In Part 4 and Part 5 of the Internetworking series we brushed on the TCP/IP, DoD, and OSI Models that are used within internetworking communications, before going any further it would be wise to touch base on these subjects again and then carry on with the TCP/IP in more depth.
Internetworking Part 4
Now is a good time to introduce the networking reference models that permit the communications within our internetworking up through the previous sessions (Part 3).
In the beginning, most computers were only able to communicate with other computers from the same manufacturer. In the 1970s the Open Systems Interconnection (OSI) reference model was created to overcome these communications problems. There are other models in use such as the DoD Reference and the Cisco Hierarchical Models, which we will discuss.
First, the OSI Model. This is a reference model, or set of guidelines, that application developers can use in the creation and implementation of applications that run on a network, which provides a framework within which network standards can be managed.
The OSI model has 7 distinct layers, which are divided in to two groups. The upper group (top 3 layers) define how the end-to-end host applications will communicate with each other. The bottom group (bottom 4 layers) define how the data is to be handles and transmitted between the hosts, end-to-end. The top group are the Application, Presentation, and Session layers; the bottom group The following operate at all seven layers of the OSI model: Network management stations (NMSs); web and application servers; gateways (not default gateways); and network hosts.
The upper layers: Application layer, Presentation layer, and the Session layer furnishes a user interface, “presents” data to the application layer, and maintains data separation between different applications.
The Application Layer: This is where you (the user) has a direct connection to the computer by inputting data, or making requests. This layer is also responsible for resolving the availability of communication and sufficiency of resources for data input. The protocols associated with this layer are HTTP, FTP, and SMTP.
The Presentation Layer: As mentioned before, this layer “presents” the data to the Application layer, which is where its name originates. It is also in control of the data translation, code formatting and conversion functions (i.e., receives generically formatted data and converst it to its original format). The protocols associated with this layer are ASCII, EBCDIC, JPEG, GIF, and MPEG.
The Session Layer: This layer’s operation is to create, organize, and disassemble between Presentation layer components. In essence, this layer can open many “seesions” and will keep all of those “sessions” and their respective data separate.
Internetworking Part 5
The lower layers, or the Transport Set, are for the transportation of the segments, packets, frames, and bits.
Transport Layer (Layer 4) provides for reliable or unreliable delivery and performs error correction before retransmit. This layer segments and reassembles data into data stream by providing end-to-end data transport service which creates a logical connection between the sending and destination hosts.
Network Layer (Layer 3) provides for logical addressing, which the routers use for path determination. This layer manages device addressing, tracks the location of devices on the internetwork, and determines the best path available.
Data Link Layer (Layer 2) combines packets into bytes and bytes into frames, provides access to media using MAC address, performs error detection – not correction. This layer provides for the transmission of data and handles error notification, topology of the network, and flow control.
Physical Layer (Layer 1) moves the bits between devices, specifies wire speed, voltage, and the pin-out of cable. Sends and receives bits, some use tones, and others can use variations of voltage or signals
Data integrity is maintained through flow control whose purpose is to govern the amount of data sent by the sender.
Connection-Oriented Communication is where the transmitting device first creates a session with its peer system through a call setup, or three-way handshake. The three-way handshake is a series of synchronization, negotiation, synchronization, acknowledgement, connection, and finally data transfer.
Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.
Odom, W. (2012). Official Cert Guide ICND1. Indianapolis: Cisco Press.