Frame Relay

Frame Relay

1. Address Field

   • DLCI(Data Link Identification number)
     • VC Identification number used to switch packets through the network.
     • Used on a hop by hop bases.
   • FECN(Forward Explicit Congestion Notification)
     • Bit set by network node experiencing congestion. Sent to the destination to inform that congestion has occurred.
   • BECN(Backwards Explicit Congestion Notification)
     • Bit set by network node experiencing congestion.
     • Sent to the source to inform sender that congestion has occurred.
   • C/R(Command/Responce)
     • Allows upper layers to identify if the frame is a command or a response.
   • DE(Discard Eligable)
     • Set by network if user has exceeded CIR and network is experiencing congestion.
   • EA(Address Field Extension)
     • Used to determine length of address field.
     • Used to determine length of address field.

2. Frame Relay Address Mapping

   • In frame relay, the end of each connection has a number to be identified with.  This number is called a Data Link Connection Identifier.
   • This address-to-DLCI mapping can be accomplished either by static or dynamic mapping.
   • Dynamic Mapping
     • This technique relies on the Inverse APR to resolve a next hop network protocol address to a local DLCI value.
     • A frame relay router sends out APR requests on its VPN so to discover the IP addresses of the remote devices connected to the frame relay network.
     • Router then uses the response's to populate an IP address-to-DLCI mapping table.
     • This contains all resolved inverse APR requests and static mapping entries.
   • Static Mapping
     • The user can choose to override dynamic inverse APR mapping by supplying a manual static mapping.
     • A static map works similar to a dynamic inverse APR by associating a specified next hop protocol address to a local frame relay DLCI

3. Congestion

   • Frame relay provides notifications that the network is becoming congested by means of the FECN and BECN bits in the data frames.
   • Used to tell end points to slow down.
   • BECN
     • Happens when a frame relay switch detects the onset of congestion based on memory buffer usage of queue length.
     • Switch waits for frames coming in the reverse direction.
     • Node sets the BECN bit within those frames to signal the upstream nodes and the sender.
   • FECN
     • Bit set in header to notify destination of congestion.
     • In response the recovery system should try to alter the flow of data from the sending system on this frame relay connection.
   • QoS Parameters
     • Limit spread of congestion to other networks.
     • Minimize frame discard.
     • Maintain agreed QoS.
     • Minimize possibility that the user takes over network resources at the expense of others.
     • Simple to implement.
     • Minimal additional network traffic.
     • Fair distribution of resource among end users.
     • Limit spread of congestion to other networks.