Exchange domain

Written on 5:10 PM by ooe

In the exchange the copper pairs from the users are terminated on a main distribution frame (MDF). This is a flexibility point for copper pairs. From there, the pairs go to the filters that are located with the DSLAM. The filter performs the same function as filters in the user's premises in that is separates the data frequencies from the POTS frequencies. POTS traffic is then returned to the MDF where it is jumpered to a line going into the narrowband remote concentrator unit (RCU) then into the rest of the telephone network.

The data traffic extracted by the splitter is decoded by a modem in the DSLAM, before being aggregated onto the link to the ATM network (each line having its own modem). The connection to the ATM network is an STM-1 connection (155 Mbit/s SDH connection). The traffic is carried within the STM-1 connection using the ATM cell structure that specifies virtual paths (VPs), virtual circuits (VCs) and classes of service, as is traffic from the users.

There will be a number of virtual paths (VPs) established from the DSLAM across the ATM network on the aggregate connection. The actual number depends upon the number and type of services supported on that DSLAM. Each service supported has at least one VP, and the ability to add additional services or additional VPs for an existing service depends upon the remaining capacity within the STM-1 aggregate connection. In all cases, the user has a single VP to and from the DSLAM through which all traffic is carried.

On the aggregate connection there will be at least one IPStream VP established to carry IPStream traffic to the RAS. Depending upon the number of DataStream service providers offering service from that DSLAM, there may be many VPs or none at all. Each DataStream VP would be connected to their service provider over the ATM network. As with DataStream, the number of VideoStream VP established depends upon the number of service providers offering service from that DSLAM. The only difference is that the VideoStream product requires a pair of VPs to support the service.

With the two sides of the DSLAM defined, it is therefore necessary for the DSLAM to cross connect traffic from the user's VCs within their VP to appropriate VCs within the correct VP on the aggregate connection.

For IPStream and DataStream the user has a single bi-directional VC within the VP. The cross connect is established at the time service is provided by selecting the next available VC within the appropriate aggregate connection VP for the service requested. All traffic for IPStream and DataStream is classed as variable bit rate (VBR).

For VideoStream, the user has a number of VCs for different types of traffic. One downstream for the video, one bi-directional for the passing of control signals, and one upstream ‘boot’ channel. The control and boot traffic is classed as VBR, but the video is classed as constant bit rate (CBR).

The size of the VP on the aggregate connection is determined by the number of users desired to connect within the VP and the contention ration. The principle of contention ratios applies to both the IPStream and DataStream products. For a 20:1 contention ration the size of the VP is 1/20th of the sum of the end user rates for the number of desired users.

For VideoStream the number of simultaneous channels required to be transmitted from the service provider determines the required size of the VP to the service provider. Where the number of channels capable of simultaneous transmission is less then the total number of users connected there is a need to manage this overbooking. In this situation all cross connections are established, but they need to prevent transmitting on more than the defined maximum. Through signalling with the user the service provider manages this overbooked situation.

The ATM network is used to transport the user traffic to an appropriate exit point. For ATM-based services of VideoStream and DataStream, the data is carried in the VPs as described above to the service provider interface. For IPStream the VP is terminated at a convenient remote access server (RAS).