Unformatted text preview: ing user. In this case, these two
SAPs share the same SAP address because these SAPs provide the same service to the
sending and receiving users.
The sending user of a service passes Service Data Units (SDUs) and Service Control
Information (SCI) to the service provider. The SDUs are the data that are delivered to the
receiving user. The service control information is used to specify how the SDUs should be
handled by the service provider.
In the case of TC-SDLP, the sending user may receive from the service provider information
on the status of the delivery of SDUs to the receiving user. Receiving
User SCI SDU SDU Space Data Link Protocol
(Service Provider) Figure 3-1: A Service Provided by a Space Data Link Protocol CCSDS 130.2-G-1 Page 3-1 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS The interactions between a service user and a service provider are defined in the form of
‘primitives’. Primitives present an abstract model of the logical exchange of SDUs and
control information between the service provider and the service user, but they do not specify
how the interactions should be implemented.
3.2 SERVICE FEATURES 3.2.1 DIRECTIONALITY All the services provided by the Space Data Link Protocols are unidirectional. The sending
user of a service can send data to the receiving user using a service provider (i.e., an entity of
a Space Data Link Protocol), but it cannot receive data from the receiving user using the
same service provider (see figure 3-1). If the sending user also requires to receive data from
some user, it must use a service of a different service provider (of the same or a different
Space Data Link Protocol) that provides services in the other direction.
3.2.2 RELIABILITY 126.96.36.199 General The reliability guaranteed by the services depends on the protocol used and, in some cases,
the service type selected by the user.
188.8.131.52.1 Reliability Provided by TC-SDLP
General Most of the services provided by the TC Space Data Link Protocol (TC-SDLP) have two
service types: the Sequence-Controlled Service (or the Type-A Service) and the Expedited
Service (or the Type-B Service). The service types determine how reliably the SDUs
supplied by the sending user are delivered to the receiving user.
Sending users of a service that has two service types specify for each SDU whether Type-A
or Type-B should be applied. Both service types are provided by the Communications
Operation Procedure-1 (COP-1) specified in reference .
184.108.40.206.2 Sequence-Controlled (Type-A) Service The Type-A Service utilizes an Automatic Repeat Request (ARQ) procedure of the ‘go-back-n’
type. It is implemented by sequence-control mechanisms, which include retransmissions of
lost data, at both sending and receiving ends of the space link. It also uses a standard report
returned from the receiving end to the sending end using a service of the protocol used in the
other direction on the space link. CCSDS 130.2-G-1 Page 3-2 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS The service provider guarantees, with a high probability of success, complete, in-sequence
delivery of Type-A SDUs supplied by a sending user at a single SAP (i.e., no S...
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