A Virtual Leased Line (VLL) service is a point-to-point data communication using ADSL network (over ATM) for communicating any data between two sites. The data speed of VLL depends on customer requirements such as 64, 128, 256, 512 Kbps.

In actual network, multiple users are connected to DSLAM ; furthermore, the service provider assigns the VLL ratio for bandwidth sharing, when each customer sends data simultaneously. This is a disadvantage of VLL service when compared with Leased line (LL) service. For the latter service, it can use a full bandwidth which is not shared with other customers. The benefit of a VLL service is that it is cheaper than a LL service.

The basic VLL configuration is shown in the diagram below.

PC-A –> ADSL Router-A –> DSLAM-A –> ATM Switch-A —>ATM-Switch-B –> DSLAM-B –> ADSL Router-B –> PC-B

From this diagram, the VLL configuration is divided into two sites : the provider site and the customer site. In the service provider, there are two DSLAMs (Digital Subscriber Line Access Multiplexer) and two ATM switches for transmitting and receiving any data traffic between two customer sites. DSLAM is used for combining data signal from multiple customers and then sends data to ATM switch. After receiving data from DSLAM, ATM switch searches the best route automatically to the other site by only configuring VPI / VCI parameters at the DSLAM connection and ATM switch connection. At DSLAM's destination side (closing to ATM-SW), the VPI / VCI of DSLAM-A must be the same as ATM SW-A's, and the VPI/VCI of DSLAM-B must be the same as ATM SW-B's.

In each customer site, there are an ADSL Router and work stations (a server, a PC, or other LAN equipment). Customers must configure their ADSL routers in VLL mode by setting the connection type to “RFC1483 Route”. At DSLAM's source side (closing to ADSL Router), the VPI / VCI of DSLAM-A must be the same as ADSL Router-A's, and the VPI/VCI of DSLAM-B must be the same as ADSL Router-B's. These parameter values depend on each telecommunication provider's assignment.

Source by Pitipong Prapamonthol