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Assignment states about the Open System Interconnection (OSI) model, Data Link Layer is it referred to “Layer 2”

Home, - The Data Link Layer

In the Open System Interconnection (OSI) model, Data Link Layer is it referred to “Layer 2”, and is responsible for the transmission of data across the physical networking system. The Data Link Layer(DLL) is functionally categorisedinto two layers, viz. (i) LLC(Logical Link Control) and (ii)MAC(media Access Control). The LLCS(LLC sublayer) is involved in managing communication between the devices connected within the single layer of network. Likewise, MAC sublayer links the Physical Network Medium with the protocol access (Baldi & Tomasin, 2016). It is noteworthy that LLC enables multiple-layer (ML) protocol to share a single physical link and it thus support fields in the link layer frame. MAC on the other hand addresses all the ports on a device, however, they can be uniquely identified with respect to each other at the DLL. An overview of the DLL (Data Link Layer)function 

 Schematic representation of the Data Link Layer functions. 

The link-layer protocol characteristics that are present in the physical medium and functionally managed by Layer 2 are (Bockelmann, 2016):

• Physical addressing - Identified devices at the LL (link-layer) and differentiate them on the basis of the physical medium. This physical addressing is also known as MAC address. 

• Network topology - Identifies how the devices are linked in a network. 

• Error notification - The notification alert higher layer protocol about any error arising on the physical link. Example include loss of signal, error in serial connection, loss of endpoint on T1 or T3 link. 

• Frame sequencing - This enables the re-ordering of the frames that are transmitted out of sequence, on the receiving end. 

• Flow control -  allows the receiving device to identify congestion and notify to the corresponding upstream or downstream neighbours.

An overview of the transfer of frames in Data Link Layer is shown in Figure 2. The Practical functionality of LLC and MAC sublayers, in need of business include the points mentioned in the next section. 

LLC sublayer -  Involves in error control, such as automatic repeat request, forward error correction, error detection and cancelling of erroneous packets with reference to all the layers within the network layer. The error control feature is provided in wireless network and telephone network modems (V.42). Another functionality is related to the flow control which is also used in modems and wireless network, but cannot be used in LAN protocol (Zou, Zhu, Wang, & Leung, 2015). 

MAC sublayer - Used for channel-access control such as CSMA/CA protocol in wireless network and Ethernet in hub or bus networks. Also used for physical addressing (MAC addressing), LAN packet switching, featuring, STP(spanning tree protocol) and SPB(shortest path bridging). Other functionalities include, data packet scheduling, queuing, quality of service (QoS) control, virtual LANs (VLAN) and Store and forwarding switching (Zou, Zhu, Wang, & Leung, 2015). 

The Transport Layer (Layer 4)

The Transport Layer serves a means of transferring data between the end users, which is transparent and reliable service to the upper layers of the network. The control of the reliability of any given link is maintained in this layer 4 by flow control system, segmentation and de-segmentation process, and error control. Typical examples of layer 4 are the i) UDP(User Datagram Protocol) and ii) TCP(Transmission Control Protocol). UDP is the simple layer protocol whichdoes not provide reliability, however, TCP is reliable which ensures that the data were received with full-features to the end user (Lennox & Holmberg, 2017). A simple linking of operations with respect to application, internet, and network access 

Both the UDP and TCP are valid transport protocols, except that UDP does not include acknowledgement for the receipt of the data. TCP involved three basic operations (Rescorla,2018):

- Tracking of the transmitted data segments. 

- Acknowledgement for the received data.

- Re-transmittance of any unacknowledged data. 

UDP on the other hand, is simple and does not include network overhead. Thus they can be used in any application that can tolerate any loss of data during transmission over the network. The preferable application includes streaming of:

(i) Video; 

(ii) Audio; 

(iii) Internet radio;

(iv) (iv) Voice over the IP (VoIP). 

While with UDP processing, the loss of data can lead to momentary loss of image or audio, which might not be noticeable by end user. 

On the other hand, TCP transmission may lead to pause and disruption which will be noticeable by the user.  Common example of TCP includes two protocol, which are commonly used in all business operations:

- FTP(File Transfer Protocol) 

- HTTP(Hypertext Transfer Protocol), 

The transmission of the data from the application to the various layers for transmission on to the medium, are built on the units called as Protocol Data Units (PDUs). Furthermore, the process is then reversed at the destination host, unit the data will be passed from layers to the destined application (Karagiannis, 2015). The limit of the data pieces (traffic) to be transmitted are mainly dependent on the networking technology and the physical medium that are used. Diving the application data into segments (PDUs) ensure that the limit of data transmitted with reference to the data from various application to the final media. 

The TCP segment headers mainly contain sequence numbers which are allowed in accordance to the transport layer function. These sequence will be reassembling the segments for the transmission and ensure that all the data reaches the destination application in exact form as initiated from sender (Karagiannis, 2015). Within the scope of UDP headers, there is no sequence number and thus are not concerned with the order in which data is transmitted. However, UDP generates less overhead compared to TCP and thus results into faster transfer of data. An overview of transport layer function is shown in Figure 5. 

Cloud and Mobile networks

This is a combination of cloud computing and mobile computing in conjunction to the wireless networks which ultimately results into opportunity of rich resources to all the operators and service providers related to cloud computing, mobile users, and network providers (Jo, Maksymyuk, Strykhalyuk & Cho, 2015).

Cloud computing architecture mainly have two components connected loosely via network – Internet, as shown in Figure 6:

(i) Front end

(ii) Back end

Front end is the client part having interface and application module, for example the web browser. Back end is the cloud itself, comprising of virtual machines, servers, deployment model, and storage memory. Notably, the back-end have the responsibility of maintain built-in security, protocols, and traffic control. 

Mobile computing network comprises of three-tier architecture (Han, Liang & Zhang, 2015):

(i) Presentation tier

(ii) Application tier 

(iii) Data tier

The integration of mobile as well as thecloud computing via networking mechanism can be understood

The benefits of such networking include:

(i) Speed and flexibility – Application can be built or revised quickly. Can be delivered to different device using varying operating system. 

(ii) Shared resources – Mobile app are not constrained with processing resource and device storage, due to cloud. Suitable to run data-intensive processes. 

(iii) Integrated data – Enable user to collect, secure, and integrate data from various sources. 

(iv) Dynamic and reliable provisioning – Users can run application without prior reservation of resource. Comprehensive data security model reduced chances of data and application lost. 

In context to business, traditional enterprises use application suitable for Windows PC, Apple iOS, and Google Android. However, using mobile cloud computing architecture, it enables the devices to access any information without the processing. This is facilitated with architecture including, Enterprise File Sync and Share (EFSS), Mobile Backend as a Service (MBaaS), and application virtualization (Chen, Zhang, Hu, Taleb,&Sheng, 2015). 

The accommodation of varieties of vendors at the end-user computing market provide the opportunity to access any type of information from any device at any location. This promise of data integration and smooth application plays a significant role in the current globalization. 

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