VCAT combines multiple lower-rate SONET/SDH transport members into one logical service; LCAS lets the network adjust how many members carry that service. Together, they can provide bandwidth in smaller increments than a single larger container and can reduce the active capacity when a member fails. They are a good fit when both endpoints and the transport network support them and the service can tolerate lower throughput during a fault—not a universally optimal choice for every network.
What VCAT does in a SONET/SDH network
Virtual Concatenation (VCAT) is a Layer 1 inverse-multiplexing method. It stripes a client signal across multiple transport members and presents their combined capacity as one logical group. IETF RFC 6344 describes VCAT as a TDM-oriented byte-striping method that works with SONET, SDH, OTN and PDH signals.
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This lets an operator build an aggregate service from several smaller server signals rather than assigning one larger signal whose capacity may exceed the client’s needs. VCAT therefore addresses a bandwidth-granularity problem: the service can use a combination of available members to approach the required aggregate rate. The actual member rates and achievable aggregate capacity depend on the network’s supported containers and equipment; the standards cited here do not establish a single capacity figure for every deployment.
How LCAS adjusts a VCAT group
Link Capacity Adjustment Scheme (LCAS) is the companion control scheme for changing a VCAT group’s capacity. ITU-T Recommendation G.7042/Y.1305 specifies increasing or decreasing the capacity of a container transported over SDH or OTN using VCAT. In practical terms, the endpoints exchange control information to manage the group’s members.
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- Map the client service. Configure the client signal to use a VCAT group carried by the transport network.
- Select the initial members. Choose enough supported members to provide the required aggregate bandwidth.
- Adjust for changing demand. Use LCAS to add members when more capacity is needed or remove them when less is needed. RFC 6344 describes this as hitless dynamic resizing of the inverse-multiplex group; actual behavior depends on compatible implementations.
- Plan for member faults. If a member fails, LCAS can reduce the active group capacity; once the failed member is repaired, capacity can be increased again.
What happens when a VCAT member fails
LCAS supports graceful degradation rather than requiring the group to keep operating at its original rate regardless of faults. The intended sequence is a reduction in active capacity after a member failure and restoration of capacity after repair. Whether the client service remains useful at the reduced rate depends on the service’s bandwidth needs and the number of members still available.
| Group condition | LCAS role | Capacity implication |
|---|---|---|
| Members available | Maintains the configured group capacity. | The service uses the aggregate capacity of its active members. |
| A member fails | Can adjust the group to remove the failed member from active capacity. | Available aggregate bandwidth falls; the service may continue at a lower rate. |
| The member is repaired | Can increase group capacity again. | Capacity can return toward the configured level. |
This is not a promise that every client application will be unaffected by a fault. LCAS adjusts transport capacity; it does not create replacement bandwidth for a failed member. To improve resilience, member signals can be routed diversely, but diverse routing requires appropriate network planning and control-plane support.
What VCAT and LCAS require to work together
- Compatible endpoints: Both ends of the VCAT group need implementations that support the relevant VCAT and LCAS functions. A mismatch can prevent the group from being established or adjusted as intended.
- Standards-compliant transport: G.7042 identifies the control information and source and sink states used by LCAS. It points to ITU-T G.707 and G.783 for the SDH information fields that carry LCAS control.
- Member-path planning: Decide whether members may share routes or need diverse paths. Diversity can reduce exposure to a shared path failure, but it does not happen automatically just because VCAT is used.
- Control-plane support where needed: RFC 6344 describes GMPLS control-plane extensions for setting up and managing diversely routed VCAT members as one group. A design that needs this automation must support the relevant GMPLS functions; VCAT and LCAS alone do not supply that control-plane coordination.
- Service tolerance for reduced rate: Confirm that the client service can remain useful with less bandwidth after a member failure, or provide another protection mechanism appropriate to its availability needs.
When VCAT with LCAS is a sensible transport choice
VCAT with LCAS is worth considering when the available transport capacity comes in discrete members, the desired client bandwidth does not map neatly to a single member, and demand may change or the network should recover capacity after a member repair. Its benefits are most useful when the endpoints and network elements interoperate, the service can handle reduced capacity during faults, and member routing is designed for the required resilience.
It may be a poor fit if the equipment does not support compatible VCAT/LCAS behavior, if the client cannot tolerate reduced throughput, or if the operational complexity of coordinating members outweighs the value of adjustable aggregate capacity. Compare it with alternatives using the same criteria: bandwidth granularity, resizing interruption behavior, response to a member failure, endpoint and network-element support, route diversity, and control-plane automation. The applicable standards do not make VCAT/LCAS the best choice in every deployment.
Which standards define the mechanism
The principal references are ITU-T G.7042/Y.1305, whose in-force edition is dated March 2006, and IETF RFC 6344, published in August 2011. The ITU-T database identifies the 2001 and 2004 G.7042 editions as superseded. G.7042 covers LCAS for VCAT capacity adjustment over SDH/OTN; RFC 6344 describes VCAT and LCAS behavior and GMPLS support relevant to member management.
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