OSI Model

OSI model (OSI stands for Open System Interconnection) is a set of standards to facilitate network communication that was introduced by the International Organization for Standardization (ISO) in 1984. OSI model is the most widely used method for describing network communication.

OSI model divides networking tasks into the following seven layers.

Layer 7: Application layer

Layer 7 OSI model (Application) defines the interface between the communications software and any applications that need to communicate outside the computer on which the application resides. This layer 7 OSI model specifies rules for the following tasks:

1. Providing network services
2. Advertising network services
3. Accessing network services

The following is the examples of layer 7 OSI model:

• Web browser
• Netware’s Service Advertising Protocol (SAP)
• Simple Mail Transfer Protocol (SMTP)
• TCP/IP Network File System (NFS)

Layer 6: Presentation Layer

Layer 6 OSI model (Session layer) main purpose is to define data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption is also defined by OSI model as a presentation layer service.

• Data translation
• Data encryption and compression

The following are the examples of protocols that implement Presentation layer of the OSI model rules:

• Netware core protocol (NCP)
• Apple Talk Filing Protocol

Layer 5: Session layer

The session layer of the OSI model defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data. The presentation layer of the OSI model can be presented with data if all flows occur in some cases.

Summary of the Session layer of the OSI model:

• Controlling communication sessions between two devices
• Establishing, managing, and releasing connections

The following are examples of protocols that implement session-layer of the OSI model rules:

• Netware’s services advertising protocol (SAP)
• TCP/IP remote procedure call

Layer 4: Transport layer

Layer 4 of the OSI model focuses on issues related to data delivery to the other computer-for instance, error recovery, segmentation of large application data blocks into smaller ones for transmission, and reassembly of those blocks of data on the receiving computer.

The following are rules specified by this transport layer of the OSI model:

• Hiding the network structure from the upper layer
• Acknowledging that a message was received
• Ensuring reliable, error-free message delivery.

The following are examples of protocols that implement Transport layer of the OSI model rules

• Netware’s Sequenced Packet Exchange (SPX) Protocol
• TCP/IP’s Transmission Control Protocol (TCP)
• TCP/IP Domain Name System (DNS)

Layer 3: Network Layer

This Network layer of the OSI model defines end-to-end delivery of packets. To accomplish this, the network layer defines logical addressing so that any endpoint can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered.

Summary of layer 3 OSI model:

• Routing data between multiple networks
• Fragmenting and reassembling data
• Identifying network cable segment

The following are examples of protocols that implement Network-layer rules of the OSI model:

• Netware’s Internetwork Packet Exchange (IPX) protocol
• TCP/IP Internet Protocol (IP)

Layer 2: Data link layer

The OSI Data link layer of the OSI model specifies rules for the following:

• Organizing bits into logical groups of information
• Detecting and sometimes correcting transmission errors
• Correcting data flow
• Identifying network devices

The following are examples of protocols that implement Data-Link later rules of the OSI model:

• High-Level Data Link Control (HDLC) for a point-to-point WAN link
• Netware’s Link Support Layer (LSL)
• Asynchronous Transfer Mode (ATM)

Layer 1: Physical layer

The Physical layer of the OSI model deals with the physical characteristics of the transmission medium. Connectors, pins, use of pins, electrical currents, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications sometimes are used to complete all details of the physical layer. Examples of the OSI model:

• RJ-45 (commonly used in LAN card or NIC computer connector) defines the shape of the connector and the number of wires or pins in the cable.
• Ethernet and 802.3 define the use of wires or pins 1, 2, 3, and 6. So, to use a Category 5 cable with an RJ-45 connector for an Ethernet connection, Ethernet and RJ-45 physical layer specifications are used.
• Ethernet protocol; IBM Tokens Ring Protocol; AppleTalk
• Fiber Distributed Data Interface (FDDI)

The physical layer is the first OSI layer of the OSI model. However, it is typically listed at the bottom to emphasize how messages are transported through the network. See also computer networking cable.

In WAN services connections, frame relay and PPP protocol operate both at Physical and Data link layer. But ISDN network operates at Physical and Data link layer as well as network layer of the OSI model.

How can you remember each layer of the OSI model, you can use this mnemonic to remember each OSI layer.

