WiMax Operator's Manual

134 CHAPTER 6 ■ BEYOND ACCESS
Routers were initially designed to switch text data and still-frame graphics only (in other
words, information that could tolerate the delays inherent in packet transmissions). Today
routers are conveying all sorts of delay-sensitive traffic including but not limited to voice;
video, including high-definition video; high-fidelity multichannel audio; interactive entertainments;
video surveillance; video conferences; and business voice-over applications such
“white boarding” and real-time, online, agent-assisted sales presentations.
For a router to provide good presentation quality with such delay-sensitive content, two
things generally have to happen. First, the content to be transmitted has to be cached as close
as possible to its final destination. Second, the router itself has to begin to operate more like
aswitch.
The latter generally involves a new packet-switching protocol that I have mentioned
before known as Multiprotocol Label Switching (MPLS). Whole books have been written about
MPLS and the various aspects of the protocol, and I will not attempt to explain it in detail here.
My aim is more modest, simply to indicate what MPLS means to the broadband wireless
operator.
MPLS is based on a concept initially known as tag switching that has been around for
almost a decade and was first associated with Cisco Systems and Toshiba, both of which developed
prestandards router/switches with tag-switching capabilities. The idea was that the basic
IP router functionality would remain intact—no separate and divergent packet-switching protocol
such as frame relay was envisioned. Instead, a switching function would be built on top
of an IP such that at least certain classes of traffic would take determinate paths through the
network—paths that could involve reserved bandwidth. These paths would be designated by
the tag or label associated with the traffic in question, and that traffic, instead of being routed
through first one path and then another according to network conditions, would take but a
single path. Each router/switch would strip the label from the designated stream of data and
rewrite a new label, indicating its next destination. There would be no consulting of routing
tables, no lookup, and no path determination procedure. In the case of the label-handling
function, the router would instead function essentially as a dumb switch.
According to MPLS, labels are assigned to traffic on the basis of common parameters such
as permissible latency, committed bit rate (if any), maximum jitter or timing errors, and so on.
Flexibility in provisioning bandwidth to meet service requirements far exceeds that associated
with asynchronous transfer mode (ATM) or frame relay, the legacy packet standards for handling
diverse types of network traffic.
MPLS itself is not nearly as comprehensive as ATM, the protocol it most resembles, however.
MPLS is basically a technique for segregating and shaping traffic according to its need for
controlled bandwidth, but unlike ATM, which performs much the same function, MPLS lacks
internal mechanisms for reserving bandwidth or enforcing network performance to ensure
that service levels are maintained. These are provided by other protocols such as the Resource
Reservation Protocol (RSVP), used for bandwidth reservation, and DiffServ, which, as the name
implies, posits instructions for differentiating various services. MPLS merely segregates the differentiated
traffic into streams and attaches the appropriate labels for handling that traffic
in transit.
MPLS is embedded in most edge routers sold today and in nearly all new core routers. It
is not common in the combined base station controllers/routers used in broadband wireless
networks, though. True, all equipment that is 802.16 compliant will have quality of service