Data Link Layer Protocol

The NOAAPORT Broadcast System (NBS) uses a subset of the high-level data-link control (HDLC) protocol for its Data-Link Layer. The Data-Link Layer, as defined by the International Standards Organization (ISO) Open Systems Interconnection (OSI) reference model, provides point-to-point communications over a physical transmission medium by creating and recognizing frame boundaries. The NBS uses the HDLC frame and HDLC framing boundaries for this purpose.

The HDLC framing code or flag sequence consists of a 0 bit followed by six 1 bits followed by a 0 bit (01111110). It is used to demarcate the boundaries of a message packet. The rest of the message packet uses the general format shown below. This consists of a beginning frame flag sequence (F), an address field (A), a control field (C), an optional information field (I), the frame check sequence (FCS), and an ending frame flag sequence (F).

Because the NBS is a point-to-multipoint system, the address field is set to the broadcast address: 11111111. The control field is always present in the frame but is not used by the Satellite Broadcast Network (SBN) protocol. As a result, the SBN protocol does not look at its value but assumes that it is present in the frame. The optional information field houses the SBN protocol and its associated data. The FCS field is used by the HDLC protocol to verify the integrity of each HDLC frame. In general, the NBS uses the HDLC frame to envelop the SBN message packets. The following table provides more details of the HDLC framing.

The 0 bit insertion/deletion required by HDLC enables the flag sequence to frame bit streams into a packet, thus allowing the data between framing flags to contain any bit pattern. This is known as transparency. At the transmitting station, a 0 bit is inserted after each sequence of five consecutive 1s. At the receiving station, the 0 bit that follows every five consecutive 1 bits is deleted.

The NBS encodes the data using nonreturn-to-zero (NRZ) encoding. This encodes a 1 as a high level and a 0 as a low level. With this encoding method, only a minimal amount of clocking information is available in the data stream in the form of transitions on bit-cell boundaries, which do not allow for self-clocking.

The Cyclical Redundancy Check (CRC)-16 FCS uses the International Telegraph and Telephone Consultative Committee (CCITT) Rec. V.41 generating polynomial X16 + X12 + X5 + 1.

The remainder of the HDLC protocol is unused.