I.introduction

AT77.04 Data Communications

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I. Introduction

Definition: The set of devices, mechanisms and procedures by which end-user equipment attached to the network can exchange meaningful information.

1.1.1 Typical functions:

A path by which electrical signals can be transmitted.

A mechanism by which bits can be converted to and from electrical signals.

The means to give meaning to groups of bits. Bits are typically grouped in units called frames, packets or messages.

Methods to overcome deficiencies in the electric path that might cause electric signals (the bits they represent) to be misinterpreted.

Techniques for selecting and maintaining a path through

the network to perform the above functions.

1.1 Communication Networks


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1.1.2 Network Structure: the general network structure includes

• Hosts (Computer, terminal, telephone or any other communicating devices)are connected to the communication subnets, provides various services to its users.

• Communication subnet (network node, a subnet, or a transport system).In general, consists of transmission links and switching systems.

▪ Transmission links carry the bits from one computer to another through

networks, connect various switching elements of subnets and define the

topology (physical connection) of the subnet. The subnet defines the three lower layers of OSI-RM.

▪ Switching system is responsible for forwarding the data to its

destination over the transmission links.After receiving data from its source, looks for a free transmission link between it and the switching element which is connected to the destination host.

If it finds a free link, it will forward the data onto it; otherwise, it will store the data in its memory and try another route

for the data. It will send the data to another switching element, which will again look for a free link until the data is delivered to its destination.


1.1.3 Type of communication service

• Point-to-point communication

• Multicast communication• Broadcast communication

(a) Point-to-point communication

Data from one host is transmitted over either direct or indirect links between nodes.

- In a direct link, nodes are directly connected via a physical communication medium during data transfer.

- In the indirect link, the data in transferred over intermediate nodes until it reaches the destination host.

The data from an incoming line arrives at the node, which stores it and waits fora free link between nodes.

If it finds a free link, the data is sent over it;

Otherwise it send it over intermediate nodes.

At each intermediate node, data is stored and forwarded to the next available node.

A communication system supporting point-to-point communication is defined as point-to-point, store-and-forward or packet-switched subnet.


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(b) Multicast communication

Data can be sent to users of a selected group.

The usual way of implementing this communication is to set the high-order bit in the address field (of data) to 1 (one).

There exists only one channel or circuit which can be shared by all connected users (of the selected group) hosts/nodes.

This type of communication suffers from the problem of contention.

To solve this problem, it requires a complex protocol at each node.


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(c) Broadcast communication

A more general form of multicast communication, where data is delivered to all hosts/nodes connected to the network.

A special code in the address field of the data is used to distinguish between point-to-point and multicast communications.

The data is sent on the network circuit and is received by every host/node connected to it.

If the address of any connected host/node matches the address contained in the data, it can copy the data into its buffer.

This type of shared communication typically defines the following network topologies: bus, satellite, radio and television.


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1.2 Switching Techniques

• Circuit Switching- Dedicated resources

• Message Switching

• Packet Switching- Shared resources- Virtual circuits- Datagrams

Figure 1.1.1 Comparison ofCircuit Switching, Message Switching, andPacket Switching


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1.2.1 Circuit Switching

A complete circuit (route or path) between source and destination nodesis established before the data can be transmitted.

The circuit between the source and destination can be established onany communication link/transmission medium (telephone lines, coaxial cable, satellite link, microwave link, etc.).

The following three steps are required to establish the connections:

1. Connection Setup. This requires a subscriber’s request for service, identifies the terminal, searches and grabs a circuit, informs the required terminal and then receives its response.

2. Data Interchange. The established link is held during the transmission of data between source and destination and sends out the billing information (depending on the distance and duration of the connection) to the subscribers.

3. Connection Termination. After the communication is completed, the link channels and shared devices are released.


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Problems with Circuit Switching

• Many data sessions are low duty factor (bursty), (message transmission time)/(message interarrival time) << 1

Same as: (message arrival rate)*(message transmission time) << 1

• The rate allocated to the session must be large enough to meet the delay requirement. This allocated capacity is idle when the session has nothing to send.

• If communication is expensive, then circuit switching is uneconomic to meet the delay requirements of bursty traffic.

• It requires a call set-up during which resources are not utilized. If messages are much shorter than the call setup time, then circuit switching is not economical (or even practical)

- More of a problem in high-speed networks.

