COMNET III CACI

Note that she has decided to reduce the available bandwidth of the office LANs to
4Mbps. She wants to argue that this is the minimum available bandwidth under the
existing case. This simplification allows her not to explicitly model the local LAN
traffic. Furthermore, she has decided to model the 64Kbps leased PVC in both cases to
allow a comparison between the results. She realizes that this link will have to be
upgraded when fully supporting ATM real time services, but for the simplicity of her
argument against management, she is only going to model the existing network and
compare it with model illustrating ATM to the desktop, all else being equal. She already
considers building a third model to illustrate the upgrade of even the inter-office link to
DS-3 speeds. The model for the existing network is depicted above in Figure 12.
The servers and the representative workstation in each office are all modeled as
COMNET III Computer & Communication nodes with their default values. The manager
is not interested in the local processing statistics for her argument. The routers in the
offices are modeled using the Novell Multi-protocol Router Plus, V2.1 and the IBM
6611, V1.1 parameter sets in COMNET III. She found those values to be representative
of the ATM routers currently installed in each office. The LANs in each office are
modeled as COMNET III token ring link and Ethernet link respectively. The token ring
link is given the 802.5 16Mb Token ring parameter set with a modified bandwidth of
4Mbps. Similarly, the Ethernet is given the 802.3 10 Base T parameter set with a
modified bandwidth of 4Mbps. The routers are connected to a transit net in the model,
using a 64Kbps point-to-point link. This link is modeled inside the transit net to
represent the capacity the company has currently leased from its service provider. The
transit net allows the manager to model the specific protocol stack of TCP/IP over ATM.
It is given two service classes: CBR and ABR. Each service class contains a single
connection type, specifying the egress router and the respective ATM protocol
respectively. The parameter settings for the transport protocol of these connection types
are identical to those outlined in the previous example.
The model also includes 4 traffic sources to represent the remote traffic in the network.
The file transfer traffic originates at both servers. The destination in each case is the
remote workstation. The size of the file transfer is set to 1Mb, occurring exactly once in
the simulation. The file transfer is given the transport protocol ‘TCP/IP’ in COMNET III
and a network service level of 10. With these settings, the file transfer of 1Mb will be
segmented into TCP/IP packets across the local LAN. Upon arrival at the ATM routers,
these TCP/IP packets will then be further segmented into ATM cells following the CBR
service class (i.e. using AAL1). The manager sets the priority of this traffic source to 80
to indicate the higher priority of CBR traffic. The voice mail traffic occurs periodically
throughout the simulation. The given inter-arrival time is 10 seconds exponentially
distributed. This value reflects the frequency with which all the workstations in each
office send voice mail to the remote office. The size of the messages is set to 10Kb,
again following the TCP/IP transport protocol. In this case, the net service level
parameter is given the value 70 to indicate that the voice mail packets are transmitted
across the ATM PVC using the ABR service class. Again, this traffic is modeled using a
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