NM-1A-OC3-POM产品简介   NM-1A-OC3-POM思科原装线卡模块

A virtual circuit (VC) is a point-to-point NM-1A-OC3-POM思科原装线卡模块connection between remote hosts and routers. A VC is established for each ATM end node with which the router communicates. The characteristics of the VC are established when the VC is creNM-1A-OC3-POM思科原装线卡模块ated and include the following:

•Class of service category:

–Constant bit rate (CBR)

–Variable bit rate non-real-time (VBR-nrt)

–Variable bit rate real-time (VBR-rt)

–Available bit rate (ABR)

–Unspecified bit rate (UBR and UBR+)

•ATM adaptation layer 5 (AAL5)

•Encapsulation type:

–Logical link control Subnetwork Address Protocol (AAL5SNAP)

–Multiplexer (AAL5MUX)  NM-1A-OC3-POM思科原装线卡模块

–Network Layer Protocol ID (AAL5NLPID)

–Integrated Local Management Interface (ILMI)

–Switched Multimegabit Data Service (SMDS)

–ITU/Q.2931 Signaling ATM Adaptation Layer (QSAAL)

–Cisco AUTO PPP over AAL5 (aal5autoppp)

–Cisco PPP over AAL5 (aal5ciscoppp)

Each VC supports the following router functions:  NM-1A-OC3-POM思科原装线卡模块

•Multiprotocol

•Fast switching of IP packets

•Flow, and Cisco Express Forwarding (CEF) switching of IP packets

•Pseudo broadcast support for multicast packets

By default, CEF switching is enabled on all OC-3 module interfaces. These switching features can be turned off with interface configuration commands. Flow must be explicitly enabled for each interface.

Permanent Virtual Circuits  NM-1A-OC3-POM思科原装线卡模块

To use a permanent virtual circuit (PVC), NM-1A-OC3-POM思科原装线卡模块configure the PVC in both the router and the ATM switch. PVCs remain active until the circuit is removed from either configuration. When a PVC is configured, all of the configuration options are passed on to the OC-3 module. You can write these PVCs into nonvolatile RAM (NVRAM); they are used when the system image is reloaded.

Some ATM switches might have point-to-multipoint PVCs that NM-1A-OC3-POM思科原装线卡模块do the equivalent of broadcasting. If a point-to-multipoint PVC exists, it can be used as the sole broadcast PVC for all multicast requests.

Like Frame Relay, ATM supports two types of interface: point-to-point and multipoint. The one you choose determines whether you need to use the configuration commands that ensure IP to ATM mappings.

Switched Virtual Circuits NM-1A-OC3-POM思科原装线卡模块

ATM switched virtual circuits (SVCs) are created and released dynamically, providing user bandwidth on demand. This service requires a signaling protocol between the router and the switch.

The ATM signaling software provides a method of NM-1A-OC3-POM思科原装线卡模块dynamically establishing, maintaining, and clearing ATM connections at the User-Network Interface (UNI). The ATM signaling software conforms to the ATM Forum UNI 4.1 specification.

In UNI mode, the user is the router, and the network is an ATM switch. This is an important distinction. The Cisco router does not perform ATM-level call routing. Instead, the ATM switch does the ATM call routing, and the router routes packets through the resulting circuit. The router is viewed as the user and the LAN interconnection device at the end of the circuit, and the ATM switch is viewed as the network.

Figure 1 Basic ATM Environment