ATM Option System Reference - Comtech EF Data

ATM Option
1.2.4 ATM Switching
• The ATM header contains a Virtual Channel Identifier (VCI) and a Virtual Path
Identifier (VPI).
- These identifiers provide the address of the next node that the ATM traffic
will pass through.
- The header is read at this node and is passed to the next node via another virtual
channel and path.
VPs and VCs are switched (or cross-connected) according to the vendor-specific
proprietary techniques of each ATM switch used.
• Usually cells are taken from input links and placed into buffers, where they can
be read out to other links. Switch size is a function of buffer speed versus message
delay tolerance.
• The ATM switch can process cells far more rapidly than the transmission rate of
ATM traffic, so there is usually very little switching delay. There are two types of
ATM switches:
- Virtual path switch: Terminates virtual path links, and translates the virtual
path identifier for a group of virtual channels. With this type of switch, all virtual
channels are switched to a new virtual path link.
- Virtual channel switch: Translates both the virtual channel and the virtual
path identifiers, and can switch a virtual channel from one virtual path to
another.
• Quality of Service (QoS) levels are negotiated at the time of service provisioning,
and are application and service dependent.
1.2.5 PVCs Versus SVCs in the Network
As of V10.1, the NetPerformer supports switched virtual circuits (SVCs) on the ATM
licensed option. SVCs are easier to manage and require less human intervention than
permanent virtual circuits (PVCs).
PVCs are predefined virtual connections. The attributes of a PVC, such as its destination,
bandwidth and QoS requirements, must be defined on every ATM switch where the PVC
is accessed, and on each end system using it (see Figure 1-1 on “ATM PVCs in the
Network” on page 1-7). The only information known to the end system is the VPI and
VCI, which are configured on the NetPerformer itself.
• This makes configuring a large number of PVCs very cumbersome for network
providers. Any change to PVC parameters requires manual intervention of the
network provider to reconfigure the NetPerformer.
1-6 Memotec Inc.