data link control protocol(3)

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Lecture 03Data-Link Protocols

Chapter 23Electronic Communications Sytems , Fifth Editon

By: Wayne Tomasi

ECE @Saint Louis University, Baguio City

Prepared by: Engr. Jeffrey Des B. Binwag

ECE @Saint Louis University, Baguio City 2

SYNCHRONOUS DLCP• SYNCHRONOUS DATA-LINK CONTROL (SDLC)– A synchronous bit-oriented protocol (BOP) developed in

the 1970s by IBM for use in system network architecture (SNA) environments

– The first link-layer protocol based on synchronous, bit-oriented operation

– Can transfer data simplex, half-duplex, or full duplex and can support a variety of link types and topologies

– Can be used on point-to-point and multipoint networks over both circuit and packet-switched networks

– Defines two types of nodes: primary stations and secondary stations


SDLC TRANSMISSION STATES– TRANSIENT STATE. Exists before and after and

after an initial transmission and after each line turnaround

– IDLE STATE. A circuit status assumed by the secondary after receiving 15 or more consecutive logic 1s

– ACTIVE STATE. Exists whenever either the primary or one of the secondary stations is transmitting information or control signals


SDLC FRAME FORMAT COMPONENTS

1. Flag Field (beginning and ending)2. Address Field3. Control Field4. Information (or text) Field5. Frame Check Sequence Field


SDLC FRAME FORMAT

FlagField8 bits

AddressField8 bits

ControlField8 bits

InformationField

Variable length

Frame Check

Sequence (CRC-16)

FlagField8 bits

F A C I FCS F

7EH 7EHSpan of CRC accumulation

Span of zero insertion

0111111001111110


SDLC FLAG FIELD VARIATIONS1. One beginning and one ending flag per frame

2. Ending flag for one frame is used as the beginning flag of the next frame (Shared flag)

FCCI FL

AGFCCA C I FL

AGFCCA C I FL

AG A

FLAG

FCCA C I FL

AG A C IFLAG

FCCI FL

AG


3. The last zero of an ending flag can be the first zero of the beginning flag of the next frame(Shared Zero)

4. Flags are transmitted continuously during the time between frames in lieu of idle ones

SDLC FLAG FIELD VARIATIONS

FLAG

FCCA C I FL

AGFLAG

FLAG

FLAG

FLAG

FLAG

Ending Flag & Starting Flag with

shared zero

011111101111110

FCCI A C I

Ending Flag & Starting Flag with

shared zero

011111101111110

FCC A C


SDLC ADDRESS FIELD• 8 bits in length• 256 possible addresses• 00000000 is designated as the null address and is never

assigned to a secondary• 11111111 is designated as the broadcast address and

used by the primary station to send a message to all secondary stations

• The rest of the 254 addresses are utilized as a unique station address intended for one secondary station only or as a group address intended for several secondary stations.


SDLC CONTROL FIELD FRAME FORMATS

• INFORMATION FRAME– Used to transmit sequenced information that must be

acknowledged by the destination station• SUPERVISORY FRAME– Used to confirm previously received information

frames, convey ready or busy conditions, and for a primary to poll a secondary when the primary does not have any numbered information to send to the secondary

• UNNUMBERED FRAME– Used to send network control and status information


• INFORMATION FRAME (Primary to Secondary)

• INFORMATION FRAME (Secondary to Primary)

SDLC CONTROL FIELD INFORMATION FRAME

b0 b1 b2 b3 b4 b5 b6 b7

nr nsP/P 0_

b0 b1 b2 b3 b4 b5 b6 b7

nr nsF/F 0_


SDLC CONTROL FIELD INFORMATION FRAME

• nr (Number received). Indicates the number of the next frame expected or the frame number of the frame containing an error for Go-back-n-frames sliding window ARQ

• P/P (Poll or not poll). Indicates if the primary is sending a poll to the secondary or not

• ns (Number sent). Indicates the frame number of the frame currently being sent

• F/F (Final or not final frame). Indicates if the secondary is sending its final frame or not

• b7 = 0, identifies the control field as an information frame

_

_

SDLC

EXCHA

NGE OF INFORMATION

FIELDS


SDLC CONTROL FIELD SUPERVISORY FRAME

b0 b1 b2 b3 b4 b5 b6 b7

nr X XFunction Code

P/P 0 1Indicates

supervisory frame

_

F/F

b4 b5 Receive Status0 0 Ready to receive (RR)

0 1 Ready not to receive (RNR)

1 0 Reject (REJ)

1 1 Not used with SDLC (Used in HDLC for Selective Reject SREJ)

Supervisory Frame Functions

_


SDLC CONTROL FIELD UNNUMBERED FRAME

b0 b1 b2 b3 b4 b5 b6 b7

X XFunction Code

P/P 1 1Indicates

unnumbered frame

_

F/F

[000(P/F)0011] Unnumbered Information (UI) [010(F)0011] Request Disconnect (RD)

[000(P)0111] Set Initialization Mode (SIM) [010(P)0011] Disconnect (DISC)

[000(F)0111] Request Initialization Mode (RIM) [011(F)0011] Unnumbered Acknowledgment (UA)

[100(P)0011] Set Normal Response Mode (SNRM) [100(F)0111] Frame Reject (FRMR)

[000(F)1111] Disconnect Mode (DM) [111(P/F)0011] TEST

[101(P/F)0011] Exchange Station ID (XID) [110(P/F)0111] Configure (CFGR)

[111(F)1111] Beacon (BCN)

Unnumbered Frame Functions

X X XFunction Code _



• UI. Used to send unnumbered information• SIM. Places a secondary station into the initialization mode• RIM. Sent by a secondary station to request the primary to send a SIM

command• SNRM. Places a secondary into the normal response mode• DM. Sent by the secondary if the primary sends numbered information

while the secondary is in the normal disconnect mode• RD. Sent by the secondary when it wants the primary to place it into

disconnect mode• DISC. Places the secondary in disconnect mode• UA. An affirmative response that indicates compliance to SIM, SNRM,

or DISC commands and acknowledge unnumbered information frames• FRMR. Used by the secondary to report any of four possible procedural

errors



• PROCEDURAL ERRORS– Invalid command–Prohibited information field–Buffer overrun–Received nr disagrees with ns


SDLC INFORMATION FIELD• The number of bits in the SDLC information field is

variable but must be a multiple of 8 bits (1 byte)• Not all SDLC frames allow an information field• The data in an information field may either be user

information or control information

SDLC Frame Check Character FIELD• Contains the error detection mechanism for SDLC• Equivalent with the BCC used in BSC• Uses CRC-16 with the generating polynomial applied on the control

in the address and information fields• Uses the CCITT V.41 specified generating polynomial:

x16 + x12 + x5 + x1

data link control protocol(3)

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