• Circuit switching usually uses a fixed data rate (e.g., 64 kbps)- Difficult to support variable data rates.


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Circuit Switching Example

L = message lengths (bits or bytes)

= arrival rate of message (messages/sec)R = channel rate (bits/sec)X = message transmission delay =L/R

- R must be large enough to keep X small- Bursty traffic => X << 1 => low utilization

• Example- L= 1,000 bytes (8,000 bits)

- = 1 message per second- X < 0.1 seconds (delay requirement)- => R > 8,000/ 0.1 = 80,000 bps

utilization = 8,000/ 80,000 = 10%

• With packet switching channel can be shared among many sessions to achieve higher utilization.


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1.2.2 Message Switching

An individual message is separately switched at each node along its route or path from source to destination.

A circuit (or path) is not established exclusively for a message; instead, message are sent using the store-and-forward approach.

Each message is divided into blocks of data by users based on the capacity of the networks, and these blocks are transmitted in a sequence.

The receiver, after receiving these blocks, constructs the original message from the blocks (by putting them in the same sequence as when it was transmitted) and sends an acknowledgement to the source.

The first block of the message contains control information regarding routing, QoS (Quality of Service) parameters, etc.

If the destination node does not accept the message (due to incomplete message, errors, or some other failure), the source node will transmit the same message again on the same link.


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1.2.2 Message Switching (cont.)

The intermediate switching nodes will store the block of messages in the event of circuit failure, heavy traffic, broken links, loss of acknowledgement, etc.

The switching nodes store the blocks of message and look for the free link to another switching intermediate node.

If it finds any free node, it sends one block at a time to that node

until all the stored blocks are sent.

If it does not find any free node, it will store the blocks and keep

on trying to find free links until it finds one and transmits the block to it.

The intermediate nodes are predefined between source and destination on the basis of certain criteria (e.g., shortest route, fastest route, heavy traffic, shareable files and programs, real-time data, etc.).

- If it does not find any intermediate node within the constraints, it will reroute

the blocks on a different alternative route.

Each block of messages is received as a complete frame and errors are checked in each block separately.

If an error is found in any block, then that block will be re-transmitted unidirectionally (simplex operation)


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Problems with Message Switching

Due to no limit on the size of the blocks of messages, the following problems may occur:

1. The switching node must have enough storage capacity for storing the blocks.

2. If a free link between source and destination is busy for a long time, then waiting and response time for the blocks to be transmitted may be significant.

Thus the technique seems to be useful for batched messages of longer duration but not suitable for interactive messages of shorter duration.

Advantages and Disadvantages of Message Switching

The message switching technique is less expensive than circuit switching and is widely used as a technique for interconnection.

This network allows the efficient utilization of a communication link.- Each of the nodes connected in the network share the load on equal probability and hence the channels are utilized to their maximum capacity.

It does not support dynamic load balancing and, as such, heavy traffic on a particular node will be handled by itself.


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1.2.3 Packet Switching

Figure 1.2.1 Concept of Packet Switching


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1.2.3 Packet Switching (cont.)

In principle, packet switching is based on the concept of message switching with the following differences:

Packets are parts of messages and include control bits (for detecting

transmission errors).

Networks break the message into blocks (or packets), while in message

switching, this is performed by users.

Due to very small storage time of packets in the waiting queue at any

node, users experience bi-directional transmission of the packets in real time.

The message is divided into blocks (or packets) of fixed size with its own control information regarding the routing, etc., across the network.

Route chosen on packet-by-packet basis.

Different packets may follow different route, results in out-of-sequence arrival at destination.

The receiver, after receiving the packets out of sequence, has to arrange the packets in the same order as they were transmitted from the source.


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1.2.3 Packet Switching (cont.)

If any node receives a garbled packet, it will request the sending node to transmit the same packet again.

The acknowledgement will be sent upon receiving the last packet.

If the destination node does not receive all the packets within the specified time, it sends a request for the data (instead of acknowledgement) to the source about the missing packets.

- In this case, the acknowledgement will not be sent out after the receiving node receives the missing packets.

The packet switching technique allows the switching nodes to transmit the packets without waiting for a complete message and also allows them to adjust the traffic they have, thus minimizing the resource requirements of the nodes.

- If any particular node is already heavily loaded, it will reject the packets until its load becomes moderate.


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1.2.3.1 Mode of Operation of Packet Switching

• Datagram Packet Switching- No need to establish the connection between the source and destination.

