CONCATENATION

Concatenation is the process of summing the bandwidth of X containers (C-i) into a larger container. This provides a bandwidth X times bigger than C-i. It is well indicated for the transport of big payloads requiring a container greater than VC-4, but it is also possible to concatenate low-capacity containers, such as VC-11,VC-12, or VC-2.

Figure An example of contiguous concatenation and virtual concatenation. Contiguous concatenation requires support by all the nodes. Virtual concatenation allocates bandwidth more efficiently, and can be supported by legacy installations.

There are two concatenation methods:

1. Contiguous concatenation: which creates big containers that cannot split into smaller pieces during transmission. For this, each NE must have a concatena- tion functionality.
2. Virtual concatenation: which transports the individual VCs and aggregates them at the end point of the transmission path. For this, concatenation functionality is only needed at the path termination equipment.

Contiguous Concatenation of VC-4

A VC-4-Xc provides a payload area of X containers of C-4 type. It uses the same HO-POH used in VC-4, and with identical functionality. This structure can be transported in an STM-n frame (where n = X). However, other combinations are also possible; for instance, VC-4-4c can be transported in STM-16 and STM-64 frames. Concatenation guarantees the integrity of a bit sequence, because the whole container is transported as a unit across the whole network .

Obviously, an AU-4-Xc pointer, just like any other AU pointer, indicates the position of J1, which is the first byte of the VC-4-Xc container. The pointer takes the same value as the AU-4 pointer, while the remaining bytes take fixed values equal to Y=1001SS11 to indicate concatenation. Pointer justification is carried out the same way for all the X concatenated AU-4s and X x 3 stuffing bytes .

However, contiguous concatenation, today, is more theory than practice, since other alternatives more bandwidth-efficient, such as virtual concatenation, are gaining more importance.

Virtual Concatenation

Connectionless and packet-oriented technologies, such as IP or Ethernet, do not match well the bandwidth granularity provided by contiguous concatenation. For example, to implement a transport requirement of 1 Gbps, it would be necessary to allocate a VC4-16c container, which has a 2.4-Gbps capacity. More than double the bandwidth that is needed.

Virtual concatenation (VCAT) is a solution that allows granular increments of bandwidth in single VC-n units. At the MSSP source node VCAT creates a continuous payload equivalent to X times the VC-n units . The set of X containers is known virtual container group (VCG) and each individual VC is a member of the VCG. All the VC members are sent to the MSSP destination node independently, using any available path if necessary. At the destination, all the VC-n are organized, according the indications provided by the H4 or the V5 byte, and finally delivered to the client

Differential delays between VCG member are likely because they are transported individually and may have used different paths with different latencies. Therefore, the destination MSSP must compensate for the different delays before reassembling the payload and delivering the service.