flag-8 bit sequence with a bit pattern 01111110 (7e)
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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)