Frame relay was developed as a fast packet switching technology designed to eliminate the delays associated with the use of an X.25 packet network. In doing so/ frame relay became a suitable transport for interactive query-response applications in which a client was located on one network and the server was located on a geographically separated network/ with both networks linked together through the use of a frame relay network.

The fact that CRC checking was employed only at switches to determine whether or not to drop a frame (instead of using full error detection and correction by retransmission) enabled data to flow end to end with minimal delay. This capability was extremely important to support query-response applications in which client-station users would enter a variety of short queries that would otherwise be significantly delayed through a conventional packet switching network and would degrade the ability of the user to type in a normal manner, adversely affecting productivity.

Although the use of a frame relay network enables access to SNA mainframes as well as LAN-to-LAN connectivity that otherwise might be awkward when using an X.25 network, frame relay uses a variable-length information field. Thus, from a technical perspective, the variable frame length can adversely affect the arrival and interpretation of the digitized-voice frame when its contents are examined and used to reconstruct voice.

Frame Length Handling

The information field in the frame relay frame is variable in length. To illustrate the problem this can cause, consider the FRAD or router which provides a connection to a frame relay network for a transmission to and from a LAN and a PBX. In this example, there is one serial port connection used to link the FRAD or router, which services the LAN and the PBX/ to the frame relay network. This means that the transfer of a file, a long response from the server to a client query from another location connected to the LAN via the frame relay network, or another lengthy data transfer will tend to fill the information field in the frame relay frame.

Effect of Variable-Length Frames

Although an Ethernet LAN limits the information field length to 1500 bytes, a Token Ring LAN operating at 16 Mbps can have an information field that can be up to approximately 18 Kbytes in length. If a LAN station obtains access to the serial interface of the router or FRAD between frames transporting digitized speech, the delay will depend on the operating rate of the serial connection to the network and the length of the frame. Concerning the former, there are significant delay differences between accessing a frame relay network at 56/64 Kbps and a Tl operating rate of 1.544 Mbps.

For example, at 64 Kbps, one 8-bit byte carried in the information field of a frame relay frame requires 125 x 10″6 seconds (8/64/000) for transmission. Thus, a 1500-byte information field in a frame relay frame that results from the transportation of data in the maximum-length Ethernet information field to the FRAD or router would require 0.1875 seconds (125 x 10~6 x 1500) to be transmitted to the network, without considering the overhead associated with control fields.

This means that, excluding frame overhead and processing time associated with the FRAD or router forming a frame for placement onto the access line connected to the frame relay network, a lengthy Ethernet LAN packet could delay a frame transporting voice by almost 0.2 second.

  • APPLICATION NOTE: FRADs or frame relay-compliant routers must be configured to place a limit on the length of frames transporting data to minimize their effect on frames transporting digitized voice.

Frame length handling

Frame prioritization

CIR selection

Frame loss handling

Echo cancellation

Frame delay handling

Silence suppression

Voice-compression method

Frame overhead

Telephony signaling

Multiplexing technique

Service-level agreements

In addition to the variable-length frame presenting problems with respect to the transportation of digitized voice, a variety of other technical issues face equipment developers. In the remainder of this section, we will examine each issue. Not only will this give you an appreciation for the obstacles equipment developers and implementers had to overcome, it will also help to clarify the technological features so you can compare equipment manufactured by different vendors.

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