HDLC High Level Data Link Control It is

HDLC- High Level Data Link Control • It is bit oriented protocol • It support full duplex and half duplex communication in point-to-point (1 to 1) and multi point (1 to many) • • Basic Characteristics Station Types Link Configurations Data Transfer modes Ajith G. S: poposir. orgfree. com

• HDLC- • • Station Types HDLC specifies the following three types of stations Primary Station Secondary Station Combined Station Ajith G. S: poposir. orgfree. com

• HDLC- Station Types • • Primary Station Has the responsibility of controlling the operation of data flow. Handles error recovery Frames issued by the primary station are called commands. Secondary station Operates under the control of the primary station. Frames issued by a secondary station are called responses. The primary station maintains a separate logical link with each secondary station. • Combined station • Acts as both as primary and secondary station Ajith G. S: poposir. orgfree. com

• HDLC- Link Configurations • Configuration refers to the relationship between the hardware devices on a link. • Following are the two configurations defined by HDLC: • Unbalanced Configuration • Balanced Configuration Ajith G. S: poposir. orgfree. com

• HDLC- Link Configurations • Configuration refers to the relationship between the hardware devices on a link. • Following are the two configurations defined by HDLC: • Unbalanced Configuration • Balanced Configuration Ajith G. S: poposir. orgfree. com

• HDLC- Link Configurations Unbalanced Configuration • Consists of a primary station and one or more secondary stations Ajith G. S: poposir. orgfree. com

• HDLC- Link Configurations Balanced Configuration • Consists of two combined stations and supports both full duplex and half-duplex transmission Ajith G. S: poposir. orgfree. com

• HDLC- Data Transfer Modes • Normal Response mode (NRM) • Asynchronous balanced mode (ABM) • Asynchronous response mode (ARM) Ajith G. S: poposir. orgfree. com

• HDLC- Data Transfer Modes • Normal Response Mode (NRM) • Secondary station can send ONLY when the primary station instruct it to do so • Used in Two common links configurations – • Point-to-Point link (one primary station and one secondary station) – • Multipoint link (the primary station maintain different sessions with different secondary stations) Ajith G. S: poposir. orgfree. com

• HDLC- Data Transfer Modes • Normal Response Mode (NRM) Ajith G. S: poposir. orgfree. com

• HDLC- Data Transfer Modes • Asynchronous Balanced Mode (ABM) • Mainly used in point-to-point links, for communication between combined stations • Either stations can send data, control information and commands Ajith G. S: poposir. orgfree. com

• HDLC- Data Transfer Modes • Asynchronous Response Mode (ARM) • More independent secondary station Can send data or control information without explicit permission to do so (note that it is still can not send commands) Ajith G. S: poposir. orgfree. com

• HDLC • • Frames HDLC Defines three types of frames Information frames(I-frames) Supervisory frames (S-frames) Unnumbered frames (U-frames) • Each type of frames Serves as an envelope for the transmission of a different type of message. • I-frames - Used to transport user data and control information related to user data • S-frames – Used to transports only control information. • U-frames – reserved for system management. • It is used to manage link itsels Ajith G. S: poposir. orgfree. com

• HDLC- • • Frames Structure Each HDLC frame contains 6 fields A beginning flag field, address field, a control field, an information field, a frame check sequence(FCS) field and end flag field Ajith G. S: poposir. orgfree. com

• HDLC- Frames Structure • HDLC Flag 8 bit: 01111110 - indicates start and ending of frames • FCS: 16 -bit CRC using generating polynomial G(x) = x 16 + x 12 + x 5 + 1 • Address field: When a primary station is sending a frame, the address field contains the receiver identity • If a secondary station is sending the frame, the address field contains the sender identity • Control field: The control field is a 1 - or 2 -byte segment of the frame used for flow and error control • Information field: The information field contains the user's data from the network layer or management information. • Its length can vary from one network to another. It is a variable length field. Ajith G. S: poposir. orgfree. com

• HDLC- Frames Structure • Control field • Determines the type of frame and defines its functionality • Control field format for different frame type Ajith G. S: poposir. orgfree. com

• HDLC- • • • Frames Structure Control Field for I-Frames i-frames for carry user data from n/w layer They contain error control and flow control First field define type If first bit of control field is 0 indicate frame is i-frame Next 3 bits called N(S) sequence no of the frame Last 3 bits called N(R) ACK no P/F poll when frame is sent by a primary to secondary final when frame is sent by secondary to primary Ajith G. S: poposir. orgfree. com

• HDLC • • • Frames Structure Control Field for S-Frames S-frame used fro flow and error control If first 2 bit of the control field is 10 means frame is s-frame The last 3 bits, called N(R) acknowledgment number (ACK) or negative acknowledgment number (NAK) depending on the type of Sframe. The 2 bits called code is used to define the type of S-frame itself. With 2 bits, we can have four types of S-frames Receive ready (RR)- 00 Receive not ready (RNR)- 10 Reject (REJ)-01 Selective reject (SREJ)-11 Ajith G. S: poposir. orgfree. com