- Route chosen on packet-by-packet basis.- Different packets may follow different routes.- Packets may arrive out of order at the destination, e.g., IP (TheInternet Protocol)

• Virtual Circuit Packet Switching- Route is chosen at start of session and it is only a logical connection.- All packets associated with a session follow the same path.- Packets are labeled with a VC# designating the route.- The VC number must be unique on a given link but can change from link to link.

Imagine having to set up connections between 1,000 nodes in a mesh, unique VCnumbers imply 1 Million VC numbers that must be represented and stored at each node.

- The network manager performs the operations of segmenting,

reassembling and maintaining the path/route until the packets are delivered to the destination, e.g., ATM (Asynchronous Transfer Mode).


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1.2.3.2 Comparison of the Mode of Operation of Packet Switching

• For Datagram, addressing information must uniquely distinguish each network node and session

- Need unique source and destination addresses.

• For Virtual Circuits, only the virtual circuits on a link need to be distinguished by addressing

- Global address needed to set-up virtual circuit.- Once established, local virtual circuit members can then be used torepresent the virtual circuits on a given link: VC number changes from link to link.

• Merits of virtual circuit

- Save on route computation.(Need only be done once at start of session)

- Save on header size.

- Facilitate QoS provisioning

- More complex and Less flexible.Figure 1.2.2 Table of Switching Node


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Virtual Circuit approach Datagram approach

Figure 1.2.3 Comparison between Circuit approach and Datagram approach


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1.3 Advantages and Disadvantages Comparison of Switching Techniques

1. The link utilization of packet switching is improved over circuit switching.- Since the link can be shared by a number of different packets.

Each node has its own queue storing those packets which are going to use the node.

After the communication link is established, the packets (of different or the same data messages) may be transmitted during the established connection.

2. In packet switching networks, stations with different data rates can communicate with each other, and the necessary conversion between different data rates is done by the network.While in circuit switching networks, both stations must have the same data rate.

3. In circuit switching, the packets may be lost (not transmitted) in the event of a busy network.In packet switching, packets will be accepted, but there may be some delay in their delivery.


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1.3 Advantages and Disadvantages Comparison of Switching Techniques(Cont.)

4. The delay in the transmission of packets at any node may become a problem in the case of packet switching, while in the case of circuit switching, a dedicated circuit has been established and hence there is no delay in the transmission.

5. Priorities can be assigned to the switching nodes in the packet switchingsuch that nodes with higher priorities will transmit the packets waiting in their respective queues before the lower priority nodes.

In the case of circuit switching, there is no concept of priority.

6. Circuit switching generates the ordered delivery of the packets, while packet switching does not give any guarantee for the ordered delivery of the packets.


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1.4 Comparison of Communication Switching Techniques

Circuit Switching Datagram Packet Switching Virtual Circuit Packet Switching

Dedicate transmission path No dedicated path No dedicated path

Continuous transmission of data Transmission of packets Transmission of packets

Fast enough for interactive Fast enough for interactive Fast enough for interactive

Messages are not stored Packets may be stored until delivered

Packets stored until delivered

The path is established for entire conversation

Route established for each packet Route established for entire conversation

Call setup delay; negligible transmission delay

Packet transmission delay Call setup delay; packet transmission delay

Busy signal if called party busy Sender may be notified if packet not delivered

Sender notified of connection denial

Overload may block call setup; no delay for established calls

Overload increases packet delay Overload may block call setup; increases packet delay

Electromechanical or computerized switching nodes

Small switching nodes Small switching nodes

User responsible for message loss protection

Network may be responsible for individual packets

Network may be responsible for packet sequences

Usually no speed or code conversion

Speed and code conversion Speed and code conversion

Fixed bandwidth transmission Dynamic use of bandwidth Dynamic use of bandwidth

No overhead bits after call setup Overhead bits in each message Overhead bits in each packet


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1.5 OSI Reference Model

Protocols: A protocol is concerned with exchanging streams of data between two entities

Key functions of protocols:

• Segmentation and Reassembly

• Encapsulation

• Connection Control

• Ordered Delivery

• Flow Control

• Error Control

• Multiplexing


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1.5.1 Segmentation and Reassembly

(a) Segmentation: Whether the application entity sends data in messages or in a continuous stream, lower level protocols may need to break up the data into blocks of some smaller bounded size.

