Date post: | 15-Dec-2015 |
Category: |
Documents |
Upload: | shaylee-lombard |
View: | 218 times |
Download: | 1 times |
For better usageUse links for data in both directions, why
Allows the receiver to wait until network layer passed to next packet.
Acknowledgement then attached to the outgoing data frame. Causing piggybacking to act.
PiggybackingThe technique of temporarily delaying
outgoing acknowledgements so they can be hooked onto the next outgoing data frame.
It gives a positive acknowledgement with retransmission protocol.
Example:When data layer waits for a long period of time
then the sender wants, the frames then get resubmitted, defeating the purpose of having acknowledgements.
Advantages of PiggybackingAllows the privilege of having distinct
acknowledgement frames.It gives better use of available channel
bandwidth.The ack field acts as an acknowledger and a
checksum.
Ack FieldFrame header cost only a few bits( One bit in
frame header)Fewer frames means a lighter processing
load of receiver.
Protocol 3Allows unidirectional data flow over an
unreliable channel.Goes in a number of sequences describing
each protocol with the framesframe_to send-tells which frames the sender is
trying to sendframe_expected- tells which frames the
receiver is expecting
Bidirectional ProtocolsConsist of a sequence numbers Stop-and-wait sliding window n=1
This restricts the numbers from 0 and 1Consist of three class
1)Efficiency 2)Complexity 3)Buffer Requirements
Sending/Receiving WindowSender maintains a set of sequence numbers
corresponding to frames it is permitted to send. The receiving end corresponds to the set of frames permitted to be acceptedMust maintain the same lower and upper limitsMust maintain the same sizeSome are fixed in size, but grow and shrink
over time frame of being sent and receivedGives date link more freedom about order of
sending and receiving
Purpose of Sequence of NumbersRepresent frames that have been sent or can
be sentNewest packet is sent at highest sequence
numberMaintains a list of acknowledgment frames
One-Bit Sliding Window ProtocolBidirectionalUse the stop-and-wait protocol as discussed
in slide 8It starts by defining variables
Frame_to_send- tells frames the sender is trying to send
Frame_expected- tells which frame the receiver is expecting.
O and 1 are the only possibilities
Protocol Using Go-Back_NAllows sender to transmits w frames before
blocking, instead of just one.With the large variety of w the sender will be
able to continuously transmit frames since the acknowledgements will arrive for previous frames.
Before the windows become full, it prevents the sender from blocking.
Receiver would discard all subsequent frames, sending no acknowledgements for the discarded frames.
Bandwidth-delay productIn order to find the appropriate value for w we
need to know how many frames can fit inside the channel.
Bandwidth in bits/sec multiplied by one-way transit time
2BD+1Link utilization-< w/1+2BD
This value is an upper bound because it does not allow for any frame processing time and treats the acknowledgement frame as having zero length, since its usually short.
PipeliningTechnique of keeping multiple frames in
flightCan cause serious issues
Unreliable communication channelFrame can become lost of damaged
Approaches To Dealing With Errors In PipeliningTo aid in the recovery the frames are
discarded by data link layerWhen the window size is to large the frames
are buffered by data link layer
Selective RepeatAnother strategy for handling errors of framesWhen bad frame that is received is discarded,
but any good frames received after it are accepted and buffered.
Only the oldest unacknowledged frames are retransmitted.
Often combined with having receiver send a negative acknowledgement(NAK)
There are a few approaches to if NAK becomes lost, or is timed out
Alternative Approaches Bandwidth and data link layer buffer spaceThe network_layer_ready- is used when the
network layer has packet it wants to sendThe library procedures include:
disable_network_layer enable_network_layer
Cumulative AcknowledgementUses protocol 5Extremely important when frames were lost
or garbledWhenever acknowledgements come in, the
data link layer check to see if any buffers can be released
Has multiple outstanding frames, and needs multiple timers
Protocol Using Selective RepeatAllows receiver to accept and buffer the
frames following a damaged or lost oneBoth sender and receiver maintain a window
of outstanding and acceptable sequence number
Data Link ProtocolsPoint to point lines in the internet with two
common situationsUsed to send packets over these links
SONETIt provides a bitstream that runs at a well-
defined rateThe bitstream is organized as fixed-sized byte
payloads that recur every 125msec whether or not there is data sent
Examples of data link protocolsSLIP(serial line internet protocol)- is used to handle
error detection link configuration, support multiple protocols, permit authentication, and more.
LCP(link control protocol)- a link control that brings lines up, testing them, negotiating options, and bringing them down again gracefully when they are no longer needed.
NCP(network control protocol)- a way to negotiate network-layer options in a way that is independent of the network layer protocol to be used.
HDLC( high-level data link control)- provides reliable transmission with a sliding window, acknowledgements, and timeouts.
ADSL( Asymmetric Digital Subscriber Loop)Connects million of home subscribes to
the internet at megabit/sec rates over same telephone local loop used for plain old telephone service
It sends bits over a local loop to a device called DSLAM( DSL Access Multiplexer)
ATM(Asynchronous Transfer Mode)Link layer based on the transmission of fixed-
length cells of informationMeans that cells do not always need to be sent in
the way that bits are continuously sent over synchronous lines
Connection-Oriented technologyTo send data over network it needs to be mapped
into a sequence of cellsEach cell carries a virtual circuit identifier in its
header and devices use this identifier to forward cells along the paths of established connections
They can divide the bandwidth of physical layer link into slices sending both voice and data
Adaption Layers To send data over an ATM network, it needs
to be mapped into a sequence of cells, this is done with an ATM adaption layer in a process called segmentation and reassembly.
Several are defined for different servicesPeriodic voice samplesPacket data
AAL5 (ATM Adaption Layer 5) is the main one used for packet data