Due tomorrow night Computer science

1. OSI Protocol Model

The open system interconnection (OSI) model is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers.

An open system is a set of protocols that allow any two different systems to communicate regardless of their underlying structure. The purpose of OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.

The OSI model defines a networking framework to implement protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

· Physical (Layer1) This layer conveys the bit stream electrical impulse, light or radio signal through the network at the electrical and mechanical level.

· Data Link (Layer2) Provides error free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume error free transmission over the link.

· Network (Layer3) controls the operation of the subnet, deciding which physical path the data should take based on network conditions priority of service, and other factors.

· Transport (Layer 4) insures that the messages are delivered error free, in sequence, and with no losses or duplications.

· Session (Layer 5) this layer allows session establishment between processes running on different stations.

· Presentation (Layer 6) formats the data to be presented to the application layer. It can be viewed as the translator for the network.

· Application (Layer 7) this layer serves as the window for users and application processes to access network services.

2. Circuit and Packet Switching

A. Circuit Switching

When using circuit switching, the path of data is decided before the transmission starts. The system decides the route of travel, based on a complicated algorithm, and transmits according to the path made. During the whole length of the communication session the two nodes communicating is dedicated, exclusive and is only disassembled when the session terminates. Circuit switching is also old, expensive, but is still in use today with PSTN, while packet switching is more modern. However, circuit switching is more reliable than packet-switching. When you have a dedicated connection then the information will get across better and more secure.

There are two types of circuit switched networks. The first circuit switched network type is based on leased lines. The second type is based on the Integrated Services Digital Network (ISDN). Leased line network types can be either point-to-point connections or multipoint leased-line connections. A benefit of leased-line networks is that the connection is not shared with anyone else. If you’re paying for a 10Mbps pipe, you’re getting all 10Mbps. A disadvantage to leased-lines is that they are expensive. A leased-line circuit is calculated by miles. Therefore, the further the circuit needs to go, the more costly it will be.

B. Packet Switching

During the packet switching method, the packets have to find their own route towards the destination. There is no “set” or “predetermined” path. After reaching a node it will then decide on the next path to take towards the next node. By doing this, the packet finds its way towards the destination by the information it carries, such as the source and destination IP address. Packet switching is more modern in today’s world however it does have some major drawbacks. While a packet is looking for the destined route, the packet could come across a congested network which would higher the latency or you could even lose the packet. Since packet switching uses other protocols the connections can be more reliable.

There are two main types of packet switched networks. Connection oriented networks use the X.25, Frame relay, and ATM protocols. Connectionless networks revolve mainly around the Internet Protocol (IP). Early IP switches were unable to handle high traffic requirements. However, equipment such as SONET, has provided technicians the ability to apply Quality of Service (QoS) control on their circuits. QoS allows technicians the ability to control which types of data have priority over the other data types. Each protocol is described in detail below.

C. X.25

X.25 is a protocol that was developed to provide accurate and reliable data communications on a public network. X.25 uses various methods, two of these methods are packet switching and virtual circuits to obtain a data rate up to 64kbps. X.25 also provides multiple error checking features which makes an excellent choice for the older networks. This protocol is widely accepted throughout the world because of its extensive error checking capabilities. Since packet switching does not use a dedicated virtual circuit and it’s actually connectionless in nature, X.25 establishes these connections. The connection is established, the data is transferred, and when it’s all said and over with, the connection is terminated.

D. Frame Relay

The frame relay technology is based on the X.25 packet switching technology. However, there is no error checking with frame relay because it uses a fast packet switching technology. Today’s digital and optical networks generally transfer data with a very low error rate. The end station or node actually handles all the errors that were generated. Frame relay does reduce the amount of overhead associated with the data transmission, this allows for faster packet-switching. The bandwidth obtained from frame relay is called Committed Information Rate which uses a T1 line. Since frame relays can handle data bursts, the customer might actually use more than the Committed Information Rate if any is available. Since customers take advantage of this the phone companies have consistently increased the customer’s usage rate.

E. Asynchronous Transfer Mode

Asynchronous Transfer Mode takes cells and cuts them into blocks of information. The cells are a fixed variable and only contain 5 bytes of header data and 48 bytes of information. This field carries the data used and the contents inside the header which allows Asynchronous Transfer Mode to work. Also, the only way Asynchronous Transfer Mode can work is with a very high transfer speed, fiber optics is preferred. The hardware which is performing all the cell switching is what makes this method a faster data transfer technology. The software determines the length of the packets which means switching needs to be done. Asynchronous Transfer Mode is faster than any existing packet switching network and frame relay.

F. Transfer Control Protocol/ Internet Protocol

TCP/IP (Transfer Control Protocol / Internet Protocol) offer 3 layers of service, application services, transfer services, and connectionless packet delivery service. IP layering allows you to replace the service without affecting anyone else. However packets can be lost, duplicated, or out of order without any notification. IP software does provide the routing of the packets. TCP explains the subdivisions of the packet that is being transmitted to another node, this includes ftp and email. The TCP segment structure accepts data from a data stream, breaks it up into chunks, and adds a TCP header. This is when the packet of information that TCP uses to exchange data with its peers. The data section is followed by the header. The contents of the data section are the payload data carried from the originating node. Inside this TCP segment is also the instructions on how to get to the other IP address.

3. Circuit and Packet Protocols

One of the major circuit switch protocols is signaling System no. 7 (SS7) or most frequently known in North America as Common Channel signaling System 7 (CCSS7). SS7 divides essential information to arrange and manage telephone calls in the Public Switched Telephone Network (PSTN) on a separate packet switched network instead of using the same circuit switched network where the telephone calls are made. The technique is also called out-of-band signaling and is different from in-band techniques. Furthermore, in circuit switching it accommodates two types of transmissions datagram and data-stream transmissions, which are used extensively in the telephone company networks, circuit switching operates much like a normal telephone call. Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

References

Duncan, P. (2004). Pablotron: News. Retrieved May 11, 2015, from http://pablotron.org

The Fiber Optic Association, Inc. (2015). The fiber optic association. Retrieved May 11, 2015, from http://www.thefoa.org