Voice over 802.11

 
Voice over 802.11
The greatest test 802.11 will have to face when being compared to the Public Switched Telephone
Network (PSTN) is its capability to transmit voice with the clarity of the PSTN. The immediate assumption
is that the quality of service (QoS) of an 802.11 network is inadequate for voice transmission. As
demonstrated in the previous chapter, QoS on 802.11 networks can be engineered to equal that of wired
networks. Its wide acceptance in enterprise networks is testimony to this. This chapter explores how
voice becomes data in voice over Internet Protocol (VoIP) and that 802.11 is wireless IP; therefore, VoIP
can travel over 802.11, delivering voice with a quality equal to that of the PSTN.
The threat to telephone companies is that it is infinitely cheaper to beam data (and voice) to customers
than it is to run a copper wire or coaxial cable to them. In addition, the potential data flow to a subscriber
over an 802.11 network is exponentially greater than that of the 56 Kbps delivered via a telco's copperwire
dial-up connection. The emergence of softswitch as a switching alternative to Class 4 and Class 5
switches makes it all the more feasible for 802.11 service providers to offer voice services independent of
the telephone company or for subscribers (especially enterprises) to be their own telephone company,
effectively bypassing the PSTN entirely.
PSTN Architecture
The PSTN, over which the vast majority of the voice traffic in North America travels, is comprised of three
elements. The first is transport, or the transportation of conversation from one central office to another.
The second is a telephone switch contained in the central office. A switch provides switching, or the
routing of calls, in the PSTN. Finally, access denotes the connection between the switch in the central
office and the subscriber's telephone or other telecommunications device. Figure 6-1 provides an
overview of this architecture.
Figure 6-1: The three components of the PSTN- access, switching, and transport
As illustrated in Figure 6-2, 802.11 is a form of access to a wider network (the PSTN, a corporate local
area network [LAN] or the wide area network [WAN], or the Internet). The Memorandum of Final
Judgment (MFJ) of 1984 opened transport to competition. The "bandwidth glut" at the time of this writing
has made transport relatively inexpensive. Softswitch technologies (such as IP private branch exchange
[PBX] or Class 4 and 5 replacements) offer a viable alternative to the switching facilities of the PSTN. The
Telecommunications Act of 1996 was to have opened the switching and access facilities of the PSTN to
competition. For a number of reasons, this has not happened. 802.11 presents a bypass technology of
the telco's copper-wire access.