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172 PUBLICNETWORKSa different digital hierarchy of transmission capacities is
used. The standards are defined in the Council of Euro-
pean Postal and Telecommunications authorities (CEPT).
The E1 standard operates at 2.048 Mbps and is analogous
to the T1 standard. The next step is a T3 (DS3) at 44.7
Mbps and the corresponding CEPT E3 standard operating
at 34.4 Mbps. Higher transmission capacities are available
using synchronous optical network (SONET) and the syn-
chronous digital hierarchy (SDH) and range from 155.52
Mbps to 10 Gbps.
Digital leased lines can be used to build a company’s
leased line private network, as shown in Figure 1, or can
be used in combination with a public network, as shown
in Figure 2. When leased lines are used to access a public
network the traffic between several sites must be multi-
plexed over the single access line. Therefore, it is impor-
tant to be sure that the leased line is fast enough to support
this traffic. For example, if a site has 15 56 Kbps leased
lines connected point-to-point with other sites and wants
to convert this to a single access line to a public network,
then the access line would require at least 840 Kbps of ca-
pacity. From Table 1, this would require a T1 line (Panko,
2001).Synchronous Optical Network
Synchronous optical network defines a hierarchy of stan-
dardized digital data rates. A compatible version, Syn-
chronous digital hierarchy has been published by the
ITU-T. SONET is intended to provide a specification for
high-speed digital transmission over optical fiber.
SONET, or SDH, is the highest speed and most
costly digital leased lines. SONET/SDH operates in mul-
tiples of 51.84 Mbps. Standards are specified as OCx for
SONET, and STMx for the SDH specification. A common
SONET/SDH speed is OC3/STM1, at 156 Mbps. Other
common rates include 622 Mbps, 2.5 Gbps, and 10 Gbps.
SONET technology can be used for access both to the pub-
lic network and within the public network.X.25
X.25 was developed during the 1970s for use in public
packet switching networks, and this standard was later
ratified by the ITU-T (Tanenbaum, 1996). X.25 was very
slow, often running at only 9600 bps, but it was fast
enough for the text-based transmissions of early net-
works. Its use is declining, but it is still popular in the U.S.
for low-speed applications such as a department store’s
point-of-sale transaction network. Also, there are many
X.25 legacy connections, particularly in Europe and in
countries where the telecommunications infrastructure is
lagging. X.25 is one of a few standards that have been set
by the ITU-T for public switched data networks. Other
standards set by the ITU-T for public networks include
ISDN, frame relay, and ATM.Frame Relay
Frame relay is the most popular technology choice within
public switched data networks today (Panko, 2001). Its
speed range matches the needs of the greatest corporate
demand, and it has very competitive pricing. Frame relay
can also be used instead of leased lines as an access tech-nology or to connect company private networks. Its low
overhead even makes it suitable for interconnecting LANs
and high-speed stand-alone systems (Stallings, 2001). Cur-
rent commercial offerings of frame relay include MCI–
WorldCom, which offers frame relay service access speeds
from 28.8 Kbps to 45 Mbps (MCI–WorldCom, 2002), and
Qwest, which offers frame relay service access speeds
from 64 Kbps to 45 Mbps (Qwest, 2002).
Typically, a company accesses a public frame relay net-
work through a leased line. Several frame relay virtual
circuits are multiplexed over a single access line to the
public network. A virtual circuit is a connection from
source to destination and represents an end-to-end path
that all packets from the same source to the same destina-
tion go through. Virtual circuits simplify forwarding de-
cisions and make the costs of the switches cheaper. A per-
manent virtual circuit (PVC) is one that is set up manually
when a company first subscribes to a public network, and
only changes when the site changes. For a large company
network, a PVC is established for every pair of sites that
would get a leased line in a private leased line network.
The frame relay protocol includes functions for detec-
tion of transmission errors and congestion control func-
tions. The frame relay protocol allows users to negotiate
a committed information rate (CIR) when a connection
is set up. The CIR is the network’s commitment to deliver
data in the absence of errors, and represents the user’s
estimate of its “normal” traffic during a busy period. Any
traffic sent above the CIR is not guaranteed to arrive, but
may arrive if the network has the capacity to deliver it.
In addition, a maximum allowable rate is defined, and all
traffic above this level is discarded (Frame Relay Forum,
2002).
Pricing for frame relay is usually divided into several
different components. First, the company needs a frame
relay access device. This is a router that has been modi-
fied to allow it to communicate with the frame relay’s first
switch. Second, the company must lease an access line
to the nearest POP of the public network. If the POP is
a long distance away then the customer must use expen-
sive, long-distance access lines. The leased line must be
fast enough to handle the available bit rate on the line.
At the POP, the leased access line connects to a port on
the frame relay switch of the public network. The fee for
the port is usually the largest single element in frame re-
lay pricing. To prevent wasting port capacity, the speed of
the leased line should be at least as fast as the port speed.
There is usually a monthly fee for each PVC and this fee
depends on the speed of the PVC. Finally, some vendors
build in other fees, such as per-bit traffic charges or fees to
set up and tear down switched virtual circuits that are es-
tablished on a call-by-call basis. Frequently there are sub-
stantial initial charges to install the access device, leased
line, port connection, or PVC. Figure 4 illustrates the pric-
ing elements in frame relay (Panko, 2001).Asynchronous Transfer Mode
Asynchronous transfer mode is now viewed to be the
universal technology for networking and will likely re-
place many other current offerings (Stallings, 2001). Just
as frame relay allows messages to be divided into many