Backhaul

Backhaul refers to the connection from an access point or base station back to a central office
facility. In a broadband wireless network, backhaul, ironically enough, generally occurs over
wireline connections, although wireless links are perfectly possible as well.
Obtaining backhaul connections at reasonable rates is essential if the network is to operate
profitably, particularly as the network expands and more and more backhaul is required.
For this reason the operator must determine the means of obtaining backhaul for given locations
before a single network element is put in place. One simply cannot build first and then
start casting about for backhaul solutions.
Backhaul itself is a rather complex subject. Several distinct physical media have been used
for providing it, and a number of different arrangements are possible with companies offering
backhaul capacity. One needs to explore all options during the planning stage in order to
make certain that the essential backhaul component is being obtained in the most costeffective
manner.
Backhaul itself represents the aggregation of network traffic, the sum of each transmission
to or from a base station and an individual subscriber node. For this reason the capacity of the
backhaul must be significantly greater than the capacities of the individual access airlinks.
Nevertheless, the capacity need not and should not equal that which would be required if all
subscribers in a cell were transmitting simultaneously, because such an eventuality is highly
unlikely. The capacity of the backhaul can be as little as a tenth of the aggregate capacity of all
individual airlinks, though a four-to-one or six-to-one ratio is more prudent. If, for instance,
100 customers are each provided with a 10 megabits per second (Mbps) connection within a
single cell, then the combined capacity of all of the individual airlinks is 1 gigabits per second
(Gbps). A 100Mbps backhaul could suffice in that situation, though 250Mbps would be ideal.
Quasi-Backhaul
It should be noted here that there are a couple of ways to set up wireless broadband networks
requiring little or no backhaul. These involve two variant network architectures, the mesh network
and the ring or point-to-consecutive-point network, both of which will be discussed at
further length later in this chapter. In a mesh, no real distinction exists between a base station and subscriber premises terminal.
Instead of each subscriber terminal transmitting back to a base station, which itself would
define a cell, mesh terminals transmit to one another, passing the signal along until it arrives at
its final destination at the central office. Each terminal will normally be endowed with some
intelligence and will include a small router and/or switch for determining which adjacent node
in the network will be selected as the intermediate destination for a message. The terminal will
be able to choose among all the other subscriber terminals within reach and will normally
compute a path that is least congested. If one of the terminals is malfunctioning, the transmitting
terminal will simply route around it. A wireless mesh is a form of peer-to-peer network, but
the peering takes place on the physical layer as well as layers two and three of the network
stack. In effect, the mesh provides its own backhaul.
To paraphrase George Orwell, in a mesh all nodes are equal, but one node is more equal
than the others, and that is known as the seed. The seed is the first node installed and communicates
directly with the central office equipment and through it with the PSTN and the public
Internet. In networks where subscribers are few and scattered, more than one seed may be
required.
The Internet itself takes the form of a logical mesh and stands as the prime exemplar of the
robustness and essential soundness of the mesh concept; reflecting on the explosive growth of
the Internet, some equipment developers familiar with its history have assumed a certain inevitability
to the mesh concept in the wireless domain as well. And perhaps such inevitability is
real. Still, mesh equipment to date has achieved little success in the marketplace, and most of
the more than 20 manufacturers that have developed it have been forced to shut their doors.
I suspect that price has been the major issue. Putting a router or switch at every terminal
cannot but increase the cost the subscriber premises package, though it should make for a lessexpensive
central office installation. Against that price must be weighed the price of backhaul,
which of course is highly variable. If backhaul providers are determined to gouge the wireless
broadband operator, then opting for a mesh configuration is a counter move. But one should
remember that there are other ways to implement wireless backhaul where the wireless broadband
operator still retains ownership of the backhaul channel, so the mesh must not be
regarded as the sole solution.
Ring architectures, the second means of eliminating the backhaul, are equivalent to
daisy chains or buses. Each node connects to two other nodes, and the whole assemblage
forms a closed ring. Rings tend to be impractical when a large number of subscriber nodes
are involved and are much less robust than meshes; on the other hand, they require less
network intelligence. Nevertheless, they are similarly expensive to implement because of the
presence of routers, switches, or add-drop multiplexers in the subscriber terminals. Much
of the point-to-point microwave equipment designed for mobile backhaul supports pointto-
consecutive-point architectures as well, and one should opt for this approach when
purchasing equipment.