General architecture of multiservice networks

• access

• aggregation

• edge, sometimes called distribution

• core

In the following diagram, this architecture is shown in layers:

Figure 1– Multiservice network architecture

The DSLAM Ethernet can be found in the access layer, marking the limit between the service provider’s network and the subscriber. Typically, the DSLAMs are located at the same sites as the external plant’s telephone switches and hubs.

The aggregation layer is an intermediate layer that makes it possible to escalate the number of access nodes connected to the edge. The aggregation of these nodes is made with switches/routers, using different topologies and technologies. Loop and hub-spoke aggregation topology options are shown below. In both cases, a dual homed scheme is used, that is, one in which each aggregation node can gain access to two edge or distribution nodes. This layer also uses topologies where the connection is to one single edge node, but in these cases, there is greater dependence on the availability of this node, and a breakdown in it may be critical if the amount of aggregate services is substantial.

Figure 2– Aggregation Topologies

The edge or distribution layer marks the limit between the L2 Ethernet transport up to the access and the upper L3 layers that provide 3Play services.

As we mentioned earlier, there are different technologies between the aggregation and edge layers.

Historically, ATM was used when the DSLAMs were ATM and there were no multicast transport requirements as is the case for IPTV. With the advent of the DSLAM with the Ethernet uplink, the new aggregation networks have also migrated to this technology.  At present, three trends in technology for aggregation are being envisaged:

Figure 3 – Aggregation Technologies

1)     Add to level 3, with IP and optionally with MPLS also. In general, this alternative involves higher associated costs because equipment with greater functionality is required precisely in the aggregation layer, which is the one that must have the greater mobility at the network level, generally consisting of a high number of nodes.

2)     Add to level 2, purely in Ethernet.  This alternative is based on the use of VLANs and Q-in-Q (with the Provider Bridging IEEE 802.1ad standard, also known as PB, QiQ or stacked VLANs). It involves lower costs, but has problems because of the complexity of managing the VLANs and scalability. At present, work is being done on a new standard that would address the problem of scalability; it is called Provider Backbone Bridging (IEEE 802.1ah, also known as MAC in MAC or MiM or PBB) and the OAM-related issues are tackled by the application of the IEEE 801.1ag standard.

3)     Add to level 2, using MPLS.  This alternative involves an intermediate cost and is based on the use of EoMPLS (also known as pseudowires—PW) and VPLS. It has a high potential for scalability and, furthermore, permits QoS facilities and enhances recovery times in case protocols such as Fast Reroute are used.

In the edge layer, there are service gateways generically referred to as Broadband Network Gateway (BNG).

In the case of access to Internet in the PPPoE modality, the gateway controlling the sessions, authorization, and authentication is called Broadband Remote Access Server (BRAS).

In the case of video, in general there is a dedicated gateway that controls access.  In some architectures, in particular when the Internet access model is with DHCP, there can be a single gateway that controls both services (video and Internet).

Below we can see the Triple Play architecture recommended by the DSL Forum,[i] indicating where this group’s most relevant work on this topic is being applied (WT-135, WT-126, and TR-101).

Figure 4 – Architecture according to the DSL Forum

Finally, we have the core layer whose function is basically to carry out large-scale routing functions and to bring together nationwide the various aggregation and distribution domains.  In this layer, there is no definition of services or users as in the previous layers.

Laura Saldanha
Gerente de Sector de Transporte y Sistemas Integrados
ANTEL-Uruguay