Ethernet Basics_GFP, VCAT, and LCAS

Which technologies go into transmitting data services over SDH networks?


Generic framing procedure (GFP) is a multiplexing technique defined by ITU-T G.7041. This allows mapping of client signals over an SDH network. There are two GFP modes:

Term Generic Framing Procedure Transparent Generic
Framing Procedure
Encapsu-lation process

GFP-F maps each variable- length client frame such as an Ethernet

MAC frame into a single GFP frame.

GFP-F is not used where frames are

 divided or reassembled.

GFP-T allows mapping of multiple fixed-length block-coded client data streams such as ESCON, FC, and FICON into a block code for

transport within a fixed-length GFP frame.

Difference Variable length, large delay,non-real time Fixed length, small delay,real time
IP, PPP, Ethernet SAN, video

From the above info, we can see that GFP encapsulation enables the SDH network to flexibly carry various types of data services.


Concatenation is an inverse multiplexing technique creating a large capacity payload container distributed over multiple smaller capacity TDM signals. There are two types of concatenation modes:

– Contiguous concatenation: Adjacent virtual containers in an STM-N frame are concatenated into one VC-4/3-Xc (X = 4, 16, 64) container.Contiguous concatenation must be enabled on all nodes that a service passes through.

– Virtual concatenation: Non-adjacent virtual containers in one or several STM-N frames are concatenated into one VC-4/3/12-Xv (X ? 64) container or VC group.Virtual concatenation can be enabled only on the source and sink nodes.

Take riding a train as an example: A family of four gets onboard only to find no four adjoining seats.

– Contiguous concatenation: We need to sit next to each other, so we need four adjoining seats.

– Virtual concatenation (VCAT): All we need is four empty seats anywhere on the train!

Let’s look at the comparisons in terms of bandwidth utilization from the different concatenation modes.

Service Bit Rate Utilization without VCAT Utilization with VCAT
FE 100 Mbps VC-4 (67%) VC-3-2v (100%)
GE 1000 Mbps VC-4-16c (42%) VC-4-7v (95%)
FC 200 Mbps VC-4-4c (33%) VC-3-4v (100%)
FC 1000 Mbps VC-4-16c (42%) VC-4-7v (95%)
ESCON 200 Mbps VC-4-4c (33%) VC-3-4v (100%)

Observably, VCAT enables greater efficiency and flexibility in transmitting data services over SDH networks.


Both the contiguous concatenation and virtual concatenation have the following vulnerabilities:

– When any physical channel fails, all the concatenated channels fail and all services are interrupted.

– If the bandwidth is adjusted after a service is set up, the service will be adversely affected.

Link capacity adjustment scheme (LCAS) is a method to dynamically increase or decrease the bandwidth of virtual concatenated containers.The LCAS protocol is specified in ITU-T G.7042. It allows on-demand increase or decrease of the bandwidth of the virtual concatenated group in a hitless manner. This brings bandwidth on-demand capability and survivability of data traffic.

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