OSI, TCP/IP and 5 Layer Model

How devices communicate by being critical building blocks for the understanding of networking. Of the 7 layers of the OSI Model, we will concentrate on Layers 1-4 where Network Engineers reside.

OSI Model

The OSI Model was developed by the ISO (Internet Organisation for Standardisation) to be a standards based model to support multi-vendor interoperability. The OSI model consists of 7 layers with each layer independent of each other. If a manufacturer develops software they need only to ensure it can integrate with the layer directly above and below it. It does not need to know about any other layers.

The Split of Concentration

In the past the top 3 layers (Application, Presentation and Session) where the domain of Developers and the bottom 4 (Transport, Network, Data-link and Physical) were the domain of the Network Engineers. Today these lines are blurred with the advent of SDN and NFV with network engineers having to code for automation tool sets etc.

OSI Layers - "The Top Three"

The top three layers are normally used by the "Developer"

Layer 7: Application Layer

• remote file access

• remote printer access

• resource sharing

• directory services etc. There are many more that use this layer. Some protocols that use this layer are:

  • HTTP(S)

  • FTP

  • DNS

  • SMTP

Layer 6: Presentation Layer

This layer is concerned with the actual format that the data is presented in. This means that the data is readable by the receiving system even-though it could come from different systems such as MacOS, Windows or Linux etc. It ensures that the data is readable and formats it to be presented to the application layer, it structures the data, negotiates the data transfer syntax (using ASCII or uni-code between 2 systems) It also provides encryption

• JPEG

• MPEG

• ASCII

• MP3 etc.

Layer 5: Session Layer

The Session Layer is concerned with " Interhost Communication" by establishing and termination of sessions between applications and this allows two application processes on different machines can establish, use and terminate a connection or a "session". Examples of protocols at this layer are PRC's (Remote Procedure Calls), NFS (Network File System), NetBIOS, PPTP, ZIP etc

OSI Layers - "The Bottom 4"

The bottom 4 layers are important and will go in depth as they pertain to us the Network Engineer

Layer 4: Transport Layer (TCP/UDP Protocols)

The Transport layer ensures end-to-end communication, reliability and flow control. This layer performs "message segmentation" whereby it takes the message from the upper layers (ie the Session Layer) and splits the message into smaller units and passes these smaller units downwards to the network layer for sending The transport layer handles transportation issues between hosts and ensures transport reliability and the transport layer at the "receiver" will reassemble the split units and pass them upwards to the upper layers.

The protocols were mostly interested in at this layer are TCP (IP protocol number 6) and UDP (ip protocol number 17). These protocol numbers mean that when on the receiving computer at Layer3 the network layer examines this number to determine which upper layer protocol should process the data next. So if the protocol number is 6 (TCP) the network layer sends the data to the TCP protocol for further processing

Layer 3: Network Layer

Layer 2: Data Link Layer

Provides node-to-node connectivity and data transfer (for example PC to switch, switch to router, router to router etc) Defines how data is formatted for transmission over a physical medium (eg copper/fibre etc). The DL layer detects and (possibly) corrects physical layer errors and uses L2 addressing (MAC addresses) as opposed to L3 addressing which uses IP addresses (IPv4 and IPv6)

Preable and SFD

Layer 1:Physical Layer

Defines the physical characteristics of the medium used to transfer data between devices. For example voltage levels, maximum transmission distances, physical connectors cabe specifications etc. Digital (1's and 0's) bits are converted into electrical signals (for wired connections) or radio (for wireless connections)