BIT ORIENTED PROTOCOLS : SDLC, HDLC, BISYNC, LAP & LAPB

BIT ORIENTED PROTOCOLS

A bit-oriented protocol is a communications protocol that sees the transmitted data as an opaque stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Bit oriented protocol can transfer data frames regardless of frame contents. It can also be stated as "bit stuffing" this technique allows the data frames to contain an arbitrary number of bits and allows character codes with arbitrary number of bits per character.

SDLC

Synchronous Data Link Control (SDLC) supports a variety of link types and topologies. It can be used with point-to-point and multipoint links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and circuit-switched and packet-switched networks.

SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the operation of other stations, called secondary. The primary polls the secondary in a predetermined order and secondary can then transmit if they have outgoing data. The primary also sets up and tears down links and manages the link while it is operational. Secondary nodes are controlled by a primary, which means that secondary can send information to the primary only if the primary grants permission.

 

SDLC primaries and secondary can be connected in four basic configurations:

• Point-to-point---Involves only two nodes, one primary and one secondary.
• Multipoint---Involves one primary and multiple secondary.
• Loop---Involves a loop topology, with the primary connected to the first and last secondary. Intermediate secondary pass messages through one another as they respond to the requests of the primary.
• Hub go-ahead---Involves an inbound and an outbound channel. The primary uses the outbound channel to communicate with the secondary. The secondary use the inbound channel to communicate with the primary. The inbound channel is daisy-chained back to the primary through each secondary.

                                                                SDLC

•  Flag- Initiates and terminates error checking.
• Address- Contains the SDLC address of the secondary station, which indicates whether the frame comes from the primary or secondary. This address can contain a specific address, a group address, or a broadcast address. A primary is either a communication source or a destination, which eliminates the need to include the address of the primary.
• Control- Employs three different formats, depending on the type of SDLC frame used:

1. Information (I) frame: Carries upper-layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs  flow and error control. The send-sequence number refers to the number of the frame to be sent next. The receive-sequence number provides the number of the frame to be received next. Both sender and receiver maintain send- and receive-sequence numbers.

A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response.

2.  Supervisory (S) frame: Provides control information. An S frame can request and suspend transmission, reports on status, and acknowledge receipt of I frames. S frames do not have an information field.

3.  Unnumbered (U) frame: Supports control purposes and is not sequenced. A U frame can be used to initialize secondary. Depending on the function of the U frame, its control field is 1 or 2 bytes. Some U frames have an information field.

• Data- Contains path information unit (PIU) or exchange identification (XID) information.
• Frame Check Sequence (FCS) - Precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If the result differs from the value in the original frame, an error is assumed.

HDLC

• High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol.
  
• HDLC provides both connection-oriented and connectionless service.
•  HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM).
  
• The original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM) are rarely used.
  

HDLC FRAMING

HDLC frames can be transmitted over synchronous or asynchronous serial communication links. Those links have no mechanism to mark the beginning or end of a frame, so the beginning and end of each frame has to be identified. This is done by using a frame delimiter, or flag, which is a unique sequence of bits that is guaranteed not to be seen inside a frame. This sequence is '01111110', or, in hexadecimal notation, 0x7E. Each frame begins and ends with a frame delimiter. A frame delimiter at the end of a frame may also mark the start of the next frame.

Frame structure

The contents of an HDLC frame are shown in the following table:

Note that the end flag of one frame may be (but does not have to be) the beginning (start) flag of the next frame.

Data is usually sent in multiples of 8 bits, but only some variants require this; others theoretically permit data alignments on other than 8-bit boundaries.

There are three fundamental types of HDLC frames.

• Information frames, or I-frames, transport user data from the network layer. In addition, they can also include flow and error control information piggybacked on data.
• Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.
• Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type.

BISYNC

Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half duplex link protocol. It replaced the synchronous transmit-receive (STR) protocol used with second generation computers. The intent was that common link management rules could be used with three different character encodings for messages. Six-bit Transcode looked backwards to older systems.

• BISYNC establishes rules for transmitting binary-coded data between a terminal and a host computer's BISYNC port.
• BISYNC is a half-duplex protocol, it will synchronize in both directions on a full-duplex channel.
• BISYNC supports both point-to-point (over leased or dial-up lines) and multipoint transmissions. Each message must be acknowledged, adding to its overhead.

• BISYNC is character oriented, meaning that groups of bits (bytes) are the main elements of transmission, rather than a stream of bits. The BISYNC frame is pictured next.

 It starts with two sync characters that the receiver and transmitter use for synchronizing. This is followed by a start of header (SOH) command, and then the header. Following this are the start of text (STX) command and the text. Finally, an end of text (EOT) command and a cyclic redundancy check (CRC) end the frame. The CRC provides error detection and correction.

Most of the bisynchronous protocols, of which there are many, provide only half-duplex transmission and require an acknowledgment for every block of transmitted data. Some do provide full-duplex transmission and bit-oriented operation.

BISYNC has largely been replaced by the more powerful SDLC (Synchronous Data Link Control).

LAP AND LAPB

Link Access Procedure (LAP) protocols are Data Link layer protocols for framing and transmitting data across point-to-point links. LAP was originally derived from HDLC (High-Level Data Link Control), but was later updated and renamed LAPB (LAP Balanced).

LAPB is the data link protocol for X.25.LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. It can be used as a Data Link Layer protocol implementing the connection-mode data link service in the OSI Reference Model as defined by ITU-T Recommendation X.222.

LAPB is used to manage communication and packet framing between data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) devices in the X.25 protocol stack. LAPB is essentially HDLC in Asynchronous Balanced Mode (ABM). LAPB sessions can be established by either the DTE or DCE. The station initiating the call is determined to be the primary, and the responding station is the secondary.

Frame types

• I-Frames (Information frames): Carries upper-layer information and some control information. I- frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send and receive sequence numbers.
• S-Frames (Supervisory Frames): Carries control information. S-frame functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames. S- frames carry only receive sequence numbers.
• U-Frames (Unnumbered Frames): carries control information. U-frame functions include link setup and disconnection, as well as error reporting. U-frames carry no sequence numbers

Frame format

Flag – The value of the flag is always 0x7E. In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit stuffing is used by both the transmitter and the receiver.

Address field – In LAPB, this field has no meaning since the protocol works in a point to point mode and the DTE network address is represented in the layer 3 packets. This byte is therefore put to a different use; it separates the link commands from the responses and can have only two values: 0x01 and 0x03. 01 identifies frames containing commands from DTE to DCE and responses to these commands from DCE to DTE. 03 are used for frames containing commands from DCE to DTE and for responses from DTE to DCE.

Control field – it serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.