The typical reasons for segmentation:

1. The communication network may only accept blocks of data up to a certain size. ARPANET, for example, accepts message up to 8063 bytes in length.

2. Error control may be more efficient with a smaller PDU size. If an error is

detected, only a small amount of data may need to be retransmitted.(PDU: Protocol Data Unit, a block of data exchanged between two entities via a protocol.

3. More equitable access to shared transmission facilities, with shorter delay, can be provided. For example, without a maximum block size, one station could monopolize a shared medium.

4. A smaller PDU size may mean that receiving entities can allocate smaller buffers.

5. An entity may require that data transfer comes to some sort of closure from time to time, for checkpoint and restart/recovery operations.


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Disadvantages of Segmentation

1. Each PDU contains a fixed minimum of control information. Hence the smaller the block, the greater the percentage of overhead.

2. PDU arrival may generate an interrupt that must be serviced.

Smaller blocks result in more interrupts.

3. More time is spent processing smaller and more numerous PDUs.

The segmented data must be reassembled into the messages appropriate to the application level.

If PDUs arrival out of order, this task is complicated.

(b) Reassembly:


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1.5.2 Encapsulation

Definition: The addition of control information to data.

Data are accepted or generated by an entity and encapsulated into a PDU containing that data plus control information.

Each PDU contains not only data but control information.

Indeed, some PDUs consist solely of control information and no data.

The control information falls into three general categories:

• Address: The address of the sender and/or receiver may be indicated.

• Error detection code: Some sort of code is often included for error

detection.

• Protocol control: Additional information is included to implement the

protocol functions.


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Encapsulation, Segmentation and Reassembly

A message is segmented into three PDUs before transmission.

Upon reception, the PDU headers are stripped off and the segmenteddata are recombined for delivery to the user.

Layer

N

Layer

N

Layer

N-1

Layer

N-1

Figure 1.5.1 Encapsulation, Segmentation and Reassembly


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1.5.3 Connection Control

(a) Connectionless data transfer: An entity may transmit data to another entity in an unplanned fashion and without prior coordination.

(b) Connection-oriented data transfer: Consists of three phases.

Protocol

Entity

Protocol

EntityConnection Request

Connection Accept

Data and Acknowledgements

Data and Acknowledgements

Terminate-connection Request

Terminate-connection Acknowledge

multiple exchange

Figure 1.5.2Three phases of Connection Establishment


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Connection-oriented data transfer: Consists of three phases.

Connection establishment

During the connection establishment phase, two entities agree to exchange data.

Typically, one station will issue a connection request to the other.

Both entities must be using the same protocol.

Data transfer

After connection establishment, the data transfer phase is entered.

During this phase, both data and control information (flow control, error control) is exchanged.

Connection termination

Finally, one side or the other wishes to terminate the connection and does so by sending a termination request.


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1.5.4 Ordered delivery:

When two entities are in different hosts connected by a network, there is a risk that PDUs will not arrive in the order in which they were sent.

- Because they may traverse different paths through the network.

In connection-oriented protocols, it is generally required that PDU order be maintained.

1.5.5 Flow control:

A technique for assuring that a transmitting entity does not overwhelm a receiving entity with data.

The receiving entity typically allocates a data buffer with some maximum length.

When data are received, the receiver must do a certain amount of processing (i.e., examine the header and strip it from the PDU) before passing the data to a higher layer user.

In the absence of flow control, the receiver’s buffer may fill up and overflow while it is processing old data.


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1.5.6 Error control:

A technique that allows a protocol to recover from lost or damages PDUs, based on the use of sequence numbers.

There are three mechanisms:

▪ Positive acknowledgement

It is the responsibility of the receiving protocol entity to acknowledge each PDU that it receives.

-This is done by sending back the sequence number of the received PDU to the other side.

If a PDU is lost in transit, then the intended receiver will obviously not acknowledge it.

▪ Retransmit after timeout

The sending entity will note the time that it sends each PDU.

If a PDU remains unacknowledged after a certain amount of time, the sender assumes that the PDU did not get through and retransmits that PDU.


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▪ Error detection

The PDU gets through but the bits have been altered by errors in

transit.

To account for this contingency, the error detections is needed.

The sending entity performs a calculation on the bits of the PDU

and adds the result to the PDU.

The receiver performs the same calculation and compares the

calculated result to the result stored in the coming PDU.

If there is a discrepancy, the receiver assumes that an errors has

occurred and discarded the PDU.


