The eight services provided by the CCSDS Principal Network (CPN) are: the internet, path, encapsulation, multiplexing service, bit stream service, virtual channel access, insert, and virtual channel data unit service. The internet and path services operate asynchronously across the entire CPN. The other services may be operated in either asynchronous or isochronous mode by the space link subnetwork (SLS). The internet service is asynchronous, non-sequence preserving with lifetime control and is meant for realtime interactive command and control operations. The internet service conforms with the ISO 8473 Connectionless Network Protocol Specification . These services can be seen in Figure 1.
Figure 1: CCSDS Services
The internet service is intended for less fixed interconnections requiring flexibility. The CCSDS internet service can be used for engineering data and data communications. The internet service is intended primarily for intermittently transferring at relatively low data rates. This service can support realtime interactive control and command operations, file transfer, and interactive operations such as electronic mail and remote terminal access.
The path service is intended for high data rate users with relatively fixed interconnections. The path service has been developed to optimize the handling of telemetry data. The CCSDS path service can be used for both high and low data rate scientific instruments. Path service users do not require the functional capability provided by an OSI-like stack. The function of the path service is to route high volumes of packetized data. The path service transfers variable length application layer service data units across the network. The path service is asynchronous and non-sequence preserving. The path service is intended for transferring large volumes of data at moderate to very high rates between fairly static source and destination locations. This service is appropriate for transfer of telemetry from scientific instruments. With the path service pre-established associations known as CCSDS "Logical Data Paths", can be configured because of the known associations between the source and destination.
The encapsulation service transmits variable length data units not formatted as CCSDS packets across the SLS. Encapsulation can be used to accommodate non-CCSDS packets that have a variable length, such as TCP/IP. The transfer is asynchronous and sequence preserving. The multiplexed service sends variable length packets that conform to version-1 CCSDS packet format. The transfer is asynchronous and sequence preserving.
The bit stream service transmits serial strings of bits whose internal structure and boundaries are unknown to the CCSDS Principal Network. The transfer is sequence preserving and may be either asynchronous or isochronous. Isochronous service is provided with a specified maximum jitter. High rate video may be transferred using the bit stream service. In the bit stream service the stream of bit is broken into blocks that are mapped into fixed length data fields.
The virtual channel access service provide a project organization the ability to transfer private service data units that are matched to the size of a virtual channel data unit. The transfer is sequence preserving and may be asynchronous or isochronous.
The insert service allows private octet aligned service data units to be efficiently transferred isochronously over the SLS. The insert service may provide a quasi-isochronous service that can be used for voice transmission. The insert service establishes an insert zone in each virtual channel data unit. The insert data is inserted into this zone.
The virtual channel data unit service allows fixed length octet aligned virtual channel data units or coded virtual channel data units to be created by an independent SLS entity and to be transferred over the space link subnet (SLS).