Network Design

April 30, 2016
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Application and Systems Architecture

Networking at a Global Scale

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OSI Network Model

The Open Systems Interconnection model (OSI model) network architecure is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers.

A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it. On each layer, an instance provides services to the instances at the layer above and requests service from the layer below.

Physical Layer

The Physical Layer within network architecture defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more. Its main task is the transmission of a stream of bits over a communication channel.

Data Link Layer

The Data Link Layer within network architecture provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802.

IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding-window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages. Simply, its main job is to create and recognize the frame boundary. This can be done by attaching special bit patterns to the beginning and the end of the frame. The input data is broken up into frames.

Network Layer

The Network Layer within network architecture provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the Transport Layer (in contrast to the data link layer which connects hosts within the same network). The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors.

Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical. It controls the operation of the subnet and determine the routing strategies between IMP and insures that all the packs are correctly received at the destination in the proper order.

Transport Layer

The Transport Layer within network architecture provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and re-transmit those that fail. The Transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.

Some Transport Layer protocols, for example TCP, but not UDP, support virtual circuits providing connection oriented communication over an underlying packet oriented datagram network .Where it assures the delivery of packets in the order in which they were sent and assure that they are free of errors. The datagram transport delivers the packets randomly and broadcasts it to multiple nodes. Notes: The transport layer multiplexes several streams on to 1 physical channel.The transport headers tells which message belongs to which connection.

Session Layer

This Layer provides a user interface to the network where the user negotiates to establish a connection. The user must provide the remote address to be contacted. The operation of setting up a session between two processes is called “Binding”. In some protocols it is merged with the transport layer. Its main work is to transfer data from the other application to this application so this application is mainly used for transferred layer.

Presentation Layer

The Presentation Layer within network architecture establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.

This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network.

It is sometimes called the syntax layer. The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Application Layer

The Application Layer within network architecture is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model.

Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.