Networking

OSI Models

There are 7 layers in OSI Model. Main advantages of using this layered architecture are, it encourages industry standardization by defining what functions occur at each layer of the model and divides the network communication process into smaller and simpler components thus aiding component development, design and troubleshooting

Troubleshooting the Network

End stations, like clients and servers, usually work with all seven layers. Networking devices are only concerned with the lower layers. Hubs work on Layer 1, switches on Layers 1 and 2, routers on Layers 1, 2

and 3, and firewalls on Layers 1, 2, 3, and 4.

When using the OSI model as a framework for troubleshooting, it is important to understand which functions are performed at each layer, and what network information is available to the devices or software programs performing these functions.  

For example, many processes must occur for email to successfully travel from the client to the server. The
OSI model divides the task of sending and receiving email into smaller, distinct steps that correspond with
the seven layers.

Layer 7 - Application

When a user sends an email message, the alphanumeric characters within the message are converted to data that can travel across the network. Layers 7, 6, and 5 are responsible for ensuring that the message is placed in a format that can be understood by the application running on the destination host. This process is called encoding. The upper layers then send the encoded messages to the lower layers for transport across the network. Transporting the email to the correct server relies on the configuration information provided by the user. Problems that occur at the application layer are often related to errors in the configuration of the user software programs.

Layer 6 & Layer 5 - Presentation & Session

Problems of these layers are most of time lack of codecs of plugins. Ex: Flash players

Layer 4 - Transport

The data that comprises the email message is packaged for network transport at Layer 4. Layer 4 breaks the message down into smaller segments. A header is placed on each segment indicating the TCP or UDP port number that corresponds to the correct application layer application. Functions in the transport layer indicate the type of delivery service. Email utilizes TCP segments, therefore packet delivery is acknowledged by the destination. Layer 4 functions are implemented in software that runs on the source and destination hosts. However, because firewalls often use the TCP and UDP port numbers to filter traffic, problems that occur at Layer 4 can be caused by improperly configured firewall filter lists.

Layer 3 - Network

The email data received from the transport layer is put into a packet that contains a header with the source and destination network IP addresses. Routers use the destination address to direct the packets across the network along the appropriate path. Incorrectly configured IP address information on the source or destination system can cause Layer 3 problems to occur. Because routers also use IP address information, router configuration errors can cause problems at this layer. 

Layer 2 - Data Link

Each network device in the path from the source to the destination, including the sending host, encapsulates the packet into a frame. The frame contains the physical address of the next directly-connected network device on the link. Each device in the chosen network path requires framing so that it can connect to the next device. Switches and network interface cards (NICs) use the information in the frame to deliver the message to the correct destination device. Incorrect NIC drivers, interface cards, and hardware problems with switches can cause Layer 2 problems to occur.

Layer 1 - Physical

The frame is converted into a pattern of 1s and 0s  (bits) for transmission on the medium. A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium can change along the path between the source and destination. For example, the email message can originate on an Ethernet LAN, cross a fiber campus backbone, and cross a serial WAN link until it reaches its destination on another remote Ethernet LAN. Layer 1 problems can be caused by loose or incorrect cables, malfunctioning interface cards, or electrical interference. 

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