Frame Delay

Question 1

Frame Relay ?

Answer 1

1. Frame Relay identifys how data moves between the DTE and DCE over local loops
2. A Frame Relay WAN is a mesh of switches interconnected by truck lines.
3. Normally a carrier-provided public network that customers subscribe to.
4. Frame Relay typically operates from 64kbps up to 45Mbps speeds.
5. Operates at physical and data-link layers only

Question 2

Deployment

Answer 2

1. Used as an access technology
2. low speeds
3. Much cheaper than a leased line.

Question 3

Frame Relay Operation

Answer 3

1. The DTE sends frames to the DCE switch on the WAN edge.
2. The frames move from switch across the WAN to the destination DCE switch on the WAN edge.
3. The destination DCE delivers the frames to the destination DTE.

Question 4

Outline the QoS parameters

Answer 4

1. User-to-Network Interface (UNI)
   - Interface between a FR subscriber device and a network device
2. Network-to-Network Interface (NNI)
   - Interface between 2 FR networks/switches
3. Virtual Circuit (VC)
   - Packet-switched networks may establish routes through the
   switches for particular end-to-end connections with QoS
   guarantees

• Logical circuit created within a shared network between two
  end devices

4. Permanent Virtual Circuit (PVC)
   - Pre-defined VC – stays in place for an agreed period of time
5. Switched Virtual Circuit (SVC)
   - VC that is dynamically established – torn down when
   transmission is complete – uses signalling protocol (Q.933)
6. Data Link Connection Identifier (DLCI)
   - VC identification number used by Frame Relay to switch
   packets through network

• Used on hop by hop basis, local significance only

Question 5

Is Flow Control on Frame Relay?

Answer 5

1. There is no flow control on Frame Relay - the network simply discards frames it cannot deliver.

Question 6

Congestion Control ?

FECN ?
BECN ?
DE ?
EA ?

Answer 6

Used to tell the end points to slow down, before packets start to be discarded.

Frame Relay provides indications that the network is becoming congested by means of the FECN and BECN bits in data frames.

Forward Explicit Congestion Notification (FECN, pronounced feacon)
- Sent in the direction of
destination - informs the destination that congestion has occurred

Backward Explicit Congestion Notification (BECN, pronounced beacon)
- Sent in direction of source - informs the sender that congestion has
occurred

Discard Eligible (DE) bit
- Set by network node if user has exceeded CIR and network is
experiencing congestion

EA - Address Field Extension bit – used to determine length of address field

Question 7

Backward Explicit Congestion Notification (BECN)

Answer 7

1. Happens when a FR switch detects the onset of congestion based on
   memory buffer usage or queue length
2. Switch watches for frames coming in the reverse direction on the
   connection
3. Node sets the BECN bit within those frames to signal the upstream
   nodes and the sender

Question 8

Main features of LMI:

Answer 8

1. Virtual Circuit Status Messages
   - The network and subscriber devices communicate information on
   which DLCIs have been configured for the link and on which
   DLCIs are currently active
2. Multicasting
   - Allows a sender to transmit a single frame but have it delivered by the
   network to multiple recipients – uses DLCIs 1019 to 1022
3. Global addressing
   - Gives DLCIs global rather than local significance, allowing them to
   be used to identify a specific interface to the Frame Relay network

Question 9

Frame Relay Strengths

Answer 9

1. Packet switched – efficient use of bandwidth
2. Can be used where cable, DSL not available, ATM too expensive
3. Well-established, widely-adopted standards that allow open architecture and plug-and-play
4. Low overhead combined with high reliability

Question 10

FR Address Mapping

Answer 10

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