- Data-link control requirements.
- A grammar of communication.
- Requirements include: framing, addressing, and error detection and
correction.
- Also requirements imposed by applications and communications
environment.
- Application requirements.
- Conversational traffic: short message exchanges with long turn-around
between exchanges.
- Low overhead, short-response time, small variance; operator-assisted
error detection and correction.
- Query traffic: short messages one way, big ones the other way.
- Piplining queries at the responder.
- Batch traffic: big messages both ways, automated communications.
- Automatic error detection and recover.
- Processor-processor traffic:
- Facility requirements.
- Station verification and identification.
- Adaptive to propagation delays.
- Hub-link polling.
- Minimizes line turn-around.
- Loop topologies.
- Pass-along polling.
- More restrictive control schemes (reception is not a permission).
- Response multiplexing.
- Major requirements.
- Central control.
- Independent transmission block length.
- No framing ambiguity.
- No excessive control.
- Line speed and delay adaptive.
- Duplex and topology adaptive.
- Adaptive redundancy for error detection and correction.
- Automatic and common error detection and correction mechanisms.
- Open ended control and addressing schemes.
- Fast and independent recovery.
- Management and control information.
- General DLC.
- The frame syntax is F A? C? Information? BC? F?
- Bit oriented, character-set independent.
- Fields start at bit-relative positions.
- Positions can be self describing or pre-determined.
- The F field is the start of frame indicator.
- The A field is the destination address.
- The C field provides meta-level protocol command information.
- An optional I field for information.
- The BC field provides for checksums or other error detection and
correction data.
- The trailing flag sequence.
- Can be replaced by fixed-length frames or delimited fields.
- The general structure can be shared among different architectures.
- SDLC objectives.
- Efficient support for
- Two-way simultaneous transmission of data and control.
- Support both batch and interactive communication.
- Delay insensitive.
- Works over half-duplex.
- Provide reliability, maintenance, and adaptability.
- Encompass services from previous DLC protocols.
- SDLC scope
- Serial-by-bit synchronous data link with central control.
- A single central station and multiple outline secondary stations.
- Formatting for block framing, addressing, commands and responses.
- Error detection and recover.
- Not included are
- Record delimiting (higher-layer function).
- Device control (lower-layer functions).
- Circuit management (higher-layer function).
- SDLC concepts.
- A grammar, independent of payload and representation.
- Reliable transmission, link-end to link-end.
- All fields are required and total 48 overhead bits.
- The flag.
- Isochronous transmission and synchronous detection.
- The flag is 0 1 1 1 1 1 1 0 and brackets the rest of the frame.
- Use bit stuffing to insert a 0 after five consecutive ones and strip
it on reception.
- Station address.
- A station has a unique address, zero or more multi-cast addresses and
a broadcast address.
- An eight-bit address to identify the designated receiver.
- Addressing may be multi-cast.
- Control.
- On transmission, indicates information transfer, supervisory
commands, and non-sequenced commands.
- For information transfer, the control field has
- FI a one-bit identifier.
- Ns, a three-bit sending sequence number.
- P/F, a one-bit polling or terminating frame indicator.
- Nr, a three-bit receiving sequence count.
- For supervisory frames, the control fields are
- FI, a two-bit identifier.
- S, a two-bit function indicator.
- P/F
- Nr
- No information fields in the frame.
- For nonsequenced frames, the control fields are
- FI, a two-bit identifier.
- M, a five-bit modifier-function indicator.
- P/Fn
- M
- Nonsequence frames don't use sequence numbers and provide other
data-link control functions.
- Broadcast or multi-cast messages, end-station control,
non-sequenced polls, identity exchange, transmission
termination.
- Secondary stations can also indicate status or make requests of
the primary station.
- Supervisory functions are send permission, retransmission
requests, acknowledgments.
- Information.
- Only provided in payload-frames (I and NS frames)
- Length-independent, constrained only by buffering requirements.
- Fully transparent via bit stuffing.
- Block-check field.
-
Polynomial remainder
as redundant information.
- Covers all bits between fields, except for the stuff bits.
- Semantics.
- Primary station initiated, with secondary stations options.
- Normal response mode invoked by the primary station.
- Asynchronous response mode triggered by the primary station.
- Asynchronous stations are still coordinated by a go-ahead frame.
- The asynchronous sender must determine the primary station ack.
- Initialization mode lets the primary station force secondary stations
into a known initial condition.
- SDLC attributes.
- Overall, simplicity.
- The flag delimiter and synchronizer.
- A simple wake-up call for receiving stations.
- Asynchronous communication (no central timing).
- End-to-end delimiters allow for arbitrary length payloads.
- Broken flags turn into abort signals.
-
Idle state flag transmission.
- A common frame format.
- Simplifies recognition and transmission.
- Provides a constant frame for the payload (network pdu).
- Functional separation between frame components.
- Frame integrity checking.
- Central control via the primary station.
- Establishes initial communications and manages error retries.
- Frame-by-frame addressing.
- Better interactive communication.
- Station error recover can occur in parallel with communication with
other stations.
- Flexible with respect to the networking environment.
- point-to-point or multi-point, duplexities, topologies, delays.
This page last modified on 14 November 2004.