Network architecture – layers

Introduction

Computer Network: Definition

Collection of autonomous computers interconnected by single technology.

Connectivity

Connectivity occurs between two computers through physical medium like coaxial cable or an optical fiber.

Physical Medium – Link 

Computers – Nodes

When a physical link occurs between a pair of nodes then it is referred as point-to-point.

When more than two nodes share a single physical link then it is referred as multiple access.

Data communication between the nodes is done by forwarding the data from one link to another. The systematic way of organizing these forwarding nodes form a switched network.

Two common types of switched network are

·       Circuit switched – e.g. Telephone System

·       Packet switched – e.g. Postal System 

Packet Switched Network

In this network nodes send discrete blocks of data to each other. These blocks can be called as packet or message.

Store and forward strategy:

This network follows this technique. It means “Each node receives a complete packets over the link, stores in internal memory and then forwards to next node”.

Circuit Switched Network:

It first establishes a circuit across the links and allows source node to send stream of bits across this circuit to the destination node.

The representation of network is given by cloud symbol

Cloud represents the network

Nodes inside the cloud (Switches) – Implement the n etwork Nodes outside the cloud (host) - Use the network

Internetwork:

Set of independent network are interconnected to form internetwork or internet. Node that is connected to two or more network is called router or gateway. It is responsible for forwarding data between the networks.

Addressing:

The final requirement is that each node must be able to say which of the other node it wants to communicate with.

This is done by assigning address to each node. When a source node wants to deliver message to destination node, it specifies the address of destination node.

Switches and Routers use this address to decide how to forward the message. This process based on address is called Routing.

Unicast – sending message to single node.

Broadcast – Sending message to all the nodes on the network. Multicast – Sending message to some subnet not to all.

Resource Sharing

Pblm: How do several hosts share the same link when they all want to use it at the same time?

Sol: Multiplexing – System resources are shared among  multiple users

Methods:

1.  Synchronous Time Division Multiplexing (STDM) Divide time into equal sized quanta

2.  Frequency Division Multiplexing (FDM) Transmit each flow at different frequency

3.   Statistical Multiplexing

First two methods are limited in 2 ways

·       If one flow does not have data to send then its time quantum remains idle, even the other flow has data to transmit.

·       No of flows are fixed and known ahead of time, it cannot be resized.

Statistical methods combine the ideas of both STDM and FDM

·       Data from each flow is transmitted on demand so no idle quantum

·       It defines upper bound on size of data and it is referred as packet.

Common communication patterns

Communication between a pair of processes is done by request / reply basis. The process which sends request is referred as client and the one which honors the request is referred as server.

This can be done using channels. Two types of channels are

·       Request / Reply channels

·       Message stream Channels

Reliability

To get the reliable network, it is necessary to find how network fails. Three classes of failures

·       Bit error

·       Packet loss

·       Physical link and node failure 

Network Architecture

Networks do not remain fixed at single point in time, but it must evolve to accommodate changes based on the technologies on which they are based and demands made by application programmer.

Network architecture guides the design and implementation of network. 

Most widely referred architecture:

·       OSI Architecture

·       Internet or TCP/IP architecture

Layering and Protocols

When the system gets complex, the system designer introduces another level of abstraction. It defines unifying model with important aspects of the system, encapsulated this model in interface objects and hide it from users

 

In network, abstraction leads to layering. Layering provides two nice features.

·       It decomposes the problem of building a network into more manageable components. Rather than implementing a monolithic piece of software that does everything implement several layers, each of which solves one part of the problem.

·       It provides more modular design. To add some new service, it is enough to modify the functionality at one layer, reusing the functions provided at all the other layers. 

Protocols

A protocol is a set of rules that governs data communication. It defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics and timing.

Each protocol defines two different interfaces.

Service interface defines to the other objects on the same computer that want to use its communication services. This service interface defines the operations that local objects can perform on the protocol.

Peer interface - to its counterpart (peer) on another machine. It also defines the form and meaning of messages exchanged between protocol peers to implement the communication service.

 

Except at the hardware level, peer to peer communication is indirect.

We can represent these protocols as protocol graph. Nodes of the graph correspond to protocols, and the edges represent a depends- on relation.

For example, the file access program on host 1 wants to send a message to its peer on host 2 using the communication service offered by protocol RRP. In this case, the file application asks RRP to send the message on its behalf. To communicate with its peer, RRP then invokes the services of HHP, which in turn transmits the message to its peer on the other machine. Once the message has arrived at protocol HHP on host 2, HHP passes the message up to RRP, which in turn delivers the message to the file application. In this particular case, the application is said to employ the services of the protocol stack RRP/HHP.

Encapsulation

Control information must be added with the data to instruct the peer how to handle with the received message. It will be added into the header or trailer.

Header - Small data structure from few bytes to few kilobytes attached to the front of message.

