ATM VPI Field Routing Explained | Generated by AI
I’ll search for information about ATM header format and VPI field to provide you with a comprehensive explanation.Question: What is the function of the VPI field in the ATM header format?
Answer:
In the ATM (Asynchronous Transfer Mode) cell header format, the VPI (Virtual Path Identifier) field is used for routing selection, which is the correct answer (Option A).
Detailed Explanation
The VPI field, in conjunction with the VCI (Virtual Channel Identifier) field, forms the routing mechanism within ATM networks:
Function of VPI: The VCI, together with the VPI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination.
Technical Details:
- The VPI is 8 bits for UNI (User-Network Interface) cells, while NNI (Network-Network Interface) cells use 12 bits for VPI (since the 4-bit GFC field is repurposed as additional VPI bits)
- The Virtual Path Identifier (VPI) identifies a path between two locations in an ATM network that provides transportation for a group of virtual channels, where a virtual channel represents a connection between two communicating ATM devices.
Two-Level Hierarchy: The Virtual Channel Identifier (VCI) can be considered to represent the second part of the two-level routing hierarchy used by ATM, where a group of virtual channels are used to form a virtual path.
Other ATM Header Fields
For reference, the other fields mentioned in the exam question serve different purposes:
- HEC field (Header Error Control): Used for error detection and correction
- GFC field (Generic Flow Control): Used for flow control
- CLP field (Cell Loss Priority): Used for congestion control
Your answer of A is correct.
References:
- https://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode
- https://www.sciencedirect.com/topics/computer-science/virtual-path-identifier
- https://www.informit.com/articles/article.aspx?p=680842&seqNum=4
- https://www.ccexpert.us/routing-switchin/atm-cell-format.html