Data Communication
This paper presents the limitations of two different reference models; OSI Referencing Model and the TCP Referencing Model. It also discusses the principles of a modem and its usage. Lastly, this paper includes the two types of T-carriers; the T1 and T3. A comparison between these two carriers is provided for at the last section.
Introduction
Computer usage is a pervasive phenomenon in the society. Various advancements in technology have been developed to pave the way for easier access and integration of information regardless of the geographic distance. The importance of sharing information resources is undeniable and thus distributed systems are designed to meet this need. However, .such systems remain a complex undertaking because of the universal unavailability of communication facilities that will connect these distributed systems. The sharing of information will become easier with the presence of standardized techniques that will be utilized by both vendors and the communication providers.
The OSI model has indeed provided a framework in terms of developing the protocol application in the worldwide scale. The distributed applications thus simplified under the seven layered model. The sender is able to present the data in the exact manner he wanted. However, it merely serves as a framework and the full functionality is still dependent on the complimenting networks. As such it can be asserted that this model does not set a standard per se. Also it is designed to promote interconnection systems. Furthermore, Interoperability is not guaranteed by the OSI compliance.
Protocols such as the TCP/IP are important layers in the network level and are part of the OSI as well. The layers represented are responsible for the addressing of and routing of data as well as their transportation to other software. However, unreliable messages are likely to be encountered in the establishment of links. Thus reliability to the information entails potential delays. More so, there are limits on the ability to serve as infrastructure to other services.
Furthermore, the computers are linked to the network based on the telecommunication systems. Modems for this part, serves the purpose of receiving data and demodulating them while restoring the original data. They are used plugged in telephone line that enables the exchange of data among computers and other machines. For this part, the T-carriers play a significant role. The T1 and the T3 are line technologies that serve as carriers for internet providers. The T1 line with an original transmission rate of 1.54 Mbps is widely used by internet service providers for internet connections. A more speedy line, the T3 provides a rate of 44.736 Mbps.
Open Systems Interconnection (OSI) Model
Incompatibilities occurring between different computers that attempt to intercommunicate paved the way for the Open Systems Interconnection Model (OSI). Under this seven layer model, the information from the sender is presented to a recipient in the exact manner the sender intends. Control data accompany the source data of the user that are put automatically so that particular functions will deal with the number of managing devices that will be encountered. The segment of data addressed to it is read by each device taking the correct action to route the message unchanged. Lastly, the message is correctly reproduced through the interpretation of the instruction addressed to it by the terminating device ( & , 2003, ).
However, OSI does not serve as the solution to any specific communication problem. It is a philosophy of information interchange among systems. The two characteristics of the model attributes to the significant problems. First is that the OSI is a conceptual and functional framework for standards and not a standard per se. Another is that it is designed for the promotion of successful interconnection systems which does not impose implementation (, 1991). Networking Technologies does not require the strict adherence of the OSI. It is serves only as a reference and thus many networking technologies implement subset of the layers and provide a limit on the functionality of the network (, 2006, ). Full functionality is then relied on the complement of other networking technologies.
Protocol specification is not part of the OSI reference model. The framework provided by the OSI entails the definition of protocols for each of the seven layers by communication experts. With this, a number of protocols are likely to occur in each layer and options within each protocol. These options provide advanced capabilities but applications for this entails a high price on overhead. Thus homogenous stack of capability is not produced by the OSI as a result of all the protocol and protocol options. The number of protocols implies the inability to preselect protocol elements that will meet the requirements for communication.
Finally, the problem with the OSI is the caution which is required of in terms of connectivity. Systems may mutually agree on protocols on each of the seven layers but this does not guarantee the compatibility. This incompatibility is beyond the realm of OSI. Moreover, connectivity and compatibility does not imply a complete interoperability. Even with the standard based approach brought about by the OSI, vendors are likely to implement additional whistles to enhance their offering (, 1991). Interoperability is not guaranteed for OSI-compliant implementations and it does not make such an easier and economical thing to be achieved (, 2006, ).
Transmission Control Protocol (TCP) Model
The Transmission Control Protocol (TCP)/Internet Protocol (IP) consist of different protocols in which two are considered the most important. The IP is the network level of the OSI model which provides addressing, routing and other function in the network. TCP on the other hand is the transport layer which connects establishment and managements and thus responsible for the transportation of data between software processes. These two protocols are important because of the critical function in the layers 3 and 4. The works of different protocols and technologies though are required of in order to create a functional network that will provide the users with their needed applications (, 2005, ).
Layers 3 and 4 of the OSI model are operated by the core protocols of TCP/IP. As such, it corresponds to the internet layer as well as the host-host transport layer. However, protocols operating these layers are not defined by the TCP/IP especially in most network implementations. Thus, the functionality of layer 2 is assumed to be provided by a wide area network (WAN) or local area network (LAN) by the TCP/IP. Such technologies are responsible for the functions of layer 2 which involves addressing the physical layer, control of media access and most importantly the framing of the datagram by layer 2 received from layer 3. The problem remains on the assumption that the internet protocol is capable of running on top of the layer 2 because there is no guarantee that one exists (, 2005, ).
