WiMax Operator's Manual

132 CHAPTER 6 ■ BEYOND ACCESS
IP in its basic form is a pure packet protocol, which requires using a router or packet
switch. This is a device that transmits packets (that is, discrete and discontinuous sequences of
data) as capacity becomes available in the network. During periods of congestion, packets are
held in buffers until an opportunity to transmit presents itself. Packets themselves are of variable
length, and the number of bits in each packet will vary according to network conditions.
By packetizing a signal rather than transmitting it in a continuous stream over an open circuit,
the router tends to use bandwidth much more efficiently than a circuit switch. A switch
must hold a circuit open even when a delay in the data stream occurs, while a packet router will
allow other traffic to occupy the open bandwidth. Such mingling of various messages within
the same channel tends to degrade transmissions that require the maintenance of precise timing
such as voice and particularly video, however, and this is the penalty that packet networks
customarily exact upon the user.
Each packet is provided with an address to enable the router receiving the transmission to
sort out the various messages. This address is quite distinct from the uniform resource locator
(URL), or Web address, and is normally unseen by the subscriber. In essence, it is the province
of the router and is utilized to plot routing paths. Since the supply of Internet addresses provided
by the dominant IP 4 is not inexhaustible, the usual practice is for a broadband network
to obtain a few permanent IP addresses, which interface with the outside world, and rely on
internal addresses to communicate with subscribers from the base station. The permanent IP
addresses are also known as global addresses because the entire universe of Internet routers
can see them, and the internal addresses are called local addresses. The outside party communicating
with the subscriber ordinarily sees only the permanent address and not the address
assigned to the individual user. Network address translation (NAT) from a global to a local
address takes place within a router maintained in the central office.
In some cases, particularly important customers such as large enterprises or government
agencies will be assigned permanent IP addresses, and indeed many will insist on this prerogative.
The wireless network operator planning to solicit such customers should be aware of
their requirements in this regard and should obtain a sufficient number of addresses to accommodate
them.
Another method of dealing with a shortage of Internet addresses is Dynamic Host Control
Protocol (DHCP), which distributes addresses on a dynamic basis among subscribers. The
whole process takes place transparently and automatically. A global Internet address is
“leased” to a user for a predetermined length of time, which could be as short as the trans-
mission itself or weeks or months. DHCP is really a form of oversubscription, or statistical
multiplexing, enabling the network operator to get by with less than a single address per user
on the theory that all users will never be online simultaneously.
DHCP is usually administered from a separate server, not from the router itself, and the
addresses themselves will normally be assigned to enterprise users who would probably own
internal routers of their own.
Incidentally, IP 6, which has not been widely adopted yet in the United States, has an
address space sufficiently wide enough that no shortage of addresses is anticipated even if separate
devices within the network such as computer peripherals and smart appliances are
assigned their own addresses. Whether the computing community at large will ever embrace
IP 6 remains to be seen.