31-05-2012, 11:35 AM
Data Link Control
14-Data Link Control.ppt (Size: 864 KB / Downloads: 30)
Announcements
Midterm: December 10, Friday, 17:40 – 19:30
in FENS L045
Exam will be closed book, closed notes
calculators are allowed
you are responsible all topics I covered in the class even if some of them are not in the book (I sometimes used other books) and not in the ppt files (I sometimes used board and showed applications on the computer)
I prepared some handouts for the topics that I covered from other books but I do not promise any completeness (especially CSMA/CD performance analysis is not be there)
Performance Metrics and Delays (Section 5.3)
Transmission time (delay)
Time taken to emit all bits into medium
Propagation time (delay)
Time for a bit to traverse the link
Processing time (delay)
time spent at the recipient or intermediate node for processing
Queuing time (delay)
waiting time at the queue to be sent out
Types of Errors
Single bit errors
isolated errors
affects (flips) one bit, nearby bits are not altered
not so common in real life
Burst errors
a sequence of bits are affected
most common case
a burst error of length B is a contiguous sequence of B bits in which the first and the last and some intermediate bits are erroneously flipped.
not necessarily all bits between the first and the last one
Cyclic Redundancy Check (CRC)
Standard CRCs (generators are standard)
checks all single, double and odd number of errors
checks all burst errors with length less than or equal to the length of FCS (n-k)
checks most of the burst errors of longer length
for bursts of length n-k+1 (length of generator), probability of an undetected error is 1/2n-k-1
for longer bursts, probability of an undetected error is 1/2n-k
Selective Reject
Also called selective retransmission
Only rejected frames are retransmitted
Subsequent frames are accepted by the receiver and buffered
Minimizes retransmissions
Receiver must maintain large enough buffer
Complex system
Issues
RR with P=1 is from HDLC standard
pure protocol just have retransmissions after timeout
as explained in Tanenbaum
Issues – Window Size
Given n-bit sequence numbers, what is Max window size?
go-back-n ARQ 2n-1
Why?
what about receiver’s window size?
It is 1, why?
selective-reject(repeat) 2n-1
Why?
Issues – Buffer Size
Go-back-n ARQ
sender needs to keep a buffer equal to window size
for possible retransmissions
receiver does not need any buffer (for flow/error control)
why?
Selective reject
sender needs to keep a buffer of window size for retransmissions
receiver keeps a buffer equal to window size
Issues - Performance
Notes on board
Appendix at the end of Chapter 14
selective reject ARQ is not in the book
Address Field
Identifies secondary station that sent or will receive frame
Usually 8 bits long
May be extended to multiples of 7 bits with prior agreement
leftmost bit of each octet indicates that it is the last octet (1) or not (0)
Frame Types
Information - data to be transmitted to user
Acknowledgment is piggybacked on information frames
Supervisory – ARQ messages (RR/RNR/REJ/SREJ) when piggyback not used
Unnumbered – supplementary link control functions. For examples,
setting the modes
disconnect
Control field is different for each frame type
Other DLC Protocols (LAPB,LAPD)
Link Access Procedure, Balanced (LAPB)
Part of X.25 (ITU-T)
Subset of HDLC - ABM (Async. Balanced Mode)
Point to point link between user and packet switching network node
HDLC frame format
Link Access Procedure, D-Channel (LAPD)
Part of ISDN (ITU-T)
ABM
Always 7-bit sequence numbers (no 3-bit)
always 16-bit CRC
16-bit address field
Other DLC Protocols (LLC)
Logical Link Control (LLC)
IEEE 802
For LANs (Local Area Networks)
Link control split between medium access control layer (MAC) and LLC (on top of MAC)
Different frame format
Two addresses needed (sender and receiver) – actually at MAC layer
Sender and receiver SAP addresses
Control field is same as HDLC (16-bit version for I and S frames; 8-bit for U frames)
No primary and secondary - all stations are peers
Error detection at MAC layer
32 bit CRC
LLC Services
3 alternatives
Connection Mode Services
Similar to HDLC ABM
Unacknowledged connectionless services
no connection setup
No flow-control, no error control, no acks (thus not reliable)
good to be used with TCP/IP. Why?
Acknowledged Connectionless Service
No connection setup
reliable communication