Packets of data are routed between computers through unreliable low service or the IP. TCP on the other hand, is a higher level that works on the computer wanting to make a connection with another and thus contacts it. They exchange few unreliable IP messages in the establishment of the link. Upon the creation of the link, messages are tagged by the sending computer with sequence data and the receiving computer sends an acknowledgement. The acknowledgement if not received prompts the resending of the data. The receiving computer is unable to send a message back with the TCP. Thus, it has to wait for the resending computer and it cannot process any of the later data until it has been resend ( & , 2003, ). As such, potential delays are the price for reliability.
Moreover, existing TCP/IP protocols do not guarantee quality of service that sets limit on the ability to serve as an infrastructure for other services such as video sharing between homes. The update to TCP falls short of the quality of service that is guaranteed ( & , 2003, )
Modulator-Demodulator/Modem
Fundamentally, a modem is a device capable of modulating an analog carrier signal in order to encode digital information. Such carrier signals are likewise demodulated in order for the transmitted information to be decoded. Simply put, the goal of the modem is the production of a signal that is easily transmittable and decoded for the original digital data to be reproduced. . The transmission of analog signals from diodes to radio can be done by means of modems. A voiceband modem is a familiar example of this. By this, the digital ‘1s and 0s of a personal computer are turned into sounds that are transmittable on Plain Old Telephone Systems (POTS). These sounds are converted into 1s and 0s once they are received on the pother line. Generally, the amount of data send on a given time, which is measurable in bits per second or “bps”, is what classifies a modem (, 2007).Internet users utilized faster kinds of modems, notable among them are the cable modems.
The speed of telecommunication systems linking the computers to a network is a limiting factor of network operation. Digital data are translated into analogue form by a Modem and then into digital form once it is received. It works by enabling the modulation of digital data with the analogue signals that are carried by the telephone system. The modem of the remote computer receives the signals and demodulates them while restoring the original data ( & , 2003, ).
Modems are used in most computers plugged into telephone line that enables the users to exchange data with other machines. The developments in modems have been towards the compression systems, sophisticated schemes for modulation, methods of correction and other functions ( & , 2003, )
T1 and T3 Carrier/Leased Lines
The T-Carrier System is a system that supports the digitized voice transmission. The T1 line with an original transmission rate of 1.54 Mbps is widely used by internet service providers for internet connections. A more speedy line, the T3 which provides 44.736 Mbps is also a common carrier for internet service providers. These leased line technologies are predominant in the North America and Japan. By leased line circuit, we mean the reservation for use by the renting enterprise paid on a monthly rate regardless of how much it uses.
T1 is considered to be a commonly used digital line technology that utilizes a telecommunications technology known as the time-division multiplexing (TDM). Four twisted lines are used by the T1 lines containing 24 channels that are capable of transferring data at the speed of 64, 000 bits per second (64 Kbps). The speed of the information transfer can be increased by combining these channels ( & , 2000).With this data is yielded with a rate of 1.5 Mbps. The stream of data is combined by the TDM with the assignment of each stream in different slot in a set and repeated transmission of fixed sequence of these time slots in a single channel of transmission (, 2004; , 2004, ).
An integrated T1 or channelized T1 is a line in which different applications is served by each channel. Fractional T1 on the other hand is a service installed in which some portion of the 24 channels is rented in a T1 line. Voice and other analog signals are sampled in the T1 system for at least 8,000 times a second and are digitized individually into 8-bit word. A 192-bit frame is being transmitted 8000 times per second with the 24 channels being digitized. A single bit separates the each frame to the next which makes it a 193-bit block. Hence, the 1.544 Mbps rate of T1 is made up by the 192 bit frame multiplied by 8,000 plus the framing bits of 8,000( & , 2005).
On the other hand, T3 circuits are the successor of T1. They are the fastest of T-carriers line and often used for internet access. It has 672 channels and the maximum rate for data transfer is 44, 736,000 bits per second or (44.736 Mbps) ( & , 2000). They are mostly used by ISP’s that connect smaller ISP’s to the internet as well as large enterprises. The sheer expense associated with it result to the mere leasing of T3 lines as fractional T3 lines.
Conclusion:
The advent of internet has open doors for an easier access and integration of information among networks. Evidently, the importance of being able to share information regardless of geographic distance is an increasing need in the modern society. For this reason, distribution systems are developed to cater to this demand. A standardized technique is deemed relevant to achieve this goal. However, the complexity of this undertaking makes it less likely to prosper because of the unavailability of universal facilities to connect such distribution systems.
This for one is the limitation of the OSI Reference model. As such, it doesn’t serve as a standard but merely a framework of promoting interconnection systems. The entire functionality remains dependent on the protocols that will be complimented by the communication experts. However, these protocols such as the TCP/IP have also their limits in providing infrastructure for other services. Moreover, these protocols are likely to transmit unreliable messages.
On the other hand, the link of computers and network are facilitated by the modems that receive and demodulates the data. The speed of the connection is influenced by the carrie4rs such as the T1 and T3. T1 is a common technology line used by internet providers as well as T3. They differ in terms of the rate they deliver.
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