PDF of this tutorialRemote DSLAMs are exactly what the name implies. The CO–based DSLAM is environmentally hardened and placed inside a cabinet, which can then be installed in the field, typically close to the existing DLC cabinet. From there, the remote DSLAM negotiates the transmission of ADSL data traffic between the ADSL CPE modems and a WAN. Remote DSLAMs also can be rack mounted in huts or controlled environment vaults (CEVs).
Advantages
Remote DSLAMs are useful for serving large numbers of ADSL subscribers. They scale easily, usually by adding line cards into a chassis. For instance, a typical remote DSLAM can serve 60 to 100 ADSL lines.
Because element management of a remote DSLAM used in the provisioning and monitoring of ADSL lines is similar to that of a CO–based DSLAM, it requires no additional management systems or training for network-operations personnel.
Remote DSLAMs can be used with any DLC system with no impact on POTS service because remote DSLAMs are independent from the deployed DLC system. The remote DSLAM simply splits POTS traffic off and sends it back to the DLC while in its analog form.
Disadvantages
A remote DSLAM can be an expensive solution. Because the remote DSLAM is external to existing DLC cabinets, installation requires that the network-access provider obtain a right of way, pour concrete for a pad, install the cabinet, power the electronics, and deploy wiring to and from the existing DLC. As a result, the initial investment in both time and money is quite significant. Although this investment can easily be amortized over a large number of subscribers, the fact that ADSL is in its infancy means that initial take rates typically cannot cost justify this level of expense. Furthermore, in small-line-size DLC environments where the potential number of subscribers is limited, a remote DSLAM may never be cost justified.
Remote DSLAMs also can impose significant problems concerning the size and configuration of cross-connect boxes. Typically, a network-access provider places one or more cross-connect boxes close to the DLC cabinet where all the subscriber tip-ring pairs are cross-connected to the tip-ring pairs going to the remote terminal cabinet. Because ADSL service can ride over the same pair of copper wires as POTS service, rerouting at least some of the pairs is required. Specifically, the pairs carrying ADSL/POTS traffic must be routed to the remote DSLAM where the POTS and ADSL signals are split. The POTS traffic must then be routed back to the cross-connect for connection to the DLC cabinet.
Figure 4. Cross-Connects Before and After Remote DSLAM Deployment
A problem often arises with these cross-connect configurations because cross-connect boxes are usually designed to support the number of pairs the DLC supports, with only limited spares. Thus, with the additional cross-connections needed to support the remote DSLAM, it may be necessary to add cross-connects or resize the existing ones. The situation is compounded further in cases where remote terminals have incorporated the use of multiple cross-connect boxes, because there is no way to forecast realistically which subscribers will want to add ADSL services.
Therefore, although remote DSLAMs offer the greatest flexibility for the largest number of subscribers, they also require considerable up-front financial and engineering commitment if they are to be seriously pursued as an option.
