The term concatenation means the bringing together or linking of separate items. In data transmission, concatenation refers to the linking of strings, files, or packets in a specific order to form a single entity for transmission. More specifically, in the context of Sonet/SDH, it refers to a procedure for transmitting a data payload that has a bandwidth greater than the capacity of the standard synchronous payload envelope (SPE) frame.
The ITU-T in its recommendation G.707 defines concatenation as "A procedure whereby a multiplicity of Virtual Containers is associated, one with another, with the result that their combined capacity can be used as a single container across which bit sequence integrity is maintained."
There are two types of concatenation—contiguous and virtual. In contiguous concatenation, a pointer is placed in each of the frames (SPEs) to be concatenated. To achieve the bandwidth efficiency promised by this method, every intermediate node through which the concatenated string passes must be programmed to support this operating mode. This requires extensive and expensive hardware and software upgrades at every system node.
In virtual concatenation, no intermediate node support is required. Instead, each SPE within a concatenated group representing the data packet for transmission is given an identifier. Provided as part of the Sonet path overhead (POH) information in the SPE, this identifier indicates the SPE's sequence and position within the group.
At the receiver, the SPEs can be reassembled in the correct order to recover the data. To compensate for different arrival times of the received data, known as differential delay, the receiving circuits must contain some buffer memory so that the data can be properly realigned.
The figure shows the transmission efficiency of a framer both with and without virtual concatenation. The top drawing illustrates how a standard OC-48 framer without virtual concatenation would handle a Gigabit Ethernet packet (GE#1) by spreading it out over seven OC-3 frames. The remaining frames in the OC-48 stream are wasted.
The lower drawing shows how the POSIC OC-48 framer would handle things. The first Gigabit Ethernet packet (GE#1) still occupies seven OC-3 frames, but the wasted space is used to interleave and transmit another Gigabit Ethernet packet (GE#3) and three Fast Ethernet packets (FE#2, FE#4, and FE#5).