Frame Relay Components - Page 5

By Cisco Press | Posted Jan 10, 2002
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PVCs
PVCs are Frame Relay VC connections that are permanently established. PVCs are used for frequent communication between end nodes, such as file sharing, file transfer, and CAD/CAM imaging.

Frame Relay PVCs use DLCIs for Layer 2 addressing.

PVCs operate in one of two modes:

  • Idle -- Tconnection between end nodes is active albeit with no data transfer occurring. PVCs are not terminated or "taken-down" when in an idle state.
  • Data Transfer -- Data traffic is being transmitted between end nodes over the VC.
Even though PVCs are generally discussed as being full-duplex, PVCs are simplex connections, each with its own DLCI/CIR assignment.

The three duplex modes are as follows:

  • Full-duplex communication involves origination and termination points transmitting and receiving at the same time; this is two-way communication full-time.
  • Half-duplex communication is origination and termination points transmitting and receiving, but not at the same time. Only one flow of traffic is allowed across the connection; this is two-way communication, one-way at a time.
  • Simplex communication is origination or termination points transmitting or receiving; this is one-way communication only.

SVCs
Unlike PVCs, which are permanently established connections, SVCs require a call setup process. SVCs are temporary connections that are traditionally used when communication between end nodes is infrequent or sporadic, such as in Voice over Frame Relay (VoFr) situations.


NOTE:   Frame Relay SVCs use E.164 or X.121 addresses for Layer 2 addressing.

Whereas PVCs are permanently established, SVCs require a call setup and termination process, defined by the following process and functions:

  1. Call setup -- Establishes the VC between Frame Relay end nodes. This includes negotiation of VC parameters, such as CIR.
  2. Data transfer -- Data traffic is transmitted between end nodes (originating and terminating) across the VC.
  3. Idle -- Like PVCs, when the VC is idle (no data traffic) the connection between end nodes remains active and available for communication. However, unlike PVCs, which do not terminate the connection, an SVC will terminate the connection if it is in an idle state for a configured time period.
  4. Call termination -- The VC between Frame Relay end nodes is terminated, or "taken down."

NOTE:  

In bidirectional mode, both ends of a VC send and respond to keepalive requests. If one end of the VC is configured in the bidirectional mode, the other end must also be configured in the bidirectional mode.

In request mode, the router sends keepalive requests and expects replies from the other end of the VC. If one end of a VC is configured in the request mode, the other end must be configured in the reply or passive-reply mode.

In reply mode, the router does not send keepalive requests, but waits for keepalive requests from the other end of the VC and replies to them. If no keepalive request has arrived within the timer interval, the router times out and increments the error counter by 1. If one end of a VC is configured in the reply mode, the other end must be configured in the request mode.

In passive-reply mode, the router does not send keepalive requests, but waits for keepalive requests from the other end of the VC and replies to them. No timer is set when in this mode, and the error counter is not incremented. If one end of a VC is configured in the passive-reply mode, the other end must be configured in the request mode.

The command to configure end-to-end keepalive (Cisco IOS 12.0.5(T) or greater) is frame-relay end-to-end keepalive mode {bidirectional | request | reply | passive-reply}.

X.121 is a hierarchical addressing scheme that was originally designed to number X.25 nodes. E.164 is a hierarchical global telecommunications numbering plan, similar to the North American Number Plan (NANP, 1-NPA-NXX-XXXX).


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Part 3 in this series will conclude the section on Frame Relay components.

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