In ATM networking, what is a virtual channel and how does it relate to a virtual path?

Introduction / Context:
Asynchronous Transfer Mode organizes traffic into small fixed size cells. To deliver these cells reliably between endpoints, ATM uses a two level hierarchy consisting of virtual paths and virtual channels. While a virtual path groups several connections, the virtual channel represents an individual logical connection. Understanding what a virtual channel is and how it fits inside a virtual path is important for exam questions about ATM architecture and connection oriented networking.

Given Data / Assumptions:

• The network technology is ATM, which is connection oriented and cell based.
• Each ATM cell header contains a virtual path identifier and a virtual channel identifier.
• Virtual paths can carry many virtual channels between the same endpoints.
• The question asks for the definition and role of a virtual channel.

Concept / Approach:
A virtual channel is a logical connection between two ATM endpoints that carries a single ordered stream of cells. It is identified by a virtual channel identifier that is meaningful within the context of a particular virtual path. Many virtual channels can be multiplexed inside one virtual path. The virtual path identifier plus the virtual channel identifier together uniquely identify a connection through the network. This structure allows ATM switches to route traffic efficiently while still maintaining separate virtual connections for different users or services.

Step-by-Step Solution:
Step 1: Recognize that ATM is connection oriented, so each data flow is associated with a specific logical connection.Step 2: Understand that the virtual channel is this logical connection and that it carries cells belonging to one flow, such as a voice call or data stream.Step 3: Recall that each cell header contains a virtual channel identifier that identifies which virtual channel the cell belongs to within a given virtual path.Step 4: Note that multiple virtual channels share the same virtual path, which defines a higher level route between switches.Step 5: Conclude that a virtual channel is a per connection logical path, not a physical link or a general broadcast mechanism.

Verification / Alternative check:
Technical descriptions of ATM often show the hierarchy as virtual path connections containing multiple virtual channel connections. Implementation diagrams illustrate cells with headers that include a virtual path identifier field and a virtual channel identifier field. When a connection is set up, the network assigns a virtual channel identifier that will be used for that particular call or session. These descriptions confirm that a virtual channel is the fundamental logical connection that rides inside a virtual path.

Why Other Options Are Wrong:
Option B is incorrect because the physical copper or fiber link belongs to the physical and data link layers and does not represent the logical channel concept used in ATM headers. Option C is wrong because a broadcast domain belongs to Ethernet style LANs and does not describe ATM virtual channels. Option D is incorrect because ATM virtual channels are not restricted to voice traffic; they can carry many types of data and are not simply high priority queues.

Common Pitfalls:
Students sometimes confuse virtual paths and virtual channels, or they think that a virtual channel is the same as the physical link between switches. Another pitfall is assuming that virtual channels always map one to one to individual user applications, but in practice, one application may use multiple channels or share channels with others depending on design. The key idea is that a virtual channel is a logical connection identified by a virtual channel identifier that carries a single stream of cells inside a virtual path.

Final Answer:
In ATM networking, a virtual channel is a logical connection that carries a single stream of cells, identified by a virtual channel identifier within a virtual path between two endpoints.