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1.5.7 Multiplexing:

A function that may be exercised when more than one layer of a communications architecture employs a connection-oriented protocol.

Three particular types of construction of (N) connections on top of (N-1)

connections are distinguished:

• One-to-one:

Each (N) connection is built on one (N-1) connection.

• Upward multiplexing:

Occurs when multiple (N) connections are multiplexed on, or share a

single (N-1) connection to make efficient use of the lower-level service.

If several transport connections are needed between a pair of hosts, these could all be multiplexed on a single network connection.

• Downward multiplexing (splitting):

A single (N) connection is built on top of multiple (N-1) connection, the

traffic on the (N) connection being divided between the various (N-1)

connections to improve reliability, performance or efficiency.


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Multiplexing of Protocol Connections

Figure 1.5.3 Type of Multiplexing


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1.6 The Layer concept

Figure 1.6.1 Tasks involved in sending letter


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Entity in Layer Concept

Figure 1.6.2 The Layer Concept


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Any layer is referred to as the (N) layer.

An (N) entity implements functions of the (N) layer and also the protocol for

communicating with (N) entities in other systems.

There might be multiple identical (N) entities, or also be differing (N) entities,

corresponding to different protocol standards at that level.

Each entity communicates with entities in the layers above and below it across an interface.

The interface is realized as one or more service access points (SAPs).

The (N-1) entity provides services to an (N) entity via the invocation of

primitives.

A primitive specifies the function to be performed and is used to pass data and control information.


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1.6.1 Primitive Types (X.210)

Request A Primitive issued by a service user to invoke some services and to pass the parameters needed to fully specify the requested service.

Indication A Primitive issued by a service provider either:

(1) To indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or

(2) To notify the service user of a provider-initiated action.

Response A Primitive issued by a service user to acknowledge or complete some procedures previously invoked by an indication to that user.

Confirm A Primitive issued by a service provider to acknowledge or complete some procedures previously invoked by a request by the service user.

Figure 1.6.3 TheInteraction Primitives

SERVICE USER SERVICE USERSERVICE PROVIDER

Request

Confirm

Indication

Response


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Example of Interaction Primitives (Confirmed type)

Consider the transfer of data from an (N) entity to a peer (N) entity in another system.

The following steps could occur:

1. The source (N) entity invokes its (N-1) entity with Data.request primitive.

Associated with the primitive are the parameters needed, such as the data to be transmitted and the destination address.

2. The source (N-1) entity prepares an (N-1) PDU to be sent to its peer (N-1)

entity.

3. The destination (N-1) entity delivers the data to the appropriate destination

(N) entity via Data.indication, which includes the data and a source address as

parameters.

4. If an acknowledgement is called for, the destination (N) entity issues a

Data.response primitive to its (N-1) entity.

5. The (N-1) conveys this acknowledgement in an (N-1) PDU.

6. The acknowledgement is delivered to the (N) entity via a Data.confirm.


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1.6.2 Logical relationship between data units in adjacent layers

Figure 1.6.4 The Logical Relationship of Data Units

(N)-PDU

(N-1)-SDU

(N-1)-PDU

(N) Layer

(N-1) Layer

(N-1) PCI

PDU: Protocol Data Unit

SDU: Service Data Unit

PCI: Protocol Control Information


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Data unit: The data that passes between entities in the form of a collection of bits.

The PDU is passed as a unit between peer entities.

An entity in the (N) layer cannot directly pass data to a peer entity at the (N)

layer.

- It must pass its (N) PDUs down to the (N-1) layer.

The (N-1) views data coming from the (N) layer as an (N-1) service data unit

(SDU).

The (N-1) entity may map an SDU arriving from above into a single PDU.

Alternatively, the (N-1) entity may segment the (N-1) SDU into multiple PDUs.

In any case, the identity of the SDU as a unit is preserved from one end of the connection to the other;

- if the SDU is segmented by the sending (N-1) entity, it must be

reassembled by the receiving (N-1) entity before delivery to the

receiving (N) entity.


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1.6.3 Seven Layers OSI Reference Model

Figure 1.6.5 Seven layers OSI Reference Model


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Figure 1.6.6 An Exchange using Seven layers OSI Reference Model


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1.6.3 Seven Layers OSI Reference Model (cont.)

Layer 7: Application layer

- Provides various network services required by user applications.

