CONNECTION-ORIENTED & CONNECTIONLESS NETWORKS
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1 CONNECTION-ORIENTED & CONNECTIONLESS NETWORKS Connection- oriented no logical connection initially exists between the DTEs and the network (idle state) In order to communicate a handshake is needed The user data are exchanged through a preestablished protocol The DTEs perform a connection release a return to the idle condition
2 Connection oriented Idle no connection Connection established Data transfer ACKs flow control error recovery Connection release Idle no connection
3 Connectionless (datagram) network goes directly from an idle condition into a data-transfer mode, followed directly by the idle condition Comparable to mailing a letter (a message is placed into the postal system with the assumption it will arrive at its destination)
4 Idle No connection Connectionless Data Transfer Idle No connection No ACKs, no flow control, no error control But one can push error control up into the application process (or a hogher-level protocol)
5 Data link protocols (DLC) They follow the steps: -Link establishment (DLC handshake with the remote DLC logic to ensure both systems are ready to exchange user data - Information transfer (DLC checks all data for possible transmission erors and sends acknowledgments back to the transmitting machine - Link termination (DLC must relinquish the control of the link)
6 Types of DLC Primary/secondary (master/slave) one DTE, DCE or DSE is designates as the primary station on the channel and controls all the other stations and dictates when and if the devices can communicate Peer-to-peer equal status to all stations
7 POLLING Primary Site 1 Secondary Site 2 POLL DATA Poll sequence ACK DATA EOT
8 Selection (some systems use polling only!) Primary Site 1 Secondary Site 2 SEL ACK DATA ACK DATA EOT Select command
9 Polling/Selection sequence Primary Site 1 Secondary Site 2 SEL NAK POLL DATA ACK DATA EOT Secondary site 2, I have data for you, can you receive? No, I cannot And may be you have anything for me? Yes!
10 Negative Response to Poll POLL NAK In newer systems: RR (Receive Ready the indication of a willingnes to receive or transmit) RNR (Receive Not Ready unwillingness)
11 How to reduce the great number of negative responses? The nonresponding station is dropped to a lower priority (dynamic polling/selection tables) General poll to any device; every device scans the ones attached to it for an active request, and transmit to the primary
12 Primary/secondary system One should remember on the hierarchical aspect of it All traffic comes into and goes out of the primary host Problems of: bottleneck (one device manages all traffic Problems of reliability if the primary site goes down, the entire network is lost (some form of backup is needed)
13 Polling/Selection Combined how to manage traffic between two DTEs on the same channel
14 Stop-and-Wait Polling/Selection One of the simplest and oldest forms Half-duplex is needed Sequencing is used
15 Traffic can be lost in the network because of: Complexities of the traffic pattern Bugs (logic problems) Failed components Some frame is damaged en route (sent during rainstorm ) TIMEOUT the sending station after not receiving a reply to its transmission within a given period, retransmits the data
16 Selective and Group Polling Selective polling one DTE is addressed with poll Group polling used on a ring, or loop technology, either a line with cluster controllers
17 Continuous ARQ Sliding Windows ARQ Automatic request for repeat technique Full duplex is needed A window is established on each link to provide a reservation of resources at both DTEs it provides both buffer space and sequencing rules
18 State variables: V( S ) send state variable V( R ) - receive state variable V( S ) the sequence number of the next frame to be transmitted; it s incremented with each frame transmitted and placed in the send sequence field in the frame V( R ) the number that is expected to be in the sequence number of the next frame
19 If something has gone awry a NAK (with the receiving sequence number containing the value of V( R ) is sent) a Reject, or a Selective Reject Than the transmitter resets its V( S ) and retransmit the frame whose sequence number matches the value of V( R )
20 Window Management OUTSTANDING: Frames 6 through 4
21 The advantages of AQR protocols over the stop-and-wait The expensive communications channel is as busy as possible The possibility to use inclusive acknowledgment The receiver can send an ACK of 5, without transmitting ACKs 1,2,3,4. It means: I have received and acknowledge everything up to and including 4; the next frame expected should have 5 in its send sequence field
22 Error Control Selective Repeat requires that only the erroneous transmission be retransmitted A better line utilization, but all successive frames must be hold at the receiving DTE to await the retransmission of the erroneous frame Reject all frames that were behind should be retransmitted as well A simpler technique, requires no frame queuing and resequencing, but its throughput is less
23
24 Other primary/secondary polling systems: Bisync (binary synchronous) HDLC LAP LAPB LAPD LLC LAPX LAPM SDLC..
25 NONPOLLING SYSTEMS RTS/CTS Xon/Xoff TDMA
26 RTS/CTS (Request to Send/Clear to Send) Low-level approach to protocols Can use physically RS-232, EIA-232
27 Xon/Xoff
28 TDMA Time Division Multiple Access
29 PEER-TO-PEER NONPRIORITY SYSTEMS TDM (Time Division Multiplexing), or Slot Each user has the full use of the channel during the slot of time Register Insertion Any station can transmit whenever an idle state exists on the link Carrier Sense (Collision) Systems Token Passing (Token Ring, Token Bus)
30 Carrier Sense Systems
31 Methods for channel seizing Nonpersistent P-persistent (p stands for probability) 1-persistent Problem: collision window (it depends on the propagation delay and the distance between the two competing stations) Longer frames less collisions Longer distance more collisions
32 CARRIER SENSE NETWORKS Condition Nonpersistent P-persistent 1-persistent Channel idle Transmit immediately Transmit with p defer with 1-p Transmit immediately Channel busy Randomized wait & sense Transmit with p defer with 1-p Continually sense Collision Randomized retransmission Randomized retransmission Randomized retransmission
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