The Atm Switch And Atm Endpoints; The Atm Reference Model - Cisco RJ-45-to-AUX Brochure

The ATM Cell Header
The ATM cells can be found in one of two formats, depending on whether the endpoints are a UNI or an NNI
connection. The two differ in one way: The NNI header does not contain a Generic Flow Control (GFC) field.
The NNI header has a Virtual Path Identifier (VPI) that occupies the entire first 12 bits. A cell header for a
UNI cell is shown in Figure 8.3.
Figure 8.3: An ATM UNI cell header.
Let's take a look at the fields in a cell header:
Generic Flow Control (GFC)—An 8−bit field that is used to provide information to identify multiple
stations that share a single ATM interface. The GFC is typically not used.
Virtual Path Identifier (VPI)—An 8−bit field used with the VCI to identify the next destination of a
cell as it passes through a series of ATM switches on its way to its destination.
Virtual Channel Identifier (VCI)—An 8−bit field used in conjunction with the VPI to identify a cell's
next destination.
Payload Type (PT)—A 3−bit field that indicates whether the cell contains user data or control data. If
the cell contains user data, the second bit in the user data indicates congestion, and the third bit
indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.
Congestion Loss Priority (CLP)—A 1−bit field that indicates whether the cell should be discarded if
it encounters extreme congestion as it moves through the network.
Header Error Check (HEC)—An 8−bit field that indicates a checksum calculated only on the header
itself.

The ATM Switch and ATM Endpoints

ATM networks use one of two types of devices for each end of the network: ATM switches and ATM
endpoints. An ATM endpoint is a device that has an ATM network interface adapter, such as a workstation,
router, Data Service Unit (DSU), or LAN switch. These devices in turn transmit data to an ATM switch,
which is responsible for receiving this data, updating the header information, and then sending the data out the
proper interface port to its intended destination.
As mentioned earlier in the chapter, in a UNI, the ATM interface connects an endpoint to a switch. In an NNI,
the interface connects two ATM switches together. The UNI and NNI connections can be used to further
divide the network into private and public networks. As the name implies, a private network connects the
ATM endpoint to a private network, whereas a public network connects an ATM endpoint to a public switch
(possibly owned by a telephone company or other WAN service provider).

The ATM Reference Model

The ATM standard uses a reference model to describe the functions of the protocol. The ATM Reference
Model has three layers (which roughly correspond to those in the OSI model) and three management planes.
The ATM Physical layer is similar to the Physical layer of the OSI model. The Physical layer controls the
transmission and receipt of bits on the physical medium.
The ATM layer and the ATM Adaptation layer (AAL) are similar to the Data Link layer of the OSI chart. The
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