be used for transmission. Thus before transportation, several small containers can be combined to form a group. Start of frame Data transmission Empty frame Figure 3.15 Transport frame Digital Hierarchies - PDH and SDH Stuffing information Position within the container Figure 3.16 Conbining container in a container group This group is then packed into a larger container, as shown in Figure 3.16. Each of these containers includes a label, which is evaluated by the receiver. Whenever necessary, stuffing information is added. The individual containers are assigned a certain position within the group. The position number determines the start of the respective container. The type of payload in the containers is unimportant for transportation as it is just the raw information that is transmitted. The stuffing information, which is included to fill up the unutilised space, can be regarded as part of the payload. Concatenation The above description was based on the assumption that the payload is smaller than the container available. If the payload to be transported is larger than the container available for it, several containers can be concatenated. They then form a continuous container chain, as shown in Figure 3.17. In this case, the payload is distributed on this container chain. An example will now be given to illustrate this. The source signal is 599.04 Mbps (broadband ISDN). Since the largest container defined can transport a signal only up to 140 Mbps, four such containers have to be concatenated. The position of the container chain on the conveyor belt is defined for the first container. The first one determines the position of all other containers 2, 3 and 4. This SDH system is analogous to a real world application. A company's production unit producing items and packing them to deliver to the market will be considered. The company is analogous to the SDH system, which is managing the hierarchy of steps taken to get the final result. The final result can be thought of as the multiplexed signal to be sent over t