Figure 9-8 Call setup procedure.
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Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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A. LIS Configuration and Operation
There are two flavors of LIS configurations: one is based on PVC and the other is based on SVC. In the PVC case each member is connected to an ATMARP server via preestablished PVCs. In the case of
using SVC, each member joins the LIS by setting up a SVC connection to the known address of the ATMARP server. Each member as part of its joining process registers with the ATMARP server by
providing its IP and ATM address as shown in Figure 9-10.
When a member of an LIS wants to communicate with another IP destination, it requests an IP to ATM address translation for the destination station from the ATMARP server by sending an ATMARP
Request. Upon receiving the ATM address of the destination in an ATMARP Response message, it will set up a SVC connection to establish the communication with the destination. This scenario is illustrated
in Figure 9-10. If the destination is located in another LIS, the ATMARP server will return the address of a default router which serves as a bridge to the other LIS domain.
As shown in Figure 9-11, several routers are configured as members of multiple LISs to serve as a bridge to carry traffic between these logically separated LISs. Both LANE and CLIP suffer from this
same drawback in terms of relying on routers to carry traffic between the logically separated subnetwork domains. This is true even if both source and destination are residing in the ATM network, i.e., it is
possible to set up a direct VC shortcut between the two stations over the ATM network. Reliance on routers create extra latency since these routers have to check every data frame header to make a routing
decision. This reliance also creates bandwidth bottlenecks since the traffic between the subnetwork domains has to be channeled through these routers. The Next Hop Routing Protocol NHRP is
specifically designed to address these problems.
RCF 1577 defines the key operational requirements for the LIS environment as:
• All members should have the same IP networksubnet number and address mask. • All members of a LIS should be directly connected to the ATM network and be able to set up
direct VCs to each other fully meshed connectivity. • The same Maximum Trasmission Unit MTU has to be used for all VCs in a LIS. The default
MTU is 9180 octets. However, this can go up to the AAL5 limit of 64K-1 octets as long as each member is configured to use the same MTU value.
The NHRP replaces the traditional routers with the NHRP Servers NHS. These NHS servers perform the address resolution to enable the source to set up direct VC short-cut to a destination in another
subnetwork domain.
Figure 9-9 Classical IP over ATM configuration.
Figure 9-10 Classical IP over ATM operation flow.
III. Multi-Protocol over ATM MPOA
It can be safe to say that MPOA is the next stage in the evolution of ATM based LAN networking. It is a result of experience gained from LANE and CLIP. The experience with LANE and CLIP revealed some
significant deficiencies in the way the standard bodies approached the problem of transition from legacy LAN to ATM-based LAN environment. One of the key deficiencies of LANE and CLIP was the fact
that it was still necessary to use classical routers to carry traffic between the logical subnetworks created by these protocols even if the logical subnetworks resided over the same ATM network. As mentioned
earlier, these routers cause excessive latency and become the bottleneck points, since every packet has to be individually processed by these routers. In addition, since these routers have to regenerate the packets
from ATM cells in order to interpret the packet header for routing purposes, this creates additional
