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174 PUBLICNETWORKSReliability
Reliability is defined as the amount of time the network
service is available. Reliability can be difficult to evaluate
because several different things can cause downtime. For
example, if a user is trying to transfer data from a server
that is down then from the user’s point of view the net-
work is down. When a packet switching node or dedicated
leased line in a large complex network does fail it affects
a large amount of transmission capacity and therefore
a large number of users. For example, MCI–WorldCom’s
frame relay outage in August 1999 lasted eight days and
affected 30% of MCI’s frame relay customers, perhaps as
many as 70,000 users (Orenstein and Ohlson, 1999).
An advantage to using a private network is that the
redundancy of the network can be designed according
to the business requirements. The major disadvantage is
that it requires investment in redundant packet switch-
ing nodes and leased lines for fault tolerance, personnel
training, disaster recover planning, and testing. These ex-
penses are often overlooked or have less priority when a
private network is designed (Snow, 2001). Or once the pri-
vate network is operational these expenses are considered
low priority. Therefore, when there is an outage the busi-
ness is not prepared for it and its effects are worse than if
a disaster recovery plan had been written.
The reliability of a public network has advantages and
disadvantages. The advantage of using a public network
is that since the cost is spread out over several subscribers
added investment in reliability can be cost effective. The
disadvantage is that a subscriber is completely dependent
on the provider for reliable service. Service level agree-
ments have to be negotiated with clear and strict penalties
if the provider does not meet the negotiated reliability. If
reliability is of high importance to a business, then they
may subscribe to two or more public network providers
for added reliability.Cost and Performance Tradeoffs
The choice between a public and private network includes
determining the tradeoffs between the cost and perfor-
mance of the network. The performance of the network
is defined by throughput and delay. The throughput is the
actual data speed seen by the user in bits per second. The
delay is the maximum end-to-end delay that a packet will
incur in the network.
The costs of the network may vary depending on the
type and volume of traffic that the network will carry. The
type of traffic on a network is classified as being either
stream or bursty (Stallings, 2001). Stream traffic is long
and relatively constant and therefore more predictable
than bursty traffic. An example of stream traffic would be
voice traffic or uncompressed video. Bursty traffic is short
and sporadic such as computer-to-computer communica-
tion in the Internet. Although sporadic, bursty traffic often
requires a large transmission capacity for brief periods of
time. Many Internet applications such as the Web and
e-mail create such bursty traffic. If there are several
bursty traffic sources that share a communications link
and the volume of the combined traffic is high then the
aggregate traffic on the link may be considered stream
traffic.Bursty traffic requires a different type of network than
stream traffic. For example, if one file is required to be
transferred from an office to a central site once a day
then a dial-up connection may be the most feasible. On
the other hand, if there is bursty traffic to be transferred
among a small number of sites and the aggregate of the
bursty sources has a high volume then a private packet
switching network would be more efficient. Leased lines
are not dependent on volume but have a constant fixed
rate for a given transmission capacity and distance. If the
percentage of use of the leased line is high enough then
the volume discount given by the constant fixed rate can
be cost effective. For example, large nationwide contracts
can negotiate T1 access lines for $200 a month while users
in metropolitan areas can get T1 access for approximately
$900 per month (The Yankee Group, 2001). Compare this
to $50 per phone time’s 24 channels that is $1,200 per
month for an equivalent amount of bandwidth.
If there is a moderate volume of bursty traffic to be
transferred among a medium to large number of sites then
a public network may be a better choice. Since the public
network provider has several subscribers, the aggregate
volume of traffic is great enough to have high use and
therefore is cost effective for the provider. These savings
are passed on to subscribers who do not have enough vol-
ume of traffic to justify a private network.
The costs for some network technologies can be ne-
gotiated with the expected performance in mind. For ex-
ample, with frame relay, the user chooses the committed
information rate in bits per second and committed burst
size (Frame Relay Forum, 2002). A frame relay network
provider will also specify a maximum end-to-end delay for
a frame in their network. These parameters are a part of
the pricing for frame relay service.
The price of a network is usually divided up into a fixed
cost and a variable cost. The fixed access cost depends
on the type of access technology that a user connects to
the POP with and the distance the user is from the POP.
There may not be a variable cost, but if there is the price is
dependent on the volume of traffic. A user may subscribe
to a certain data rate from the network for a fixed cost
and if the user exceeds the limit, the user is charged for
the additional usage.Support
Support is defined as the quality of a provider’s techni-
cal and logistical help. In one survey the complaint most
cited was the lack of support (Greenfield, 2001). Networks
are complex and they do break and fail. A good network
provider should be fast to respond and correct problems.
A business should consider where the nearest technician
would be coming from to service their sites. Service level
agreements will define minor and major problems and the
type of responses that the network provider will provide.Control
An organization relies on its network to operate its busi-
ness (Stallings, 2001). Management requires control of
the network to provide efficient and effective service to
the organization. There are tradeoffs between a private
and public network when considering control. There are