WiMax Operator's Manual

72 CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE
limit their number. By obliging commercial operators to collocate on few approved sites, that
proliferation has been checked, and, coincidentally, a new industry has emerged.
Broadband wireless operators can certainly elect to occupy such a site, but they may not
necessarily be welcome or able to afford the lease if they are. Tower owners do not charge uniform
rates, but a rough average is $1,000 a month. This is nothing to a cellular operator with
tens of thousands of customers, but for a struggling wireless broadband operator it is a sig-
nificant expenditure especially when that operator is expected to sign a 30-year lease in the
bargain. Another issue with towers is that they are primarily intended for hosting lowerfrequency
mobile services rather than networks operating above 2GHz, and they may not
provide optimal elevation since the mobile services lack line-of-sight requirements.
If the broadband wireless network operators are disinclined or unable to gain access to a
tower, they can always attempt to negotiate roof rights on a multistory building. Here it is difficult
to generalize, because rates can vary tremendously. In some instances, if the network
operator is willing to provide data services to the tenants of the structure in question, recurrent
fees may be reduced or even waived.
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.