3 Packet-Switch Public Data Network (PSPDN) It enables data to be sent digitally over the network. It consists of a number of packet switching exchanges (PSEs) which are linked together to form a mesh topology (Note that each PSE has at least two connections to the network). Computers may be connected to PSPDN directly or connected via packet assember/disassember (PADs) A PAD takes asynchronous traffics and converts it into packets which can then be sent through the PSPDN and vice versa.
5 Why Uses Packet? What is the advantage of breaking up a stream of data and sending it in packets? Most computers tend to send data in bursts. Advantages Better use of transmission capacity. Lower transmission costs. Disadvantages Variable propagation delay. Higher processing costs.
6 How To Avoid Mixing of Packets From Different Users? How to avoid packets from different users got mixed up as they pass through the network? Use the address of the destination terminal to identify the packets (ITU-T X.121). Not feasible because Unacceptable overhead (56 bits / packet) Each PSE would have to work out where to send the packet. The solution is to establish a virtual circuit through the network which need last only for the duration of the call.
7 Virtual Circuit In a connection-oriented service, call request packet is used to establish a virtual circuit from one end to the other. The virtual circuit exists through a series of PSEs, each linked to the next. Each link between PSEs has a number of logical channels, one of which will be assigned to the virtual circuit. The PSE uses a routing table to work out where packets arriving from a given link and logical channel should be forwarded.
9 Throughput = the rate at which a PSE can switch packets, measured in packets per second. Two types of network services Connection-oriented network service – a path is established before data are transferred. Connectionless network service – no path is established before data are transferred.
10 X.25 ITU-T X.25 is a network access protocol. X.25 specifies interface between the user and a PSPDN for a connection- oriented network service. procedures of using packets in order to setup virtual circuits In a network that use X.25 internally, all packets consist of 2 parts: header and data. Each packet header is made up of 3 octets. 1 st octet: GFI (General Format Identifier) which indicates the type of format used by the address block. For data packet, GFI indicates the no. of bits in the sequence, which is either 3 (modulo-8) or 7 (modulo-128). LCGN(Logical Channel Group No.) indicates the type of call. 2 nd octet: LCN (Logical Channel Number) which will be modified as packet gets forwarded through a PSE. 3 rd octet: PTI (Packet Type Identifier) which specifies the function of the packet.
13 Handshaking One device makes a request and then waits to be informed of the outcome of that request. Signal sequence diagram
14 Data Transfer Consider the transfer of data along a virtual circuit once it has been setup. Data PTI is the only one where bit 1 is 0. P(R) and P(S) are known as sequence numbers, and are used to regulate the flow of packets by a technique known as flow control. M(More) bit is used to indicates block of data split over several data packets.
15 Flow Control P(R) and P(S) fields are used to implement flow control: Each data packet sent has a modulo-8 sequence number. The value of the P(R) field in a data packet sent by A indicates the value of the next sequence number it expects to be received from B, and acknowledges all packets from B that have P(S) <= P(R) – 1. Window size is the no. of packets that can be sent without receiving an acknowledgement. Window size affects the no. of packets that can be in transit in the network and therefore affects the traffic loading of the networking. Piggy-backing – packets in one direction along the virtual circuit carry sequence and flow control information for packets in the reverse direction. A protocol that use the concept of a window to control the no. of packets in transit is called a sliding window protocol.
16 How can flow control information be sent from B to A if no data are being transmitted in that direction? B will send a packet of type RR(Receive Ready) or RNR(Receive Not Ready). REJ(Reject) packets are used by a DTE to request the DCE to retransmit unacknowledged data packets.
17 Packet Level VS Link Level Packet level concerns with transporting packets between DTEs. Link level concerns with transporting packets between a DTE & a DCE, or between PSEs. Low error rate in link level. Link level and packet level have much in common, but they differ in the scale of their respective operations. Link level uses frames to convey packets between PSEs. Bit stuffing is used to preserve the unqiueness of the special flag pattern (01111110).
18 Frame Type There are three categories of frame Information frame (I-frame) Supervisory frame (S-frame) RR frame (receive ready) RNR frame (receive not ready) REJ frame (reject) Unnumbered frame
19 Go-Back-N ARQ Protocol ARQ stands for Automatic Repeat reQuest. Go-back-N means that, if a frame is found to be in error, it will repeated together will all the other unacknowledged frames. A transmitter is notified of transmission problems either when it receives a negative acknowledgement (in the form of an REJ frame) or when timeout.
21 Aims of OSI Any application process irrespective of its origin or manufacture should be able to communicate freely with any application process in any other computer that supports the same standards. Provide a common framework, which will allow existing and evolving application processes to cooperate and carry-out their particular (distributed) information, data, processing task irrespective of the type of computer on which they are running. eg: Access remote file Email
22 Principal Functions of OSI Layers Physical: Transmission of bit streams over physical media. Data Link: Reliable transfer of information between two directly linked nodes. Network: Routing and transmission of data across one or more sub-networks. Transport: Provision of reliable end-to-end transmission independently of physical network. Session: Setting up, closing down and controlling the structure of an extended communication (session) between two systems. Presentation:Ensuring that data presented to applications are in the correct form (correct syntax). Application: Ensuring use of appropriate semantics for for information interchange between applications.
24 Services Each layer uses the services of the layer below and provide services to the layer above. Services are requested and provided through the exchange of primitives and associated parameters. Each layer uses request and response primitives to obtain services from the entities in the layer below. The entities of each layer use indication and confirm primitives to provide services to layer above. (vertical communication)
27 Protocol Protocol is used to describe: the way service request primitives and their parameters are transformed into appropriate values; the way negotiations take place to establish the values of the parameters; the way the parameters are used for communication between peer entities. The use of peer protocols for horizontal communications between peer entities provides virtual channels between the same layers of two systems. Protocols for one layer are independent of those for other layers. The decoupling of protocols provides great flexibility, potential for evolution, and economy in software generation
31 Features of Frame Relay Frame relay requires that two conditions be met: End device must be running an intelligent higher-level protocol. Transmission line must be virtually error-free. Simplified X.25 No flow and error control. Physical and Data Link layers only. Frames provide multiplexing at Data Link layer. Congestion control Higher speed possible
37 Data Link Connection Identifier (DLCI) DLCI serves the same function as the Logical Channel Number (LCN) as the packet level in X.25 DCLI allows virtual connections to be multiplexed over a single physical connection.
38 Congestion Control – Discard Control CIR= Committed Information Rate B c = Committed Burst Size Time interval T c = B c / CIR B e = Excess Burst Size Between B c and B c + B e Mark DE bit Over B e Discard
41 Congestion Control – Forward Explicit Congestion Notification Algorithm Compute the fraction of frames for which the FECN bit is set over some measurement interval. If more frames have the FECN bit of one than have a FECN bit of zero, reduce the flow of frames from the source. If congestion condition persists, institute additional reductions. When congestion condition ends, gradually increase the flow of frames.
42 Congestion Control - Backward Explicit Congestion Notification Algorithm When the first frame with the BECN bit set is received, reduce the information rate to CIR. If additional consecutive frames with the BECN bit set are received, institute additional reductions. If a consecutive sequence of frames with the BECN bit set to zero are received, gradually increase the flow of frames.