All narrowcast service groups are assigned to streaming zones so that all DEPI enabled transport streams are uniquely identified within an RXD device using a session ID.
An operator can define the range of Narrowcast Session IDs from 0x80008000-0x8000DFFF allowing for 57,344 unique Narrowcast Session IDs. However, due to various operational and service providing use cases that drive a large number of Narrowcast Service Groups and the services provided within them (number of transport streams, and so on), a maximum of 57,344 unique transport stream session IDs may not be adequate.
To overcome this issue, operators can manage Streaming Zones, where each Streaming Zone has its own pool of unique Narrowcast DEPI Transport Stream session IDs (0x80008000-0x8000DFFF) assigned to it. A Streaming Zone can then be applied to one or more Narrowcast Service Groups. Since sessions IDs need to be unique within an RXD, an RXD device can only belong to Narrowcast Service Groups assigned to the same Streaming Zone.