FLAG-8 bit sequence with a bit pattern 01111110 (7E)

ADDRESS-address of secondary station that is either the originator or destination of the frame (or the station acting as the secondary in the case of combined stations)

CONTROL-1 or 2 byte segment used for flow and error control.

Information-Users data from network layer or network management information. Length can vary from network to network, but is always fixed within each network

FCS-Frame Check Sequence contains either a 2 or 4 byte ITU-T CRC.

FLAG ADDRESS CONTROL FCS FLAGINFORMATION

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

VARIES2 OR 4BYTES

1 BYTE

HDLC FRAME TYPES

FLAG ADDRESS CONTROL FCS FLAGUSERINFORMATION

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

VARIES2 OR 4BYTES

1 BYTE

I-Frame

FLAG ADDRESS CONTROL FCS FLAG

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

2 OR 4BYTES

1 BYTE

S-Frame

FLAG ADDRESS CONTROL FCS FLAGMANAGEMENTINFORMATION

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

VARIES2 OR 4BYTES

1 BYTE

U-Frame

Information Frame

If the first bit of the control field is a zero, then the frame is an information Frame

N(S)- These three bits define the sequence number of the frame in travel

P/F-When the bit is set, this means either poll or final. It means poll when the frame is being sent by the primary station to a secondary station.

N(R)- These three bits correspond to the value of the ACK when piggybacking is used

FLAG ADDRESS CONTROL FCS FLAGUSERINFORMATION

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

VARIES2 OR 4BYTES

1 BYTE

0

N(S)

p/f

N(R)

Supervisory Frame

FLAG ADDRESS CONTROL FCS FLAG

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

2 OR 4BYTES

1 BYTE

1 p/f

CODE N(R)

0

If the first two bits of the control field are 10, this means this is an S-frame

The second 2 bits of the control field is a code that defines four types of S-frames

00-RR S-Frame (receive ready) Acknowledges a safe and sound frame or group of frames

01-RNR S-Frame (Receive Not Ready) Acknowledges the receipt of a frame or group of frames, and announces that the receiver is busy and cannot receive more frames.

10-REJ S-Frame (Reject) This is a NAK frame. NAK as used in Go Back NARQ

11- SREJ S-Frame(sSelective Reject) NAK frame used in Selective repeat ARQ

Unnumbered FrameFLAG ADDRESS CONTROL FCS FLAGMANAGEMENT

INFORMATION

1 BYTE 1 ORMOREBYTES

1 ORTWO

BYTES

VARIES2 OR 4BYTES

1 BYTE

1 p/f1

CODE CODE

Bit-Oriented protocol(cont’d)• U-Frame control command and response

Command/response Meaning

SNRMSNRMESARMSARMESABMSABMEUPUIUARDDISCDMRIMSIMRSETXIDFRMR

Set normal response modeSet normal response mode(extended)Set asynchronous response modeSet asynchronous response mode(extended)Set asynchronous balanced modeSet asynchronous balanced mode(extended)Unnumbered pollUnnumbered informationUnnumbered acknowledgementRequest disconnectDisconnectDisconnect modeRequest information modeSet initialization modeResetExchange IDFrame reject

Bit-Oriented protocol(cont’d)

• U-Frame~ can be divided into five basic functional

category –Mode setting–Unnumbered-Exchange–Disconnection–Initialization Mode–Miscellaneous

Bit-Oriented protocol(cont’d)

• Mode Setting Command

~ are sent by the primary station, or by a combined station wishing to control an exchange, to establish the mode of the session

• Unnumbered-Exchange

~ are used to send or solicit specific pieces of data link information between device

• Disconnection• Initialization Mode• Miscellaneous

Bit-Oriented protocol(cont’d)• Example 1 : Poll/Response

Bit-Oriented protocol(cont’d)• Example 2 : Select/Response

Bit-Oriented protocol(cont’d)• Example 3 : Peer Devices(1)

Bit-Oriented protocol(cont’d)• Example 3 : Peer Devices(2)