Trailer – Information will be added at the end of t he message Payload or message body – Data send by the program

In this case data is encapsulated with new message created by protocol at each level.

In this example HHP encapsulates RRP’s message by attaching a header of its own. Then HHP sends the message to its peer over some network, and then when the message arrives at the destination host, it is processed in the opposite order.

Multiplexing and De-Multiplexing

The fundamental idea of packet switching is to multiplex multiple flows of data over a single physical link. This can be achieved by adding identifier to the header message. It is known as multiplexing or demux key. It gives the address to which it has to communicate.

The messages are de-multiplexed at the destination side. In some cases same demux key is used on both sides and in some cases different keys are used.

OSI Architecture

ISO defines a common way to connect computer by the architecture called Open System Interconnection (OSI) architecture.

Network functionality is divided into seven layers.

Organization of the layers

The 7 layers can be grouped into 3 subgroups

1. Network Support Layers

Layers 1,2,3 - Physical, Data link and Network are the network support layers. They deal with the physical aspects of moving data from one device to another such as electrical specifications, physical addressing, transport timing and reliability.

2. Transport Layer

Layer4, transport layer, ensures end-to-end reliable data transmission on a single link.

3. User Support Layers

Layers 5,6,7 – Session, presentation and applicatio n are the user support layers. They allow interoperability among unrelated software systems

The upper OSI layers are almost always implemented in software; lower layer are a combination of hardware and software, except for the physical layer, which is mostly hardware.

An Data exchange using the OSI model

At each layer, a header is added to the data unit except layer 7 and 1. At layer 2 a trailer is added as well.

Functions of the Layers 

Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

The physical layer is concerned with the following:

·       Physical characteristics of interfaces and media – The physical layer defines the characteristics of the interface between the devices and the transmission medium.

·        Representation of bits - To transmit the stream of bits, it must be encoded to signals. The physical layer defines the type of encoding.

·       Data Rate or Transmission rate - The number of bits sent each second.

·       Synchronization of bits - The sender and receiver must be synchronized at the bit level. Their clocks must be synchronized.

·       Line Configuration - In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices.

·       Physical Topology - The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh, bus, star or ring topology.

·       Transmission Mode - The physical layer also defines the direction of transmission between two devices: simplex, half-duplex or full-duplex.

 

Data Link Layer 

It is responsible for transmitting frames from one node to next node

The other responsibilities of this layer are

·       Framing - Divides the stream of bits received into data units called frames.

·       Physical addressing – If frames are to be distributed to different systems on the n/w, data link layer adds a header to the frame to define the sender and receiver.

·       Flow control- If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the Data link layer imposes a flow ctrl mechanism.

·       Error control- Used to detecting and retransmit damaged or lost frames and to prevent duplication of frames. This is achieved through a trailer added at the end of the frame.

·       Access control -Used to determine which device has control over the link at any given time.

NETWORK LAYER

This layer is responsible for the delivery of packets from source to destination. It is mainly required, when it is necessary to send information from one network to another.

The other responsibilities of this layer are

·       Logical addressing - If a packet passes the n/w boundary, we need another addressing system for source and destination called logical address.

• Routing – The devices which connects various networks calle d routers are responsible for delivering packets to final destination.

TRANSPORT LAYER

·       It is responsible for Process to Process delivery.

·       It also ensures whether the message arrives in order or not.

The other responsibilities of this layer are

·       Port addressing - The header in this must therefore include an address called port address. This layer gets the entire message to the correct process on that computer.

• Segmentation and reassembly - The message is divided into segments and each segment is assigned a sequence number. These numbers are arranged correctly on the arrival side by this layer 
• Connection control - This can either be connectionless or connection-oriented. The connectionless treats each segment as an individual packet and delivers to the destination. The connection-oriented makes connection on the destination side before the delivery. After the delivery the termination will be terminated. 
•  Flow and error control - Similar to data link layer, but process to process take place.

SESSION LAYER

This layer establishes, manages and terminates connections between applications.

The other responsibilities of this layer are

·       Dialog control - This session allows two systems to enter into a dialog either in half duplex or full duplex.

·       Synchronization-This allows to add checkpoints into a stream of data.

PRESENTATION LAYER 

It is concerned with the syntax and semantics of information exchanged between two systems.

The other responsibilities of this layer are

·       Translation – Different computers use different encoding system , this layer is responsible for interoperability between these different encoding methods. It will change the message into some common format.

·       Encryption and decryption-It means that sender transforms the original information to another form and sends the resulting message over the n/w. and vice versa.

·       Compression and expansion-Compression reduces the number of bits contained in the information particularly in text, audio and video.

APPLICATION LAYER

This layer enables the user to access the n/w. This allows the user to log on to remote user.

The other responsibilities of this layer are

·      FTAM (file transfer,access,mgmt) - Allows user to access files in a remote host.

·      Mail services - Provides email forwarding and storage.

• Directory services - Provides database sources to access information about various sources and objects.

Summary of layers