Forelesning nr 4, høsten 2001
Øvingsoppgaver
Oppgave 7.11, 7.17 og 7.26
Gjennomgås onsdag 19. september
Forelesning nr 4, høsten 2001
Flow ControlEnsuring the sending entity does not
overwhelm the receiving entity Preventing buffer overflow
Transmission time Time taken to emit all bits into medium
Propagation time Time for a bit to traverse the link
Forelesning nr 4, høsten 2001
Stop and WaitSource transmits frameDestination receives frame and replies
with acknowledgementSource waits for ACK before sending next
frameDestination can stop flow by not send ACKWorks well for a few large frames
Forelesning nr 4, høsten 2001
FragmentationLarge block of data may be split into small
frames Limited buffer size Errors detected sooner (when whole frame
received) On error, retransmission of smaller frames is
needed Prevents one station occupying medium for
long periods
Stop and wait becomes inadequate
Forelesning nr 4, høsten 2001
Link utnyttelseTransmission time = tid det tar å sende en
rammeSettes = 1Propagation delay = tid det å sende en bit fra
A til BSettes = a
Forelesning nr 4, høsten 2001
Sliding Windows Flow ControlAllow multiple frames to be in transitReceiver has buffer W longTransmitter can send up to W frames
without ACKEach frame is numberedACK includes number of next frame
expectedSequence number bounded by size of field
(k) Frames are numbered modulo 2k
Forelesning nr 4, høsten 2001
Sliding Window EnhancementsReceiver can acknowledge frames without
permitting further transmission RNR (Receive Not Ready)
Must send a normal acknowledge to resume
If duplex, use piggybacking If no data to send, use acknowledgement
frame If data but no acknowledgement to send, send
last acknowledgement number again, or have ACK valid flag (TCP)
Forelesning nr 4, høsten 2001
Error DetectionAdditional bits added by transmitter for
error detection codeParity
Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
Even number of bit errors goes undetected
Forelesning nr 4, høsten 2001
Cyclic Redundancy CheckFor a block of k bits transmitter generates
n bit sequenceTransmit k+n bits which is exactly
divisible by some numberReceive divides frame by that number
If no remainder, assume no error
Forelesning nr 4, høsten 2001
CRC Cyclic Redundancy CheckM= 10110101, melding som skal overføres
CRC generatorpolynom P= x4+x+1=10011
FCS (Frame Check Sequence) er rest av divisjonen
2n * M : P n=4
101101010000 : 10011 = 10101010
Gir rest på 1110, dvs FCS = 1110
Sender M + FCS = 101101011110
Forelesning nr 4, høsten 2001
101101010000:10011=1010101010011 01011 00000 10110 10011 01011 00000 10110 10011 01010 00000 10100 10011 01110 00000 1110
FCS = 1110
Utregning
Overfører:
101101010000 (2nM)
+ 1110 (FCS)=101101011110
Forelesning nr 4, høsten 2001
Error ControlDetection and correction of errorsLost framesDamaged framesSamlebegrep: Automatic repeat
request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and
retransmission
Forelesning nr 4, høsten 2001
Automatic Repeat Request (ARQ)Stop and waitGo back NSelective reject (selective retransmission)
Forelesning nr 4, høsten 2001
Stop and WaitSource transmits single frameWait for ACKIf received frame damaged, discard it
Transmitter has timeout If no ACK within timeout, retransmit
If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1
Forelesning nr 4, høsten 2001
Go Back N Based on sliding windowIf no error, ACK as usual with next frame
expectedUse window to control number of
outstanding framesIf error, reply with rejection
Discard that frame and all future frames until error frame received correctly
Transmitter must go back and retransmit that frame and all subsequent frames
Forelesning nr 4, høsten 2001
Go Back N - Damaged FrameReceiver detects error in frame iReceiver sends rejection-iTransmitter gets rejection-iTransmitter retransmits frame i and all
subsequent
Forelesning nr 4, høsten 2001
Go Back N - Lost Frame (1)Frame i lostTransmitter sends i+1Receiver gets frame i+1 out of sequenceReceiver send reject iTransmitter goes back to frame i and
retransmits
Forelesning nr 4, høsten 2001
Go Back N - Lost Frame (2)Frame i lost and no additional frame sentReceiver gets nothing and returns neither
acknowledgement nor rejectionTransmitter times out and sends
acknowledgement frame with P bit set to 1(P=1 betyr kommando)
Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )
Transmitter then retransmits frame i
Forelesning nr 4, høsten 2001
Go Back N - Damaged AcknowledgementReceiver gets frame i and send
acknowledgement (i+1) which is lostAcknowledgements are cumulative, so
next acknowledgement (i+n) may arrive before transmitter times out on frame i
If transmitter times out, it sends acknowledgement with P bit set as before
This can be repeated a number of times before a reset procedure is initiated
Forelesning nr 4, høsten 2001
