White Paper

Net to Net Technologies Page 1 of 6 Author – Charlie Hampson champson@nettonet.com

Positioning the BRAS in the Next Generation Access Network Traditionally, ATM has been used in access networks with PVC’s providing connectivity from the CPE device through the DSLAM to the Broadband Remote Access Server (BRAS). This design worked well enough in the early days when the primary if not sole application was lowest cost Internet browsing, but with the reality of next generation services this model is now struggling to deliver the bandwidth and scalability required by these new applications.

Every competitive carrier and incumbent needs to deploy a network that is capable of delivering these new services, more specifically networks able to support the triple play of services - Voice, Video and Data - if they are to compete in the new market - it is no longer possible to profitably deliver simple Internet access.

Positioning the BRAS in the Next Generation Access Network.................1

Figure 1. Traditional ATM DSL Access Network .......................................2 The Problem with the BRAS in the Core ...........................................................2 Moving the BRAS to the Edge...........................................................................3

Figure 2. Distributed mini-BRAS architecture............................................4 Migrating to Native IP ........................................................................................4

Figure 3. Next Generation Ethernet Access Network................................5

Figure 1 shows the traditional way of connecting subscribers to the network, using ATM edge switches to aggregate the PVC's and an ATM based BRAS to terminate all the PVC’s to an IP router or forward the sessions on to the service provider. Note that the service provider may not necessarily own the access network, as in the wholesale model common to many incumbent operators.

White Paper

Net to Net Technologies Author – Charlie Hampson champson@nettone

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The Problem with the BRAS in the CoreThe main problem with adding triple-play servicescalability. The scalability issues are mainly down

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• Secondly, most triple-play video services use channels over the network. The ATM based aoriented so in addition to their serviced PVCs via their own PVC all the way through to the B

Any multicast IP traffic will have to be distributeif all the users on a particular DSLAM are watchirequire a separate stream connected through theWith video streams typically running at 3-5 Mbsoon add up and become unsustainable.

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White Paper

Net to Net Technologies Page 3 of 6 Author – Charlie Hampson champson@nettonet.com

For example, if a DSLAM has 500 ports and every user is watching a TV channel then a massive 2.5 Gbps of bandwidth will be required on the backbone just to connect up this single DSLAM. It is clear from these figures that if triple play services are to be viable, a multicast protocol will need to be used in the access network to reduce the bandwidth requirements. Although the ATM standards do allow for the use of an ATM based multicast, this has never really been an option mainly due to the extreme complexity and high costs involved. IP is the obvious choice for multicast video distribution and this technology has been tried and tested for some time. Various solutions may be implemented in the access network including Gigabit Switch/Routers, MPLS etc; whichever option is chosen it should support IP multicast.

Using these IP multicast based solutions we can resolve the issues of multicast support in the access/metro network, but there still remains the problem of the ATM based DSLAMs out at the central office.

Moving the BRAS to the Edge Equipment vendors have invested much time and effort in developing these platforms and don’t wish to abandon them just yet. In response to this problem many DSLAM vendors are attempting to move the BRAS functionality closer to the DSLAM, in some cases integrating the BRAS into the DSLAM itself. This solution allows a migration to an Ethernet or IP based core network as the ATM PVCs from the subscriber are terminated near the DSLAM in a mini-BRAS.

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White Paper

Net to Net TechnologieAuthor – Charlie Hamp

of a connection oriented protocol (ATM) has always been problematic, and implementing multicast protocols on top of this presents an even greater challenge.

By using pure IP based DSLAMs it is possible to handle triple play services including multicast traffic with little or no configuration and leverage the inherent scalability of Ethernet.

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White Paper

Net to Net Technologies Page 6 of 6 Author – Charlie Hampson champson@nettonet.com

In the DSL access network, users are connected permanently to telephone lines, so there is not the same requirement for authentication but it may be useful to assign certain addresses or attributes depending on the user connecting. This is possible using DHCP with option 82. When the client connects to the network, the DSLAM will add a tag onto any DHCP requests which identify the DSLAM, slot number and port number of the connected customer. This tag can then be used by the back office systems to assign particular IP addresses or limit the number of addresses assigned etc.

By using Ethernet in the DSLAM and the access network, new PoP’s may be deployed faster than ever before.

Native Ethernet IP DSLAMs such as the Net to Net Technologies IP DSLAMs require no configuration to start passing traffic and can easily be configured to support new services as they are added.

By using an Ethernet based backplane, new technologies may be deployed in existing chassis without having to do a forklift upgrade. This results in a lower initial investment, lower cost of ownership and ability to deploy new services in the shortest timeframe.

In fact, using the new BSX8000-5 Broadband Service Switch, the DSLAM may replace a locally deployed gigabit capable Ethernet access switch and provide GbE connectivity to other equipment colocated in the PoP – simplifying the deployment of network elements and providing integrated management.