he channel. All the data streams to be sent over the channel are first collected. These data streams may be of different capacities, e.g. an ATM signal, a PDH stream or an ISDN BRI (basic rate interface) channel. In order to send these signals over the same channel they need to be multiplexed using the SDH system. In the SDH system the signals are put in their respective containers according to their capacities just like items of different sizes are put into boxes that are meant for them only. Next, on the outside of the boxes is written the name, etc., of the respective item it contains, which makes it easier for the receiver Transmission Network Planning and Optimisation 1st container 2nd container 3rd container 4th container Position Concatenation Start of frame Figure 3.17 Concatenated containers to decipher the item inside the box. Similarly, the signal containers are labelled using certain overheads for easy unpacking. If the box is big enough nothing further needs to be added as the box carrier is just meant for a particular capacity and can carry a box of only a particular capacity. However, if there is still some space left inside the carrier, more boxes can be clubbed together in a bigger box, which is again labelled and put on the carrier. In this way the capacity of the carrier is used to its fullest and is how the SDH proceeds. If, say, two PDH signals along with an ATM signal need to be sent then the two PDH signals would not occupy the full channel. Thus to utilise the channel properly the PDH and ATM signals are put in their respective containers and are then combined at the next level of hierarchy. This bigger container with all the overheads included can then be shipped to the destination where it is unpacked. SDH Multiplex Elements An analogy between the SDH system and the real world clears the picture of higher order multiplexing, but the way these containers are made, labelled and moved up in the hierarchy is still hazy. In the following subsections there is a discussion on different types an