ICND1 break down — An OSI Model Review

Posted on July 15, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |


Computers in the network are manufactured by different vendors and use different types of programs to operate. If they want to communicate with each other, they must follow a common set of rules for data communications. The rules that define how systems talk to one another are called protocols.

The Open Systems Interconnection (OSI) reference model, released in 1984, is a framework for building protocols and to help people understand the process around network communications and communications standard in itself. The main goal of OSI reference model is to allow different vendors’ networks to interoperate.  Some benefits of the OSI reference model are:

  • It breaks network communication into smaller, simpler parts, which also make learning easier.
  • changes at a certain layer do no affect the other layers.
  • various types of hardware & software can communicate with each other.
  • networks are more extensible.

The OSI reference model have seven layers, as shown below.

Layer 7 — The Application Layer

This is the layer where the end users themselves interact with the network. The application layer provide services to applications outside the OSI reference model and establishes agreement on procedures for error recovery and control of data integrity.

Notice Authentication runs at Application layer, whereas Encryption runs at Presentation layer.

Protocols and services that run at L7 include:

  • Email protocols SMTP and POP3
  • Telnet
  • HTTP
  • File Transfer Protocol (FTP)
  • Simple Network Management Protocol (SNMP)

Layer 6 — The Presentation Layer

This layer answers one simple question: “How should this data be presented?” In addition to properly formatting data, encryption occurs at this layer.

Layer 5 — The Session Layer

Layer 5 is the “manager” of the two-way communication between two remote hosts. This is the layer that handles the creation, maintenance, and tear-down of sessions between the two communicating hosts. The session layer also responsible for session regulation, efficient data transfer, class of service, and exception reporting of session layer, presentation layer and application layer problems.

Whereas the upper three layers — application, presentation and session layers are concerned with application issues, the lower four layers — transport, network, data link and physical layers are concerned with data-transport issues. Whereas the application protocols are running in the upper three layers, the data-flow protocols are running in the lower four layers.

Layer 4 — The Transport Layer

The transport layer segments data from the sending host and reassembles the data into a data stream on the receiving host. The transport layer provide data-transport service that shields the upper layers from transport implementation details. Besides establish, maintain, and terminate virtual circuits, the transport layer also optionally provide services such as transport error detection and recovery and information flow control. The Transport Layer also responsible for session multiplexing. Session multiplexing is an activity by which a single computer, with a single IP address, is able to have multiple sessions occur simultaneously. A session is created when a source machine needs to send data to a destination machine.

TCP and UDP both run at the Transport layer, and we’ve got to know both of those protocols inside and out to pass the CCNA and CCENT exams.

Layer 3 — The Network layer

It’s at Layer 3 of the OSI model that you and I as network admins begin to have a great deal of interaction with the network. Internet Protocol (IP) runs at this layer, and since routers operate here at L3, this layer is often called “the routing layer”.

In a nutshell, routing is a two-question process:

  • What valid paths exist from the local router to a given destination?
  • What is the best path to take to get there?

Layer 2 — The Data Link Layer

The data link layer defines how data is formatted for transmission and how access to the network is controlled. This layer defines how devices on a common media communicate with each other, including addressing and control signaling between devices.

The IEEE refined the standards and divided the Data Link layer into two sublayers: the LLC and the MAC sub layer.
- LLC sublayer

LLC is short for Logical Link Control. The Logical Link Control is the upper sublayer of the Data Link layer. LLC masks the underlying network technology by hiding their differences hence providing a single interface to the network layer. The LLC sublayer uses Source Service Access Points (SSAPs) and Destination Service Access Points (DSAPs) to help the lower layers communicate to the Network layer protocols acting as an intermediate between the different network protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.) This layer is also responsible for frames sequencing and acknowledgements.
The LLC sublayer is defined in the IEEE standard 802.2.

- MAC sublayer

The Media Access Control layer takes care of physical addressing and allows upper layers access to the physical media, handles frame addressing, error checking. This layer controls and communicates directly with the physical network media through the network interface card. It converts the frames into bits to pass them on to the Physical layer who puts them on the wire (and vice versa)

IEEE LAN standards such as 802.3, 802.4, 802.5 and 802.10 define standards for the MAC sublayer as well as the Physical layer.

Other standards on this layer include: X.25 and Frame Relay

Switches operate at Layer 2.

  • High Data Link Control (HDLC)
  • Point-to-Point Protocol (PPP)
  • Frame Relay

Frame Check Sequence (FCS) is used for error detection at this layer.

Layer 1 — The physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

When things get a little complicated in networking, I like to remind myself that “it’s all ones and zeroes!” Whatever data our end users are creating, it’s going to eventually be “translated” into a series of 1s and 0s.

Once that is done, it’s the Physical layer that handles the actual data transmission. Anything to do with a physical cable – the pins, the connectors, the electrical current itself – is running at the Physical layer.

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Note:  The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP) [RFC826], specifically IPv4, to map IP network addresses to the hardware addresses used by a data link protocol. The protocol operates below the network layer as a part of the interface between the OSI network and OSI link layer.

For detail see this OSI Model tutorial.

ICND1 and ICND2 break down

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