- Serves as a window between application processes which are using OSI-RM and defines its aspect for various application entities.

Example of Standards:

• CCITT X.400 message handling (MHS) (for PSDNs)

• CCITT FTAM (file transfer access and management ) (for PSDNs)

• CCITT X.420 interpersonal messaging (for PSDNs)

• CCITT X.500/520 directory services (for PSDNs)

• CCITT TTX telex service (for PSTNs)

• CCITT T100, T101 videotex (for PSTNs)

• Network file system (NFS), file transfer protocol (FTP), remote login (Rlogin),

remote procedure call (RPC), simple mail transfer protocol (SMTP), simple network management protocol (SNMP), and others are provided by TCP/IP communication interface (UNIX).


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Layer 6: Presentation layer

- Provides suitable syntax for data transmission, means of data formatting (data compression, etc.), data encryption for security, privacy and authentication.

- Provides efficient data transfer across the layers of the networks.


• ISO 8822 presentation service and syntax notation (for LANs)

• ISO 8823 presentation layer protocol (for LANs)

• ISO 8824, ISO 8825

• CCITT X.216, X.226 (for PSDNs)

• CCITT T50, T51, T61 (for PSTNs)


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Layer 5: Session layer

- Obtains virtual end-to-end message service from transport layer.

- Provides directory assistance, access rights, billing function, etc.

- Standardization has not proceeded well here, since transport to application are all in the operating system and do not really need standard interfaces.


• ISO 8326 session service specification (for LANs)

• ISO 8327 session layer protocol (for LANs)

• CCITT X.215/X.225 (for PSDNs)

• CCITT T62 (for PSTNs)

• Remote procedure control (RPC) library (UNIX)


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Layer 4: Transport layer

- The network layer provides a virtual end-to-end packet pipe to the transport layer.

- The transport layer provides a virtual end-to-end message service to the high layers.

The functions of the transport layer are:1. Break messages into packets and reassemble packets size

suitable to network layer.2. Multiplex sessions with same source/destination nodes.3. Re-sequence packets at destination. 4. Recover from residual errors and failures.5. Provide end-to-end flow control.

Example of Standards:• ISO 8072 transport service specification (for LANs)• ISO 8073 transport layer protocol (for LANs)• CCITT X.214/X.224 (for PSDNs)• CCITT T70 (for PSTNs)• Transmission Control Protocol (TCP) (UNIX)


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Figure 1.6.7 Network layer

Layer 3: Network layer- Accepts incoming packets from the transport layer and transit packets from the DLC layer.

- Routes each packet to the proper outgoing DLC or (at the destination) to the transport layer.

- Adds its own header to the packets received from the transport layer. This header provides the information needed for routing (e.g., destination

address).

Example of Standards:• CCITT’s X.25 includes lower three layers known as level 1,2,3.

• X.21 has been defined for level 1, high-level datalink control (HDLC for level 2 and packet level standard for level 3.

• CCITT X.213 (circuit-switched network) (for PSDNs)• CCITT X.25-3 (packet-switched network)• CCITT T30 (for PSTNs)• ISO connectionless mode network Internet Protocol (IP) (UNIX)

• CCITT I.450 and I.451 (for ISDNs)


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Layer 2: Data link layer

-Responsible for reliable and error-free transmission of packets across a single link.

• Framing: Determine the start and end of the packets

• Error detection: Determine which packets contain transmission errors

• Error correction: Retransmission schemes (Automatic Repeat Request

(ARQ)

- Provides synchronization and delimiting between sender and receiver.

Example of Standards:• High level data link control (HDLC: bit-oriented)• ANSI’s X3.28 (character-oriented)• CCITT X.21-2 (circuit-switched network)• CCITT X.25-2 (packet-switched network)• CCITT X.212/222 (for PSDNs); CCITT T71 (for PSTNs)

• Protocols for HDLC: ISO 3309 HDLC procedures (frame structure),

ISO/DIS 5335 HDLC procedures (elements of procedures), ISO/DIS 6159 HDLC unbalanced classes of procedures, ISO/DIS 6256 HDLC balanced classes of procedures-Ethernet, logical (UNIX)

• CCITT I.440 and I.441 (for ISDNs).


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Layer 1: Physical layer

- Responsible for transmission of bits over a link.- Connects the network to the transmission medium and also generates the electromagnetic signals.

- Offers two types of transmission configurations: point-to-point and multi-point.

