WiMax Operator's Manual

138 CHAPTER 6 ■ BEYOND ACCESS
Telephone Switches
Telephone services were commonly offered by the first-generation millimeter wave wireless
operators but seldom to date by networks operating in the lower microwave regions. Chapter 7
covers voice telephony and the protocols and equipment required to implement it, but here I
will venture a few words as to how telephone switches figure into the central office environ-
ment.
In traditional local exchanges, the switch was the primary piece of hardware occupying
the central office. In broadband networks where the emphasis is on data, switches are of secondary
importance and are lacking in many networks.
As I stated earlier, I do not think provisioning circuit telephone services is a good business
for a wireless broadband operator to enter, particularly a startup with limited financial
resources. Class 5 circuit switches cost millions of dollars, and, considering that most wireless
broadband networks start small and remain fairly small, acquiring at most a few thousand subscribers,
paying millions of dollars for a device that is likely to yield a few dollars per subscriber
per month simply does not make economic sense—not in a highly competitive service environment
where infrastructure equipment has a far shorter life span than in the past. The full
return on investment for a class 5 switch would require decades, by which time it would be
completely obsolete. Also, given that telephone switches are not designed to interface with
radios and that complex procedures for mapping voice channels onto airlinks, providing dial
tone and ring tones, and assigning telephone numbers are required, the prospect of doing circuit
voice appears distinctly uninviting—except perhaps in developing nations in regions
where wireline telephone services are completely unavailable (such fixed wireless telephone
services are known as wireless local loop). Cable operators, for reasons best known to themselves,
have frequently offered circuit voice, and they have had to put in complete cable
telephony hardware and software platforms to enable such services as well as the aforementioned
class 5 switches. If anything, circuit over broadband wireless is even more problematic.
Softswitches
IP telephony is a different matter. Equipment costs thousands rather than millions of dollars,
and many manufacturers are already building broadband wireless radios with inherent support
for IP voice. Therefore, the barrier to entering the IP voice market is much lower than is the
case for traditional circuit voice. Still, it is a business not to be embraced without careful consideration
and one that demands hard choices in respect to basic infrastructure equipment.
Two fundamental equipment-related issues confront the broadband wireless operator
considering IP voice services. The first has to do with the choice of a platform, and the second
has to do with the scope and positioning of the services.
IP telephony today suffers not from a lack of standards but a plethora of them. H.323,
Universal Datagram Protocol (UDP), Session Initiation Protocol (SIP), Simple Object
Access Protocol (SOAP), Media Gateway Control Protocol (MGCP), and Media Gateway
Control (MEGACO), among others, vie for the allegiance of the IP voice carrier, and in no case
are these all supported on a single hardware platform. IP telephony also suffers from a diversity
of approaches in embodying those several standards in hardware. Often, though not always,
signaling and converting IP traffic into circuit traffic takes place within different devices,
and some manufacturers favor highly distributed architectures where numerous aggregation
boxes are scattered about the network. In the case of SIP-enabled networks, special telephone