Posts Tagged ‘Convergence Sublayer’

Recently I was asked if WiMAX base stations come with MPLS support. This might have sounded an innocuous question for someone from an IP background. If you are from a wireless background like I am, it might sound a little strange. A WiMAX base station provides wireless connectivity at the physical layer. In particular, WiMAX provides last mile connectivity to the end user. It can also be used to provide point-to-point (PTP) backhaul links.

From this perspective, anything above the physical layer can run transparently on WiMAX. While this is so, WiMAX has defined Convergence Sublayers (CS) at its interface which can then be mapped correctly to 802.16 MAC layer before the packets are sent on the wireless channel. The supported CS Specifications include ATM and Packet (IPv4, IPv6, Ethernet, 802.1Q-VLAN) CS. So where does MPLS fit in, if at all?

MPLS is a technology that sits between Layer 2 and Layer 3. It can be seen to be outside the scope of a WiMAX base station and certainly outside the scope of the WiMAX standards. This is where we, as engineers, have to look at the whole thing from a deployment and operational angle.

Firstly, operators want MPLS because of the many advantages it offers. It leverages on both IP and Ethernet, technologies which are cheap and ubiquitous. It offers QoS. It provides multipoint connectivity but in a simpler way than IP. Its faster to switch at Layer 2 using labels than perform routing decisions at Layer 3. The attractiveness about MPLS in the coming years is that it is set to enable the move towards all-IP transport networks. When IP replaces TDM and ATM architectures, MPLS is set to play a major role.

So operators are interested in MPLS. Before they install new devices into their network, they want to know if it supports MPLS. The problem is that there is a clear distinction between core networks and access networks. MPLS is usually limited to the core. However, there has been significant push towards bringing MPLS to the access networks. This enables end-to-end traffic engineering, right up to the WiMAX base station. 3ROAM offers such a base station with MPLS built-in. Likewise, New Edge Networks is another company that is taking MPLS beyond the core to edge networks.

What if the WiMAX base station did not support MPLS? In this case, an MPLS-enabled network would terminate at an MPLS edge router (ingress or egress). This router would then be co-located and connected to the WiMAX base station. The problem for the operator in this situation would be to have a router for every base station. This is simply not cost-effective.

In general, WiMAX base stations operate in bridge mode (Layer 2) or routing mode (Layer 3). If a base station has to be MPLS-enabled, it has to work in Layer 3 mode. In other words, the base station doubles as an ingress/egress router. It does more than simply provide wireless connectivity.

Sprint has in its long-term roadmap this architecture in mind for backhauling of its WiMAX network. Sprint’s WiMAX base stations would be  MPLS-enabled and the backhaul between such a base station and its ASN Gateway would be IP over MPLS. One of Sprint’s providers for its WiMAX backhaul is Ciena which uses PBB-TE. This may very well carry IP over MPLS right up to the base station. The backhaul itself is wireless with equipment supplied by DragonWave.

To answer the question with which we started, WiMAX base stations may very well support MPLS for cost-effectiveness from operator’s point of view. Practically, it makes a difficult case since the MPLS-enabled network is likely to be from a different provider than the base station itself. Integration becomes an issue. It is well-known that managing an MPLS network is a challenge and requires a steep learning curve. Nonetheless, MPLS may be that small factor to choose one base station over another when an operator is unable to decide otherwise.

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