Network Protocols

Protocols are agreed upon standards.  Without protocols computers would be unable to talk to each other.  In networking terms protocols are a lot like languages.  Imagine if this website was in german and you only spoke english.  Without access to a dictionary there would be no chance to understand this website.  When computers talk to each other they are required to speak the same language or have a translator (dictionary).   When working with networking protocols a model is used to define function and role this is known as the OSI (open systems interconnect) model.  The model was first defined in 1977, since a lot has changed since 1977 it can be hard to fit newer protocols into the OSI model.  There are two different versions of the OSI model a 7 layer and a 5 layer.  Since the 7 layer incorperates the 5 layer this article will explain the 7 layer model.  From the top down the layers are: Application, Presentation, Session, Transport, Network, Data Link, and Physical.  Each layer provides a method for communication between it’s adjacent layers.

Layer 7: Application

The application layer directly interfaces the application.  It provides the data in a method that the application expects.  It also sends requests for information to the presentation layer. Examples of Layer 7 are:

  • A web browser
  • A mail client
  • A FTP client

Layer 6: Presentation layer

The presentation layer takes the request or information from either side of it’s adjacent layers and translates them into usable form.  It will break information from the application layer into encapsulated sessions for the session layer.  It will also re-assemble the session layer data into application usage.  Common examples are:

  • SSL
  • TLS

Layer 5: Session Layer

The session layer controls dialogue and connections (sessions) between computers  It handles communication between the local and remote applications.  t provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. Common examples are:

  • NetBIOS

Layer 4: Transport Layer

The transport layer provides transparent controls for link through flow control, segmentation/desegmentation, and error control.   The transport layer also may be responsible for resending lost packets.  Common Transport layer protocols are:

  • TCP
  • UDP
  • IPSec
  • IPX

Layer 3: Network Layer

The Network layer ensures quality of service and addressing for end to end communication.  This is the layer at which most routers operate.  Common examples are:

  • IP
  • RIP
  • ARP
  • ICMP

Layer 2: Data Layer

The data layer provides a method to deal with errors that happen in the phsyical layer it also provides node to node communication unlike the network layer that provides end to end communication.  Data can also be broken up to accomidate the needs of the phsyical layer. Common examples are:

  • Ethernet (802.3)
  • Wireless (802.11 a/b/g)
  • Frame Relay
  • Token Ring

Layer 1: Phsyical Layer

The phsyical layer defines the required electrical and phsyical needs for communication this may include wave modulation, fiber optic cables, cat 5 cable and phone lines.

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