Thursday, August 19, 2010

The access network equation

My last entry provoked several emails on the subject of the terms last/first mile vs. access networks. While answering these emails I found it useful to bring in an additional term – the backhaul network. Since these discussions took place elsewhere, I thought it would be best to summarize my explanation here.

Everyone knows what a LAN is and what a core network is. Simply put, the access network sits between the LAN or user and the core. For example, when a user connects a home or office LAN to the Internet via a DSL link, we have a LAN communicating over an access network with the Internet core. Similarly, when a smartphone user browses the Internet over the air interface to a neighboring cellsite, the phone connects over an access network to the Internet core.

However, the access network itself naturally divides into two segments, based on fundamental physical constraints. In the first example the DSL link can’t extend further than a few kilometers, due to the electrical properties of twisted copper pairs. In the second case when the user strays from the cell served by the base-station, the connection is reassigned to a neighboring cell, due to electromagnetic properties of radio waves. Such distance-limited media are the last mile (or first mile if you prefer).

DSLAMs and base-stations are examples of first aggregation points; they terminate last mile segments from multiple users and connect them to the core network. Since the physical constraints compel the first aggregation point to be physically close to its end-users, it will usually be physically remote from the core network. So an additional backhaul segment is needed to connect the first aggregation point to the core. Sometimes additional second aggregation points are used to aggregate multiple first aggregation points, and so on. In any case, we label the set of backhaul links and associated network elements the backhaul network.

We can sum this discussion up in a single equation:
* access network = last mile + backhaul network

I’ll discuss the consequences of this equation in future blog entries.