Nokia’s implementation supports IMA functionality as specified in ATM Forum’s Inverse Multiplexing for ATM (IMA) Specification Version 1.1 (af-phy-0086.001, March 1999). The following capabilities are supported:
TX Frame length — Only IMA specification default of 128 cells is supported.
IMA version — Both versions 1.0 and 1.1 of IMA are supported. There is no support for automatically falling to version 1.0 if the far end advertises 1.0 support, and the local end is configured as 1.1. Because of potential protocol interoperability issues between IMA 1.0 implementations, it is recommended that IMA version 1.1 is used whenever possible.
Alpha, beta, and gamma values supported are defaults required by the IMA specification (values of 2, 2, and 1 respectively).
Clock mode — Only IMA specification default of common clock mode is supported (CTC).
Timing reference link — The transmit timing reference link is chosen first among the active links in an IMA group. If none found, then it is chosen among the usable links or finally, among the unusable links.
Cell Offset Configuration — The cell offsets for IMA links are not user configurable but internally assigned according to the recommended distribution described in the IMA spec.
TX IMA ID — An internally assigned number equal to the IMA bundle number.
Minimum Links — A configurable value is supported to control minimum member links required to be up for an IMA group to stay operationally up.
Maximum Group Bandwidth — A configurable value is supported to specify maximum bandwidth available to services over an IMA group. The maximum may exceed the number of minimum/configured/active links allowing for overbooking of ATM shaped traffic.
Symmetry mode — Only IMA specification default of symmetric operation and configuration is supported.
Re-alignment — Errors that require a re-alignment of the link (missing or extra cells, corrupted frame sequence numbers), are dealt with by automatically resetting the IMA link upon detection of an error.
Activation/Deactivation Link Delay Timers — Separate, configurable timers are supported defining the amount of delay between detection of LIF, LODS and RFI-IMA change and raising/clearing of a respective alarm to higher layers and reporting RXIFailed to the far end. This protocol dampening mechanism protects those higher layers from bouncing links.
Differential delay — A configurable value of differential delay that is tolerated among the members of the IMA group is supported. If a link exceeds the configured delay value, then LODS defect is declared and protocol management actions are initiated as required by the IMA protocol and as governed by Link Activation and Deactivation procedures. The differential delay of a link is calculated based on the difference between the frame sequence number received on the link and the frame sequence number received on the fastest link (a link on which the IMA frame was received first).
Graceful link deletion — The option is supported for remotely originated requests only. To prevent data loss on services configured over an IMA group, it is recommended to initiate graceful deletion from the far end before a member link is deleted or a physical link is shutdown.
IMA test pattern — Nokia’s implementation supports test pattern procedures specified in the IMA specification. Test pattern procedures allow debugging of IMA group problems without affecting user data. Test pattern configurations are not preserved upon a router reboot.
Statistics — Nokia’s IMA implementation supports all standard-defined IMA group and IMA link status and statistics through proprietary TIMETRA-PORT-MIB. Display and monitoring of traffic related interface/SAP statistics is also available for IMA groups and services over IMA groups on par with physical ATM interfaces and services.
Scaling — Up to 8 member links per IMA group, up to 128 groups per MDA and all DS-1/E-1 links configurable per MDA in all IMA groups per MDA are supported.