• HDLC- Frames Structure • Control Field for S-Frames • Receive Ready (RR) it is ready to receive an information frame and/or acknowledge previously received frames 00 • Receive Not Ready (RNR) primary or secondary station is not ready to receive any information frames or acknowledgments 10 • Reject (REJ) request the retransmission of frames 01 • Selective Reject (SREJ) A station to request retransmission of specific frames 11 Ajith G. S: poposir. orgfree. com

• HDLC- Frames Structure • Control Field for U-Frames • Used to exchange session management and control information between connected devices. • First 2 bits 11 • U-frame codes are divided into two sections: 1. 2 -bit prefix before the P/F bit 2. 3 -bit suffix after the P/F bit. • Together, these two segments (5 bits) can be used to create up to 32 different types of U-frames. Ajith G. S: poposir. orgfree. com

• HDLC- Frames Structure • Control Field for U-Frames Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL-BALANCED (LAPB) • It is a bit oriented synchronous protocol used to manage packet framing between DTE and DCE. • DTE End systems such as terminals, PCs etc. • DCE Communication devices such as modems and packet switches. • LAPB is capable of placing frames in the correct order as well as checking packets for errors. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL-BALANCED (LAPB) • DCE = Data Circuit-terminating Equipment • The equipment that performs functions, such as signal conversion and coding, at the network end of the line between the data terminal equipment (DTE) and the line • DTE = Data Terminal Equipment • An end instrument that converts user information into signals for transmission or reconverts the received signals into user information. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL-BALANCED (LAPB) Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL-BALANCED (LAPB) • LAPB operates like HDLC, however it is restricted to using the ABM transfer mode i. e. point-to-point links. • 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. • LAPB has been derived from HDLC and shares the same frame format, frame types, and field functions as HDLC. • It differs from HDLC in the representation of address field. • The address field can contain only one of two fixed (DTE or DCE) addresses. • A minimum of overhead is required to ensure flow control, error detection and recovery. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL-BALANCED (LAPB) Ajith G. S: poposir. orgfree. com

HDLC- High Level Data Link Control • FLAG: The binary pattern 01111110 is used to mark the beginning of the LAPB frame. • The transmitter in a LAPB connection uses a bit stuffing technique to make sure that the flag pattern appears only in the flag fields Ajith G. S: poposir. orgfree. com

HDLC- High Level Data Link Control • ADDRESS: it separates the link commands from the responses and can have only two values: 0 x 01 and 0 x 03. • 01 identifies frames containing commands from DTE to DCE and responses to these commands from DCE to DTE. • 03 is used for frames containing commands from DCE to DTE and for responses from DTE to DCE. Ajith G. S: poposir. orgfree. com

HDLC- High Level Data Link Control • CONTROL: The control field is used to indicate command response frames and indicates whether the frame is an Iframe, an S-frame, or a U-frame. Control frames vary in length depending on the frame type. • INFORMATION- Contains the upper layer data • FCS-Handles error checking and ensures the integrity of the transmitted data Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL - D CHANNEL (LAPD) • LAPD is used on the ISDN D channel for packet delivery. • ISDN- Integrated Services Digital Network • ISDN emerged as an alternative to traditional dialup networking. • It is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone networks • ISDN standards are constructed using the Open System Interconnection seven-layer reference model. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL - D CHANNEL (LAPD) • • • LAPD is a Layer 2 (data link) protocol. It is evolved from the LAPB protocol. LAPD handles the handshaking, signaling and control for all the voice and data calls that are set up through the ISDN D channel. • D channel is the data channel which is used for communications between switching equipment in the ISDN network and the ISDN equipment at your site. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL - D CHANNEL (LAPD) • LAPD works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced ( no master/slave relationship). • Each station may initialize, supervise, recover from errors, and send frames at any time. • The objective of LAPD is to provide a secure, error-free connection between two end-points so as to reliably transport Layer 3 messages. Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL - D CHANNEL (LAPD) Ajith G. S: poposir. orgfree. com

LINK ACCESS PROTOCOL - D CHANNEL (LAPD) • • The LAPD Flag and Control fields are identical to those of HDLC. • • Terminal endpoint identifier (TEI) � 7 bits The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit(EA) of the first byte is set, the address is 1 byte if it is not set, the address is 2 bytes. a two-part address for dealing with two levels of multiplexing multiple user devices sharing the same physical interface multiple types of traffic within each user device The first Address-field byte contains the service access point identifier (SAPI), which identifies the port at which LAPD services are provided to Layer 3� 6 bits given to each user device manually or automatically unique IDs given to each device A TEI of all ones indicates a broadcast. SAPI and TEI together identify a logical connection –DLCI(Data Link Control Identifier) SAPI values are unique within a TEI The C/R bit indicates whether the frame contains a command or a response� 1 bit Ajith G. S: poposir. orgfree. com