Information Technology Reference
In-Depth Information
ATM Cell Header Fields
Table 5-4
Field
Description
GFC
Generic flow control
VPI
Virtual path identifier
VCI
Virtual channel identifier
PTI
Payload type identifier
CLP
Cell loss priority
HEC
Header error control
Using a fixed length cell has the advantage of allowing the switch to use fixed length buffers.
Using fixed length buffers speeds up the switching process as compared to using variable length
buffers to switch variable length frames.
Links between ATM devices can be classified as UNI or NNI. The configuration of a link as
UNI or NNI affects the cell header type and the connection signaling across the link.
A link configured as a UNI connects ATM ESs, such as routers, hosts, or switches to an ATM
switch.
A public UNI is typically defined as connecting an ATM ES to an ATM switch in a public
service provider's network.
A private UNI is typically defined as connecting an ATM ES to an ATM switch in a private
enterprise network.
A link configured as an NNI connects ATM switches.
A private NNI is typically defined as connecting two private ATM switches. The PNNI protocol
provides support for call signaling, QoS, and dynamic call routing. PNNI is discussed in more
detail later in this chapter.
A public NNI is typically defined as connecting switches from two different ATM service
providers. A protocol known as Broadband Inter-Carrier Interface (BICI) can support services
on a link between public networks.
Figure 5-16 shows the fields that make up a UNI cell header.
The first 4 bits make up the GFC field. The GFC was originally intended to provide flow control.
However, it is not currently used and is always set to the value 0000.
The next 8 bits make up the VPI. The VPI identifies a virtual path (VP) connection. Conceptually,
the VP can be thought of as containing multiple virtual channels (VCs), (also known as virtual
circuits). Using eight bits, the VPI can identify 256 VPs.
The next 16 bits make up the VCI. Along with the VPI, the VCI uniquely identifies a cell as
belonging to a specific connection. Using 16 bits, the VCI can identify 65,536 VCs.
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