WiMax Operator's Manual

CHAPTER 6 ■ BEYOND ACCESS 133
Routers
Routers themselves come in several forms that are determined by the function of the device in
the network. Core routers are huge, costly devices placed at major Internet access points and
hubs and handle the enormous volumes of long-distance Internet traffic. Edge routers are
much smaller devices used in metropolitan access networks that constitute “the edge” from
the perspective of the long-distance service providers. Yet smaller routers are located within
enterprises, and the smallest of all are located in residences and small businesses. Most wireless
broadband base station controllers made today happen to incorporate edge routers. To
see how routers fit into a public network, refer to Figure 6-1.
Figure 6-1. Wireless network with router, courtesy of Alvarion
Routers at base stations are switches, but they frequently perform much more complex
functions than traditional class 5 and class 4 circuit telephone switches. Whereas circuit
switches simply follow instructions and send messages to the next node in a predetermined
network path, routers may be capable of making instantaneous decisions as to an overall route
through the Internet based on communications from other routers. Thus, one packet may take
a different route from another, and the packets may arrive at their final destination out of
sequence, whereupon they will be held in a queue until all the packets have arrived. Information
in the packet header will enable the destination router to assemble them in correct order.
Routers make traffic decisions based on routing tables that are continuously and automatically
updated in large core routers and in the edge routers utilized in metro networks. These
tables list locations of core routers across the Internet and the regions served by them, and they
function somewhat analogously to the numbering system used in the PSTN.
Dynamic routing of this sort customarily takes place in edge routers and core routers but
frequently not in enterprise and residential routers where static routing schemes tend to prevail.
The presumption is that the configuration of the local network will be a given and that
network traffic patterns will be entirely under the control of the local area network (LAN)
administrator.