ch3-data link layer
High-level data link control protocol (HDLC) Point-to-Point protocol (PPP)
Different DLC protocols have been defined and they include (2)
1,1,2,1,3 1 0 Type P/F Next used to indicate that the control field is performing control functions
Supervisory Format
parity checking cyclic redundancy checking
2 error detection schemes
stop-and-wait scheme sliding window scheme
2 flow control schemes are used for the data link layer
Stop-and-wait automatic repeat request (ARQ); the stop-and-wait flow control protocol Go-Back-N ARQ; the sliding window flow control protocol Selective repeat ARQ; the sliding window flow control
3 error control schemes used in the data link layer
is used mainly on P2P links and each station has equal status and performs both primary and secondary functions
Asynchronous Balanced Mode (ABM)
in which the secondary station does not have to wait to receive explicit permission from the primary station to transfer any frames.
Asynchronous Response Mode (ARM)
1. multiply M(x) by x^r = shift M rbits to the left 2. divide x^r M(x) by G(x) to generate R(x); 1+1=1-1=0, 1-0=0-1=1, 1+0=1 3. quotient is discarded, and the message is concatenated w/ the R(x)(reminder), an the concatenated bit stream is transmitted 4. upon receiving the bit stream, the receiver divides it by G(x); nonzero reminder-> in error or accepted
CRC operations
Information Format Supervisory Format Unnumbered Format
Control Field Format: 3
the source resends that frame and all the frames that have been transmitted since that frame was sent as well as any new frames, provided the total number of frames being sent does not exceed N.
Go-Back-N ARQ
-Flag; 01111110 -Address; identifies the primary or secondary stations involvement in the frame transmission or reception -control; HDLC uses the control field to determine how to control the communications process. -data -checksum; contains a 16-bit cyclic redundancy check, which it is used for error detection. -flag
HDLC Frame Format
-Normal Response Mode -Asynchronous Response Mode -Asynchronous Balanced Mode
HDLC operates in three modes:
1,3,1,3 0 Seq P/F Next the frame is used to transmit end-user data between two devices
Information Format
an unbalanced or asymmetric mode in which one end of the line is the master and the other end is the slave
Normal Response Mode (NRM)
Flag 1 Address 1 Control 1 Protocol 2 Data(x) Checksum 1/2 Flag 1: Flag:01111110 Address:11111111, the standard broadcast address Control:00000011 Protocol: cxxx-LCP, 8xxx-NCP, 0xxx Frame check sequence (FCS): 16 bits (2 bytes) -> 32-bit (4-byte)
PPP Frame Format
only the frame in error is retransmitted
Selective-Repeat ARQ
the source sends a frame and waits for a response, which can be a positive acknowledgment (ACK) or a negative acknowledgment (NAK) that can also be in the form of a timeout
Stop-and-Wait ARQ
1,1,2,1,3 1 1 Type P/F Modifier used to indicate that a frame is being used for control purposes
Unnumbered Format
-Poll/Final Bit (P/F): ignored when it is set to 0 + used to provide dialogue b/w the primary station and secondary station. -Types: Type 0:Receive Ready (RR) Type 1:Reject (REJ) Type 2:Receive not Ready (RNR) Type 3:Selective Reject (SREJ)
control field format: Poll/Final Bit (P/F) + Type
-Seq: the sequence number of the current frame -Next: the piggybacked acknowledgment + the value is the sequence number of the next frame expected by the station transmitting this frame
control field format: set and next
CRC detects the presence of transmission errors by adding a few bits called CRC bits, checksum bits, frame check sequence bits to each packet
cyclic redundancy checking
a leading flag a header the data to be transmitted (or payload) the checksum: for error detection in the frame at the receiver a trailing flag
framing consists of :
used to preserve the uniqueness of the flag in a frame; flag 01111110 it inserts a 0 into the outgoing bit stream after every 5 consecutive 1s in the data ex. m=01101111100 actual data sent is m=011011111000
purpose of Bit stuffing
to ensure that data passes up to the next layer has been received exactly as transmitted (i.e., error free, w/out loss and in the correct order) and also practice flow control, which ensures that data is transmitted only as fast as the receiver can receive it
the job of data link control protocols (DLC)
- Set Normal Response Mode (SNRM) - Set Asynchronous Response Mode (SARM) - Set Asynchronous Balanced Mode (SABM) -Disconnect (DISC) - Set Initialization Mode (SIM) - Unnumbered Information (UI) - Unnumbered Acknowledgment (UA)
unnumbered format frames have 5 bits (2 from Type and 3 from Modifier) that provide up to 32 additional commands and 32 additional response functions.