Selective RejectAlso called selective retransmissionOnly rejected frames are retransmittedSubsequent frames are accepted by the
receiver and bufferedMinimizes retransmissionReceiver must maintain large enough
bufferMore complex logic in transmitter
Forelesning nr 4, høsten 2001
Selective Reject -Diagram
RR = Receive ReadyREJ = RejectSREJ = Selective reject
Forelesning nr 4, høsten 2001
HDLC Station TypesPrimary station
Controls operation of link Frames issued are called commands Maintains separate logical link to each
secondary station
Secondary station Under control of primary station Frames issued called responses
Combined station May issue commands and responses
Forelesning nr 4, høsten 2001
HDLC Link ConfigurationsUnbalanced
One primary and one or more secondary stations
Supports full duplex and half duplex
Balanced Two combined stations Supports full duplex and half duplex
Forelesning nr 4, høsten 2001
HDLC Transfer Modes (1)Normal Response Mode (NRM)
Unbalanced configuration Primary initiates transfer to secondary Secondary may only transmit data in response
to command from primary Used on multi-drop lines Host computer as primary Terminals as secondary
Forelesning nr 4, høsten 2001
HDLC Transfer Modes (2)Asynchronous Balanced Mode (ABM)
Balanced configuration Either station may initiate transmission
without receiving permission Most widely used No polling overhead
Forelesning nr 4, høsten 2001
HDLC Transfer Modes (3)Asynchronous Response Mode (ARM)
Unbalanced configuration Secondary may initiate transmission without
permission form primary Primary responsible for line rarely used
Forelesning nr 4, høsten 2001
Frame StructureSynchronous transmissionAll transmissions in framesSingle frame format for all data and
control exchanges
Forelesning nr 4, høsten 2001
Flag FieldsDelimit frame at both endsFlagg = 01111110May close one frame and open anotherReceiver hunts for flag sequence to
synchronize
Forelesning nr 4, høsten 2001
Bitstuffing - transparens
Brukes i HDLC for å ungå problemer med like datafelt og flagg.
Mellom sending av start og stop flagg, senderen vil alltid sette inn en 0er etter fem 1ere: Etter å ha detektert start flagget gransker den bit strømmen: når det kommer et mønster på fem 1ere, ser den på det
6. er dette en 0er, slettes det er dette en 1er og det syvende en 0er, er dette et
godkjent flagg. dersom både 6 og 7 er 1ere, et abort signal sendt.
Forelesning nr 4, høsten 2001
Address Field Identifies secondary station that sent or will receive
frameUsually 8 bits longMay be extended to multiples of 7 bits
LSB of each octet indicates that it is the last octet (1) or not (0)
All ones (11111111) is broadcast
Forelesning nr 4, høsten 2001
Control FieldDifferent for different frame type
Information - data to be transmitted to user (next layer up)
Flow and error control piggybacked on information frames
Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control
First one or two bits of control field identify frame type
Forelesning nr 4, høsten 2001
Poll/Final BitUse depends on contextCommand frame
P bit 1 to solicit (poll) response from peer
Response frame F bit 1 indicates response to soliciting command
Forelesning nr 4, høsten 2001
Information FieldOnly in information and some
unnumbered framesMust contain integral number of octetsVariable length
Forelesning nr 4, høsten 2001
Frame Check Sequence FieldFCSError detection16 bit CRCOptional 32 bit CRC
Forelesning nr 4, høsten 2001
HDLC OperationExchange of information, supervisory and
unnumbered framesThree phases
Initialization Data transfer Disconnect
Forelesning nr 4, høsten 2001
Other DLC Protocols (LAPB,LAPD)LAPB - Link Access Procedure, Balanced
Part of X.25 (ITU-T) Subset of HDLC - ABM Point to point link between system and packet
switching network node (mellom DTE og DCE)LAPD - Link Access Procedure, D-Channel
Beskrevet i Q.921 spesifikasjonen ABM Always 7-bit sequence numbers 16 bit address field contains two sub-
addressesOne for device and one for user (next layer up)
Forelesning nr 4, høsten 2001
LAPD commands and responses
I - Information
RR – Receive ReadyRNR – Receive Not ReadyREJ – Reject
SABME – Set Asynchronous Balanced ModeDM – Disconnect ModeUI – Unnumbered InformationDISC – DisconnectUA – Unnumbered acknowledgeFRMR – Frame RejectXID – Exchange Identification
Forelesning nr 4, høsten 2001
LAPD – TimersT200 = 1s, Time to wait for an ack before
initiating recoveryT201 = 1s, Minimum time between TEI
identity check messagesT202 = 2s, Minimum time between TEI
identity check messagesT203 = 10s, Maximum time with no
frames exchanged
Forelesning nr 4, høsten 2001
Other DLC Protocols (LLC)Logical Link Control (LLC)
IEEE 802 Different frame format Link control split between medium access layer
(MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed
Sender and receiver
Error detection at MAC layer32 bit CRC
Destination and source access points (DSAP, SSAP)