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Configuring Frame Relay Services

2-2 114058 Rev. A

Frame Relay assumes that networks use transmission lines with low error rates,

such as digital transmission media. Consequently, Frame Relay provides only

basic error detection with no error recovery. This minimizes the processing

required for each packet, allowing Frame Relay networks to operate at higher

speeds with fewer network delays.

Because Frame Relay performs only basic error checking, endstations running

upper-layer protocols such as Internet Protocol (IP) are responsible for resending

packets that did not transmit correctly the ﬁrst time.

Frame Relay Packets

Figure 2-2 illustrates the structure of a Frame Relay packet. The packet’s header

ﬁeld includes the following:

• Data link connection identiﬁer (DLCI)

The DLCI is the permanent virtual circuit (PVC) identiﬁcation number. The

Frame Relay network uses the DLCI to direct basic data ﬂow.

• Command/Response bit (C/R)

ITU-T (formerly CCITT) standards do not use this bit.

• Forward explicit congestion notiﬁcation (FECN) and backward explicit

congestion notiﬁcation (BECN)

The FECN and BECN indicate congestion on the network. See “Congestion

Control” later in this chapter for information about how the Frame Relay

software uses these bits.

• Discard eligibility (DE)

The DE bit allows the router to mark speciﬁc frames as low priority (discard

eligible) before transmitting them to the Frame Relay network.

• Extended address bit (EA)

The EA bit signals whether the next byte is part of the address. This bit

indicates the last byte of the DLCI.

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Avaya Configuring Frame Relay Services Manual de usuario Pagina 26