Xenon Remote Switching Architecture
Xenon will be designed to support two key changes in the switch design:
- Flexible Remote Switching
- Standard Interfaces between the Remote Unit and the Parent
Both the requirements are addressed to a large degree by a standard protocol (V5.2) that was developed by ETSI. We will be discussing this protocol in the following section.
V5.2 Remote Switching
V5.2 supports a very flexible Remote Switching architecture. The switching functionality is divided into two components, the LE (Local Exchange) and AN (Access Network). The LE takes care of the switching functionality and interfacing with other switches, while the AN takes care of the subscriber side connectivity. The V5.2 specifications define a protocol between the LE and ANs. Since a standard protocol is being used here, the LE and AN could have been obtained from different vendors. In fact, the ANs could connect to subscribers in totally different ways. The ANs can use land-line or radio interface to the subscribers.
The flexibility of the V5.2 Remote Switching architecture is shown in the figure above. There are three ANs shown above and each has different traffic characteristics:
Residential Area (AN2) : AN2 is connected by just three PCM trunks to the LE but has a very large number of subscribers. This type of configuration would be suitable for residential areas, where the traffic generated by each subscriber is small.
Business Area (AN3) : AN3 is handling very high traffic from the subscribers connected to it. Thus AN3 needs more PCM trunks to the LE.
Mixed (AN1) : AN1 represents a mix of residential and business subscribers.
If we compare this architecture with client-server machines, LE is more like a switching server and the AN are switching clients. LE provides centralized control over the system while the AN handle the distributed subscriber interface.
The V5.2 protocol groups subscribers and the PCM trunks connecting them to the LE as Interfaces. Each V5.2 Interface defines a group of trunks and subscriber ports. Message communication for the Interface takes place over control channels defined for the interface. Generally, a primary and a secondary control channel are defined for communication.
The figure below shows an AN connected to the LE by two interfaces, Interface1 and Interface2. Both the interfaces have a primary and a secondary control channel. Each control channel takes one timeslot on the digital trunk. Thus only 30 calls can be established on a trunk carrying a control channel, while 31 calls can be established on a trunk with no control channels.
All message communication between interfaces uses an Interface Id as a reference number. The Interface Id is a 24 bit number which is configured along with the interface definition on the AN and the LE. As a part of the interface definition the trunks Link numbers and the location of the control slot are also specified.