WiMax Operator's Manual

CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE 75
Circuit Fiber Connections: SONET/SDH: Circuit time intervals are all multiples of the basic 56 kilobit
per second (Kbps) voice channel. The chief service offering within SONET networks is the T1 at
approximately 1.5Mbps, and the equivalent SDH offering is the E1 at approximately 2Mbps.
As indicated earlier, multiple T1/E1 services may be supported by some of the newer SONET
equipment platforms, but most carriers have not installed such equipment and will not provide
fine increments of bandwidth. In the United States the next level up from the T1 is the DS3
at 45Mbps, and the next increment above that is the OC-3 at 155Mbps. Someone desiring a
20Mbps or 100Mbps backhaul is simply out of luck.
Of course, one can always choose to lease more capacity than is absolutely needed, but the
pricing of DS3s and OC-3s is steep—thousands of dollars a month for the former and well into
the five figures for the latter. For a startup wireless broadband network attempting to build a
business, such large-capacity circuit backhaul connections are a heavy burden, a burden made
heavier by the fact that they are quite inefficient for the transport of packet traffic and carry a
great deal of network overhead.
Packet Fiber: Metro Ethernet or IP: Another option for backhaul is a direct packet connection using
either the Ethernet or IP protocol. Fast packet services over fiber have been offered by a number
of startups such as Cogent and Yipes in the United States and more recently by some of the
incumbents. Quite a number of carriers are now offering such services in East Asia as well, particularly
in mainland China, Hong Kong, and Taiwan. Certainly in the case of the startups, the
pricing has been far more attractive than is the case for circuit services with monthly fees as low
as $1,000 for a 100Mbps burstable connection, but in most cases such services are not delivered
with rigid service-level agreements in respect to jitter, latency, minimum throughput,
bandwidth reservation, and so forth. Direct IP services better support quality of service than do
pure Ethernet services, which is something to consider when evaluating fast packet backhaul
providers.
Where available, packet services over fiber are often preferred to circuit connections
strictly on the basis of price, but the problem is that they are not too generally available, and
in nearly all cases they are offered by entities that would prefer to have the customers whose
transmissions are being backhauled. Then, too, fiber-based access services are expensive to
provision, and the provider of metro Ethernet or direct IP-based services may simply not be
able to lay fiber to the building being served by the broadband wireless operator.
Dark Fiber: Another option, sometimes present and often not, is the lease of so-called dark fiber
from a public utility or public transit system. Gas and electrical utilities, and, to a lesser extent,
transit districts and railroads, own extensive amounts of optical fiber that they use for monitoring
purposes and internal communications. Such private fiber networks are generally grossly
underutilized—largely dark in the parlance of the telecommunications industry—and the
owners are often amenable to leasing capacity, in which case the lessee may be assigned an
individual wavelength or an IP address. Since utilities are not primarily in the business of selling
fiber capacity and do not regard such networks as major profit centers, they can often be
persuaded to lease capacity at reasonable rates. Be forewarned, however, that using leased
dark fiber for backhaul or for any other purpose is a different proposition than purchasing a
T1 or fast packet service from an optical services provider. Public utilities often do nothing
other than provide access to an optical pipe. They do not provide termination or optical
conversion equipment, and they offer no guarantees in respect to network redundancy or