All People Seems To Need Data Processing

All – Application layer

People – Presentation layer

Seems – Session layer

To – Transport layer

Need – Network Layer

Data – data link layer

Processing – Physical layer

Protocol Implementation

The OSI model is only a theoretical way of looking at the network communication. Each layer specifies standards to follow when implementing a network. However, OSI model layers do not actually perform any real tasks.

As a network administrator you should be familiar with the OSI model, because it is the most widely used method for talking about network communication. The advantages of using OSI model to discuss networking concepts are as follows:

1. Provides a common language or reference point between network professionals.
2. Divides networking tasks into logical layers for easier comprehension.
3. Allows specialization of features at different levels
4. Aids in network troubleshooting
5. Promotes standards interoperability between networks and devices
6. Provides modularity in networking features (developers can change features without changing the entire approach).

However, there are limitations of the OSI model:

1. OSI model layers are theoretical and do not actually perform real functions
2. Industry implementations rarely have a layer to layer correspondence with the OSI model layers.
3. Different protocol within the stack perform different function that help send or receive the overall message
4. A particular protocol implementation may not represent every OSI model layer (or may spread across multiple layers).

In practice, network communication tasks are performed by protocol implementations. Protocols are the actual vendor-specific or industry-standard software processes that perform necessary network communication tasks.

Most vendors and industry standard implementations use a layered approach. Instead of using a single protocol, they create a collection of protocols – a protocol suite or protocol stack – to perform all necessary communication tasks.

Protocols within a suite protocol have the following characteristics:

1. Each protocol performs one or more network communication tasks
2. Protocols can perform tasks in different OSI model layers
3. Multiple protocols in the same suite can often perform the same task
4. Some protocol suites allow a choice of a specific protocol within the suite to perform a specific task or enable a specific feature
5. Protocols must work together receiving and sending data to other protocols

There are 3 categories protocols divided according to the general functions they perform.

• Services
• Data transportation
• Physical connections

The following network protocol will describe more detail information about how these categories correspond to the OSI model.

Application-level protocols work at the upper layers of the OSI model, Application; Presentation; and Session. They provide data exchange and application-to-application communication.

Transport-level protocols (Transport and Network layers) establish communication sessions between computers, ensure that data is transmitted reliably, and provide routing between networks.

Physical-level protocols correspond to the lower layers of the OSI model (Data link, and Physical). They address information, perform error-checking and send transmission requests.

Many protocols exist at more than one protocol level, so protocol may not to match up exactly with networking models. This is because some protocols are designed to fulfill particular communication tasks, which do not always correspond to a model.

Device communication

Network devices can communicate with each other as long as they are using the same protocol stack, even though they use different operating systems. Data sent from one device travels down the protocol stack, across the transmission media and then up the protocol stack on the other device through the following general process.

1. The message is broken into packets
2. Each protocol in the stack adds control information to the packet (each packet contain header; Data; and Trailers), enabling specific protocol features such as encryption and error checking. The header and trailer information added at each layer is meant to be read by the layer at the same level in the receiving computers.
3. At the physical layer, the packets are converted to the correct electrical format for transmission.
4. The corresponding protocols at each layer in the receiving device remove the added headers and trailers. The packets are then resembled into the original messages.

Data encapsulation process

Encapsulation is the process of breaking a message into packets, adding control and other information and transmitting the message through the transmission media with the following five steps processing.

1. Upper layers prepare the data to be sent through the network
2. The transport layer breaks the data into pieces called packets, adding sequencing and control information.
3. The network layer converts the segments into packets, adding logical network and devices addresses.
4. The data link layer converts the packets into frames, adding physical device addressing information.
5. The physical layer converts the frames into bits for transmission across the transmission media.

To remember the sequence process easily, use this:

1. Upper layer – DATA
2. Transport layer – SEGMENTS
3. Network layer – PACKETS with logical addresses.
4. Data link layer – FRAMING with physical address (MAC address)
5. Physical layer – BITS

Understanding OSI model will provide a common language or reference point between network professionals. Even for antivirus and internet security software developers basically work based on OSI model, directly or indirectly.

See also:

• Wide area network technologies
• LAN topologies
• Management of information security