This table shows the effect of copy

share   with different  latencies   dela  s .   Notice that althou  h we didn t

estrict the bandwidth at all   our connection is 100Mb  s   b   the time we

each  150ms    London  to  California  t   e  round  tri  s    the  maximum

bandwi  th  used  is   ust  1.5Me  abits    er  second.    This  is  a  case  where

buying bandwidth can t speed this transfer up.

 

f   ou  think  we  are  overstatin   the  case   we  know  of  several    ro ects

ncludin   a document retrieval   ro ect   our Word document exam  le is a

sim  le document retrieval   rocess  that failed utterl   in the WAN althou  h

it had successfully passed several LAN-  ased  tests.  It  sim  l

roved  so

slow as to be unusable.

 

Table 2: Test of Data Transfer Times for a 62mb file using NetBIOS

In all Cases Using a 100 Megabit connection

Throw More Bandwidth at the Problem

 

We all know about bandwidth at home you probably started with dial-

connections to access the inte

were proving too slow, moved to broadband ADSL or cable  Problem solved.

 

So why not buy more bandwidth? Beyond a certain point i  does not matter

how much bandwidth you have as the transfer is limited by the nee   to   et

n acknowled  ement back from the receivin   end ever   time   ou send a

certain amount of data.

So how do you test application performance over the

WAN?

 

Clearly, testing in the LAN is an essential first step. After all, if an

application will not work over the internal network, it s very unlikely to

perform over the WAN. However, as discussed, it does not follow that an

application that performs in the LAN will necessarily perform well or in

extremis even work at all in non-LAN environments.

 

OK, so now you re (hopefully!) convinced of the importance of testing in

the WAN. However, a request to conduct testing of a new and untried

application in your live WAN environment carrying business-critical data is

likely to be declined.

 

A possible alternative is to confine your testing over the live WAN to out-

of-office hours. The problem is that most live networks are actually busy

(or even busier) at night as the company performs network back-ups etc.

 

It is also going to be impractical to get the right people in place at the

required locations in order to conduct the initial test and subsequent

retesting.