TDM (E1/T1) connections as the data rate requirement for backhauling is increasing. For these BTS, carriers pay huge amounts for TDM leased lines. They are highly motivated to cost reduce backhauling by moving it to less expensive Ethernet pipes. Circuit emulation enables carriers to achieve such cost reductions by transporting TDM signals over an Ethernet network.
A major challenge in any circuit emulation service (CES) applications is clock recovery at the far end of the link. An interworking function (IWF) supports differential and adaptive clock recovery. But for more demanding applications, such as synchronization of wireless base stations, the hybrid timing generator can be applied.
The article examines the protocol details of CES and discusses the issues of clock recovery; describes network and device architectures of CES solutions. As well summarizing the benefits of FPGA based implementations.
Network trends
The continued growth in Internet usage and the ever-increasing number of applications demanding higher bandwidth are obvious trends that are forcing changes in today's telecom networks. The majority of networks currently in use by telecom operators are Time Division Multiplexing (TDM) based.
These networks were originally used for transporting voice traffic and for leased lines applications. But in the last few years data traffic on these networks has become dominant. To better handle data traffic, operators are migrating these networks from TDM-based to packet-based. Some operators are planning the next generation, an all-packet or all-IP network that will replace these existing networks.
One of the primary challenges in today's telecom network designs is the smooth migration from a TDM-based network to a packet-based network. A key driver for this migration is to create a homogeneous network that uses packet technology in the core backbone, the metro and the access network.
This single network could provide major commercial benefits with reduced capital and operational expenditure.
While an all-IP network is optimal for data services, there is still a need to transport TDM services such as voice and leased lines. The TDM equipment in use by business or government can not and will not be replaced at the same time as network operators project their introduction of all-IP networks.
Therefore the all-IP network operator will need to add features that allow transport of TDM traffic over a packet network.
The application of circuit emulation services (CES) allows a smooth migration from TDM traffic to packet traffic and vice versa. Instead of using fixed bandwidth, TDM links per individual end-service; the TDM traffic is packetized and aggregated with other TDM or IP traffic onto a single network.
However, traditional TDM networks not only transport the data, but are also used to maintain network synchronization. Today's packet systems do not support network synchronization; as a result network operators have created overlay networks for synchronization or are being forced to use expensive, GPS-based, reference clocks.
Impact on wireless networks
As the demand for new data services on wireless networks grows, the strain on supporting the backhaul network is becoming clearly visible. A typical radio base station (RBS) can support a few hundred voice calls in parallel.
On the backhaul link to a mobile switching center (MSC), a bandwidth of about four E1 lines or a single T1 line is reserved per RBS. While a moderate mobile data connection today can provide 384 kb/s, the next generation networks already offer downlink speeds ranging from 1.8 to 14.4 Mb/s (UMTS HSDPA phase 1).
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