ISO/OSI Ref Model

The Differences Between SAN and NAS

Storage Area Networks (SANs) and Network Attached Storage (NAS) both provide networked storage solutions. A NAS is a single storage device that operates on data files, while a SAN is a local network of multiple devices.

The differences between NAS and SAN can be seen when comparing their cabling and how they’re connected to the system, as well as how communication to the device occurs.

SAN vs. NAS Technology

A NAS unit includes a dedicated hardware device that connects to a local area network, usually through an Ethernet connection.

This NAS server authenticates clients and manages file operations in much the same manner as traditional file servers, through well-established network protocols.

To reduce the costs that occur with traditional file servers, NAS devices generally run an embedded operating system on simplified hardware and lack peripherals like a monitor or keyboard.

A SAN commonly utilizes Fibre Channel interconnects and connects a set of storage devices that are able to share data with one another.

SAN vs. NAS Usage Models

The administrator of a home or small business network can connect one NAS device to their local area network. The NAS maintains its own IP address comparable to computers and other TCP/IP devices. Using a software program that is provided with the NAS hardware, the network administrator can set up automatic or manual backups and file copies between the NAS and all the other connected devices.

The NAS holds many gigabytes or terabytes of data. Administrators can add additional storage capacity to their network by installing additional NAS devices, although each NAS operates independently.

Administrators of large enterprise networks may require many terabytes of centralized file storage or extremely high-speed file transfer operations.

Where installing an army of many NAS devices is not a practical option, administrators can instead install a SAN containing a high-performance disk array to provide the needed scalability and performance.

SAN/NAS Convergence

As internet technologies like ​TCP/IP and Ethernet have proliferated worldwide, some SAN products are making the transition from Fibre Channel to the same IP-based approach NAS uses. Also, with the rapid improvements in disk storage technology, today’s NAS devices now offer capacities and performance that once were only possible with SAN.

These two industry factors have led to a partial convergence of NAS and SAN approaches to network storage.

Application Layer – OSI Model

It is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.

 

Application Layer in ISO-OSI Model


FUNCTIONS OF APPLICATION LAYER:

  1. Mail Services : This layer provides the basis for E-mail forwarding and storage.
  2. Network Virtual Terminal : It allows a user to log on to a remote host. The application creates software emulation of a terminal at the remote host. User’s computer talks to the software terminal which in turn talks to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals and allows user to log on.
  3. Directory Services : This layer provides access for global information about various services.
  4. File Transfer, Access and Management (FTAM) : It is a standard mechanism to access files and manages it. Users can access files in a remote computer and manage it. They can also retrieve files from a remote computer.

Presentation Layer – OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

 

Presentation Layer in ISO-OSI Model


FUNCTIONS OF PRESENTATION LAYER:

  1. Translation : Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
  2. Encryption : It carries out encryption at the transmitter and decryption at the receiver.
  3. Compression : It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Session Layer – OSI Model

Its main aim is to establish, maintain and synchronize the interaction between communicating systems. Session layer manages and synchronize the conversation between two different applications. Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

 

Session Layer in ISO-OSI Model


FUNCTIONS OF SESSION LAYER:

  1. Dialog Control : This layer allows two systems to start communication with each other in half-duplex or full-duplex.
  2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Transport Layer – OSI Model

The main aim of transport layer is to be delivered the entire message from source to destination. Transport layer ensures whole message arrives intact and in order, ensuring both error control and flow control at the source to destination level. It decides if data transmission should be on parallel path or single path

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer and ensures that message arrives in order by checking error and flow control.

 

Transport Layer in ISO-OSI Model


FUNCTIONS OF TRANSPORT LAYER:

  1. Service Point Addressing : Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.
  2. Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
  3. Connection Control : It includes 2 types :
    • Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
    • Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
  4. Flow Control : In this layer, flow control is performed end to end.
  5. Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

Network Layer – OSI Model

The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.

It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

 

Network Layer in ISO-OSI Model


FUNCTIONS OF NETWORK LAYER:

  1. It translates logical network address into physical address. Concerned with circuit, message or packet switching.
  2. Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
  3. Connection services are provided including network layer flow control, network layer error control and packet sequence control.
  4. Breaks larger packets into small packets.

DATA LINK Layer – OSI Model

Data link layer is most reliable node to node delivery of data. It forms frames from the packets that are received from network layer and gives it to physical layer. It also synchronizes the information which is to be transmitted over the data. Error controlling is easily done. The encoded data are then passed to physical.

Error detection bits are used by the data link layer. It also corrects the errors. Outgoing messages are assembled into frames. Then the system waits for the acknowledgements to be received after the transmission. It is reliable to send message.

 

FUNCTIONS OF DATA LINK LAYER:

  1. Framing: Frames are the streams of bits received from the network layer into manageable data units. This division of stream of bits is done by Data Link Layer.
  2. Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.
  3. Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.
  4. Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.
  5. Access Control: Protocols of this layer determine which of the devices has control over the link at any given time, when two or more devices are connected to the same link.

 

Data Link Layer in ISO-OSI Model

PHYSICAL Layer – OSI Model

Physical layer is the lowest layer of all. It is responsible for sending bits from one computer to another. This layer is not concerned with the meaning of the bits and deals with the physical connection to the network and with transmission and reception of signals.

