The Internet Encyclopedia (Volume 3)

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CHOOSING APRIVATENETWORK OR APUBLICNETWORKPROVIDER 173

5. Sometimes
   Traffic
   Charges and
   Other
   Charges
   2. T1 Leased Access
   Line to POP

Customer Site B Customer Site C

Customer Site A

1. Access Device

POP

CIR=56 Kbps

Switch

Switch

Switch

PVC 2

PVC 1

4. PVC
   Charges

Public Switched Data Network

3. Port
   Speed
   Charge

PVC 1 and PVC 2, multiplexed

PVC 2

PVC 2

PVC 1

Figure 4: Pricing elements in frame relay services. Source:Buisness Data Communications and Net-

working, 3/E(Panko, 2001). Reprinted by permission of Pearson Education Inc., Upper Saddle River,

NJ.

frames that can be sent across a switched network, ATM

uses cell relay. Like frame relay, ATM multiplexes many

logical connections over the same physical interface,

sending information in fixed size 53-byte cells. ATM can

support data, video, voice, and Internet traffic on a single

access line.

The use of cells in ATM allows many important features

to be defined for a virtual channel. For example, users

can negotiate the ratio of cells lost to cells transmitted,

cell delay variation and parameters such as the average

rate, peak rate, burstiness, and peak duration for a virtual

channel (ATM Forum, 2002). The ATM service can use per-

manent virtual channels for static connections. ATM also

allows switched virtual channels to be set up dynamically

on a call-by-call basis.

Four classes of ATM service have been defined

(Stallings, 2001):

Constant bit rate:The network provider ensures that this

rate is available, and the customer is monitored to be

sure the rate is not exceeded.

Variable bit rate (VBR):A sustained rate for normal use

is defined, and a faster burst rate for occasional use

is also defined. The faster rate is guaranteed, but not

continuously. The ATM Forum divides VBR into real-

time VBR (rt-VBR) and nonreal-time VBR (nrt-VBR)

(ATM Forum, 2002). With rt-VBR the application has

tight constraints on delay and delay variation, but the

rate is allowed to vary according to parameters spec-

ified by the user. The nrt-VBR is for applications that

are bursty, but do not have tight constraints on delay

and delay variation.

Available bit rate (ABR):The user has a guaranteed min-

imum capacity. When additional capacity is available

on the network, the user may burst above this without

risk of cell loss.

Unspecified bit rate (UBR):Cells are delivered with best

effort, meaning that any cell may be lost. The main

difference between UBR and ABR is that ABR provides

feedback to the user so that the user can control the

amount of data being sent and reduce the risk of loss.

ATM is a high-performance service and is expensive.

In the range of speeds where ATM speeds overlap with

frame relay, frame relay is more attractive because it is

cheaper. However, as customer needs increase, ATM be-

comes a more attractive option. ATM is widely used within

high-speed public networks and by companies that need

higher speed private networks. Most ATM public switched

data network providers currently offer speeds from 1

Mbps to 156 Mbps, with higher speeds coming. These

public networks require access lines ranging from T1 to

a SONET OC-3 line. MCI–WorldCom offers ATM access

speeds from 1.544 Mbps to 622 Mbps (MCI–WorldCom,

2002). Qwest offers ATM access speeds from 1.544 Mbps

to 155 Mbps (Qwest, 2002).

CHOOSING A PRIVATE NETWORK OR

A PUBLIC NETWORK PROVIDER

There are several categories to consider when one decides

whether to use a private network or a public network. If

a public network is chosen, then these same categories

can help in choosing a network provider. A survey ISPs

conducted in 2001 found that the top three areas that dif-

ferentiated the best ISPs from the rest were reliability, per-

formance, and low cost (Greenfield, 2001). Subscribers to

ISPs in the survey also considered support to be impor-

tant. In addition, network control is a factor in deciding

whether to choose a private network or a public network.

Other factors mentioned in the survey include breadth of

service, security, installation, repairs, and remote access.