DLCIs

Last Updated : Jun 03, 2015 |
Navigation: WAN Port | DLCIs
Prolog information
Metadata
Keywords:
Resource Id: WAN_DLCI:18100
DLCIs are created for Frame Relay connections. These settings are not mergeable. Changes to these settings will require a reboot of the system.
Field
Description
Frame Link Type
Default = PPP
Data transfer encapsulation method. Set to the same value at both ends of the PVC (Permanent Virtual Channel). The options are:
  • None
  • PPP Using PPP offers features such as out of sequence traffic reception, compression and link level connection management.
  • RFC 1490 RFC 1490 encapsulation offers performance and ease of configuration and more inter-working with third party CPE.
  • RFC1490 + FRF12 Alternate encapsulation to PPP for VoIP over Frame Relay. When selected all parameters on the Service | PPP tab being used are overridden.
DLCI
Default = 100 This is the Data Link Connection Identifier, a unique number assigned to a PVC end point that has local significance only. Identifies a particular PVC endpoint within a user's physical access channel in a frame relay.
RAS Name
Select the RAS Service you wish to use.
Tc
Default = 10
This is the Time Constant in milliseconds. This is used for measurement of data traffic rates. The Tc used by the system can be shorter than that used by the network provider.
CIR
(Committed Information Rate) Default = 64000 bps This is the Committed Information Rate setting. It is the maximum data rate that the WAN network provider has agreed to transfer. The committed burst size (Bc) can be calculated from the set Tc and CIR as Bc = CIR x Tc. For links carrying VoIP traffic, the Bc should be sufficient to carry a full VoIP packet including all its required headers. See the example below.
EIR
(Excess Information Rate) Default = 0 bps This is the maximum amount of data in excess of the CIR that a frame relay network may attempt to transfer during the given time interval. This traffic is normally marked as De (discard eligible). Delivery of De packets depends on the network provider and is not guaranteed and therefore they are not suitable for UDP and VoIP traffic. The excess burst size (Be) can be calculated as Be = EIR x Tc.

Example: Adjusting the Tc Setting

G.729 VoIP creates a 20 byte packet every 20ms. Adding typical WAN PPP headers results in a 33 byte packet every 20ms.
For a Committed Information Rate (CIR) of 14Kbps, with the Time Constant (Tc) set to 10ms; we can calculate the Committed Burst size:
Bc = CIR x Tc = 14,000 x 0.01 = 140 bits = 17.5 bytes.
Using 10ms as the Tc, a full G.729 VoIP packet (33 bytes) cannot be sent without exceeding the Bc. The most likely result is lost packets and jitter.
If the Tc is increased to 20ms:
Bc = CIR x Tc = 14,000 x 0.02 = 280 bits = 35 bytes.
The Bc is now sufficient to carry a full G.729 VoIP packet.
Notes
  1. Backup over Frame Relay is not supported when the Frame Link Type is set to RFC1490.
  2. When multiple DLCIs are configured, the WAN link LED is switched off if any of those DLCIs is made inactive, regardless of the state of the other DLCIs. Note also that the WAN link LED is switched on following a reboot even if one of the DLCIs is inactive. Therefore when multiple DLCIs are used, the WAN link LED cannot be used to determine the current state of all DLCIs.
  3. When the Frame Link Type is set to RFC1490, the WAN link LED is switched on when the WAN cable is attached regardless other whether being connected to a frame relay network.
Related links
WAN Port