This layer defines electrical and physical details represented as 0 or a 1. How many pins a network will contain, when the data can be transmitted or not and how the data would be synchronized.

 

FUNCTIONS OF PHYSICAL LAYER:

  1. Representation of Bits: Data in this layer consists of stream of bits. The bits must be encoded into signals for transmission. It defines the type of encoding i.e. how 0’s and 1’s are changed to signal.
  2. Data Rate: This layer defines the rate of transmission which is the number of bits per second.
  3. Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver are synchronized at bit level.
  4. Interface: The physical layer defines the transmission interface between devices and transmission medium.
  5. Line Configuration: This layer connects devices with the medium: Point to Point configuration and Multipoint configuration.
  6. Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring and Bus.
  7. Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex, Half Duplex, Full Duplex.
  8. Deals with baseband and broadband transmission.

 

Physical Layer in ISO-OSI Model

ISO/OSI Model in Communication Networks

There are n numbers of users who use computer network and are located over the world. So to ensure, national and worldwide data communication, systems must be developed which are compatible to communicate with each other. ISO has developed this. ISO stands for International organization of Standardization. This is called a model for Open System Interconnection (OSI) and is commonly known as OSI model.

The ISO-OSI model is a seven layer architecture. It defines seven layers or levels in a complete communication system.

 

Diagram of ISO-OSI Model


Feature of OSI Model :

  1. Big picture of communication over network is understandable through this OSI model.
  2. We see how hardware and software work together.
  3. We can understand new technologies as they are developed.
  4. Troubleshooting is easier by separate networks.
  5. Can be used to compare basic functional relationships on different networks.

Functions of Different Layers :

Layer 1: The Physical Layer :

  1. It is the lowest layer of the OSI Model.
  2. It activates, maintains and deactivates the physical connection.
  3. It is responsible for transmission and reception of the unstructured raw data over network.
  4. Voltages and data rates needed for transmission is defined in the physical layer.
  5. It converts the digital/analog bits into electrical signal or optical signals.
  6. Data encoding is also done in this layer.

 

Layer 2: Data Link Layer :

  1. Data link layer synchronizes the information which is to be transmitted over the physical layer.
  2. The main function of this layer is to make sure data transfer is error free from one node to another, over the physical layer.
  3. Transmitting and receiving data frames sequentially is managed by this layer.
  4. This layer sends and expects acknowledgements for frames received and sent respectively. Resending of non-acknowledgement received frames is also handled by this layer.
  5. This layer establishes a logical layer between two nodes and also manages the Frame traffic control over the network. It signals the transmitting node to stop, when the frame buffers are full.

 

Layer 3: The Network Layer :

  1. It routes the signal through different channels from one node to other.
  2. It acts as a network controller. It manages the Subnet traffic.
  3. It decides by which route data should take.
  4. It divides the outgoing messages into packets and assembles the incoming packets into messages for higher levels.

 

Layer 4: Transport Layer :

  1. It decides if data transmission should be on parallel path or single path.
  2. Functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer
  3. It receives messages from the Session layer above it, convert the message into smaller units and passes it on to the Network layer.
  4. Transport layer can be very complex, depending upon the network requirements.

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.

 

Layer 5: The Session Layer :

  1. Session layer manages and synchronize the conversation between two different applications.
  2. Transfer of data from source to destination session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

 

Layer 6: The Presentation Layer :

  1. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
  2. While receiving the data, presentation layer transforms the data to be ready for the application layer.
  3. Languages(syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role of translator.
  4. It perfroms Data compression, Data encryption, Data conversion etc.

 

Layer 7: Application Layer :

  1. It is the topmost layer.
  2. Transferring of files disturbing the results to the user is also done in this layer. Mail services, directory services, network resource etc are services provided by application layer.
  3. This layer mainly holds application programs to act upon the received and to be sent data.

Merits of OSI reference model:

  1. OSI model distinguishes well between the services, interfaces and protocols.
  2. Protocols of OSI model are very well hidden.
  3. Protocols can be replaced by new protocols as technology changes.
  4. Supports connection oriented services as well as connectionless service.

Demerits of OSI reference model:

  1. Model was devised before the invention of protocols.
  2. Fitting of protocols is tedious task.
  3. It is just used as a reference model.

Reference Models in Communication Networks

The most important reference models are :

  1. OSI reference model.
  2. TCP/IP reference model.

Introduction to ISO-OSI Model:

There are many users who use computer network and are located all over the world. To ensure national and worldwide data communication ISO (ISO stands for International Organization of Standardization.) developed this model. This is called a model for open system interconnection (OSI) and is normally called as OSI model.OSI model architecture consists of seven layers. It defines seven layers or levels in a complete communication system. OSI Reference model is explained in other chapter.

 

ISO-OSI Model with seven layers

 


Introduction to TCP/IP REFERENCE Model

TCP/IP is transmission control protocol and internet protocol. Protocols are set of rules which govern every possible communication over the internet. These protocols describe the movement of data between the host computers or internet and offers simple naming and addressing schemes.

TCP/IP Reference model is explained in details other chapter.

 

TCP/IP Model with four layers