- Supports different types of the line configurations: half-duplex and full-duplex in serial and parallel modes.

Example of Standards:• EIA-232-D; 25 pins, EIA RS-449 (RS-422-A and RS-423-A); 37 pins.• EIA-530, EIA-366• CCITT X.21-1 (circuit-switched network); 9 pins• CCITT X.21 bis (for the PSDNs)• CCITT X.25-1 (packet-switched network)• CCITT V.24 (list of definitions for the interchange of circuits between DTE and DCE for the PSTNs)

• 5.28 (electrical characteristics for unbalanced)

• 5.21(general-purpose interface between DTE and DCE) for Ethernet, token

bus (UNIX)

• CCITT I.430 and I.431 (for ISDNs).


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1.6.4 OSI Operation

Figure 1.6.8 The Concept of OSI Operation

Application

Presentation

Session

Transport

Network

Data link

Control

Physical

Application

Presentation

Session

Transport

Network

Data link

Control

PhysicalBits

X Y

Communication path

data (I field)

data

data

data

data

data

F A C FCS F

NH

TH

SH

PH

AH

AP data

Outgoing frame

construction

Incoming frame

reduction


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1.6.4 OSI Operation (cont.)

Figure 1.6.9 Application layer (7)


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Figure 1.6.10 Presentation layer (6)


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Figure 1.6.11 Session layer (5)


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Figure 1.6.12 Transport layer (4)


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Figure 1.6.13 Reliable process-to-process delivery of message


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Figure 1.6.14 Network layer (3)


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Figure 1.6.15 Source-to-Destination Delivery


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Figure 1.6.16 Data Link layer (2)


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Figure 1.6.17 Hop-to-Hop Delivery


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Figure 1.6.18 Physical layer (1)


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1.6.5 Summary of Layer Functions

Figure 1.6.19 Summary of Layer Functions


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1.6.6 Addressing

Figure 1.6.20 Addresses in TCP/IP

Four levels of addresses are used in an internet employing the

TCP/IP protocols: physical, logical, port, and specific.


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Example 1 (Physical Address)

A node with physical address 10 sends a frame to a node with physical address 87.

The two nodes are connected by a link.

At the data link level this frame contains physical link addresses in the header.

These are the only addresses needed.

The rest of the header contains other information needed at this level.

The trailer usually contains extra bits needed for error detection.

Figure E.1 Data Link layer using Physical Address


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Example 2 (IP Address)

Figure E.2 Network layer using IP Address


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We want to send data from - a node with network address A and physical address 10 , located on one LAN, - to a node with a network address P and physical address 95, located on another LAN.

Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses have only local jurisdiction.

What we need here are universal addresses that can pass through the boundaries of LANs.

The network (logical) addresses have this characteristics.

The packet at the network layer contains the logical addresses, which remain the same from the original source to the final destination (A and P, respectively).

They will not change when we go from network to network.

However, the physical addresses will change when the packet moves from one network to another.


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Example 3 (Port Address)

Figure E.3 Transport layer using Port Address


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Data coming from the upper layers have

- service-point (port) addresses j and k

(j is the address of sending applications, and

k is the address of the receiving application).

Since the data size is larger than the network layer can handle, the data are

split into two packets, each packet retaining the service-point addresses (j

and k).

Then in the network layer, network addresses (A and P) are added to each packet.

The packets may travel on different paths and arrive at the destination either in order or out of order.

The two packets are delivered to the destination network layer, which is responsible for removing the network layer headers.

The two packets are now passed to the transport layer, where they are combined for delivery to the upper layer.


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TCP/IP and the OSI Model

Figure 1.6.21 Comparison of TCP/IP and OSI Model


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Internet Sub-layer

A sub-layer between the transport and network layers is required when various incompatible networks are joined together.

This sub-layer is used at gateways between the different networks.

It looks like a transport layer to the networks being joined.

It is responsible for routing and flow control between networks, so looks like a network layer to the end-to-end transport layer.

In the internet, this function is accomplished by using the Internet Protocol (IP)

- IP is often used as the network layer protocol, hence only one protocol is needed.


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Internetworking within TCP/IP

Figure 1.6.22 Internetworking with TCP/IP


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Encapsulation in TCP/IP

Figure 1.6.23 Encapsulation in TCP/IP

I.introduction

Documents

code conversion

procedures

bandwidth

block call

oriented data

smaller pdu

end message

delivered