The Role Of The DSLAM

We have now followed the path of data along the digital subscriber line from its commercial or residential source, via the local loop, to the CO. The local loop here terminates at the Main Distribution Frame (MDF), to be picked up by one of the CO's many DSL modems. If the form of DSL allows for the carrying of both analog and digital signals, a POTS splitter will separate out the signals. The analog signal will follow its time-honored path along the copper-wire infrastructure. For the digital signal, however, one step before the signal can be shot along to its destination.

The CO must now collect all the disparate digital signals from its modems and combine them into a single signal, via multiplexing. The aggregate signal then loads onto backbone switching equipment, traveling through an access network (AN) - also known as a Network Service Provider (NSP) - at speeds of up to 1 GBPS and emerging at a destination CO. At this point, the signal is then fragmented into its component parts and transmitted via telco modems to its final residential or commercial receivers. The device that performs these functions of signal combination and fragmentation is called the Digital Subscriber Line Access Multiplexer, or DSLAM.

The average DSL customer will never have to purchase a DSLAM. For the CO looking to make itself DSL-compatible, there are a number of features to consider in selecting which DSLAM best suits your needs and the needs of your subscribers. Alternatively, if you plan to enter the market of DSLAM manufacturers, these are features you should consider in constructing your product.

Chiefly, there is the question of multiservices support. As mentioned in the previous chapter, DSL technology is evolving at lightning speed. A DSLAM is a massive investment. To obtain the best value for your dollar, you should seek (or design) a system that allows for adaptation in the face of increasing application diversity. A similar concern is that of DSL code support: make sure your prospective DSLAM is flexible in the matter of line code and line protocol deployment. Remember that the newer flavors of DSL are particularly dependent on coding for proper transceiver functioning, and a good DSLAM should reflect this. Your DSLAM should also meet compatibility requirements for the various Network Management Systems (NMS) platforms, for better control and monitoring of performance.

Apart from these internal concerns, there are also two external, hardware-related issues that bear on DSLAM selection. The first is DSLAM line aggregation. The more DSL lines you can aggregate on a single output for network connection to a DSLAM, the greater the economy of space and scale. The greater the savings, the more cheaply you can supply your customers with DSL services and the more potential DSL subscribers you will have. Second is maintainability. The fastest, most flexible DSLAM in the world will fall to pieces rapidly if it is denied proper upkeep. To protect yourself against this, make sure your prospective DSLAM meets Network Equipment Building System (NEBS) standards of compliance.

Figure 3-1: DSL-based Services Reference Diagram

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