DATA LINK LAYER IN NETWORKING

Final Project Presentation By Jonathan Negash

Table of Content• Executive Summary Slides 4 - 5• Problem Slide 6• Background Slides 7 - 8• Data and Details Slides 9 - 10• Data Framing Slides 11 - 12• Addressing Slide 13• Error Detection and Handling Slide 14 - 15• Solution Slide 16• Conclusion Slide 17• Reference Slide 18

Executive Summary Executive Summary

Executive Summary

• The International Organization for Standardization (ISO) began an effort to standardize computer networking in 1977. Prior to that time, networking was mostly government sponsored or vendor driven by companies such as IBM and Digital Electronics Corporation (DEC). While these standards were being developed, networking itself exploded. Rather than being actual specifications of protocols themselves, as originally intended, the rapid advances convinced ISO instead to create a reference model as a guide for vendors so network products could interoperate. This is the chief benefit of the reference model created.

• Countless articles, papers, and books have been written about the OSI model and its seven layers. It facilitates a greater understanding of computer networking and is a framework for describing and comparing networking technologies. This is what makes it of great interest to industry and academia.

Executive Summary

• This white paper focuses on just one of those layers, the Data Link Layer (DLL). It is this layer than creates links between devices and assures data transport from node to node in a network. Within the paper, TCP/IP, which today defines the Internet is examined within the context of the OSI Data Link Layer. It should be noted that TFP/IP protocols do not map cleanly to the model, but the OSI model continues to serve as a conceptual framework to guide vendors.

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Problem

• In a networked computer system, many parties (nodes or devices) contend for the same resources. When the same resource is requested simultaneously by multiple parties, collisions occur. Also, with multiple connections between devices, assurance of integrity in data transfer, both outbound and inbound become crucial. The Data Link Layer (DLL) acts as a “traffic cop,” arbitrating among the various parties. DLL provides the data transfer across the physical layer. The DLL responds to service request from the network layer (upper layer) and issues service requests to the physical layer (lower layer). The more robust the Data Link layer is, the more reliable data transmission can be.  

Background

•  The Open System Interconnect (OSI) model is a widely-recognized networking model that describes the seven (7) layers of a networked system. The second layer is the data link layer (abbreviated as DLL), a protocol layer. The set of devices connected at the data link layer is what is commonly considered a “network.” DLL performs several vital functions including: establishing and terminating the logical link between two nodes (devices) in a network, performing frame management (e.g., assigning frames, creating frame boundaries, and frame error checking and correction), and managing media rights and access. The data link layer is conceptually divided into two sublayers: the logical link control (LLC) and the media access control (MAC). The split follows the IEEE 802 Project for defining networking technologies.

• The Logical Link Control (LLC) are the functions required for establishing and controlling links between local devices on a computer network. Working at layer 2, these links hide the details of the linking from layers above (3-7) so they can work seamlessly at the higher layers.  

Background

•  The Media Access Control (MAC) are the procedures that allow multiple device to share network media, such as network cables and switches. Since multiple device can request the same media, this sublayer manages the resource to avoid conflicts, acting as the broker (i.e., “traffic cop”).

• In addition to LLC and MAC, the DLL is responsible for three other major functions:

• Data Framing – DLL encapsulates higher-level messages into frames that are sent across layer 1, the physical layer (hardware and wires).

• Addressing – labeling information with a particular destination, using a unique hardware address (MAC address) for each devise to assure that data is routed to and from the appropriate device properly.

• Error Detection and Handling – intercepting errors that occur at layer 1 and determining the proper disposition.

Data and Details

The Basics of Protocols• Protocols are the method by which computers on a network

communicate. Network protocols are sets of rules that govern the exchange of data between computers. Protocols specify how and when computers should talk, and how they should respond when spoken to. They 1) specify how one computer finds another, 2) how to send data back and forth, and 3) how to send formatted messages for specific applications. As examples, a computer may find another computer using an Internet Protocol (IP) address; one computer sends or receives a “packet” of data using Transport Control Protocol (TCP); a web application may use a specially formatted application message protocol such as HTTP.  

Data and Details

• Protocols are stacked. At the lowest level (physical layer) are low-level protocols that transmits over the physical layer, for instance over a wire (e.g., Ethernet). Above this is, at the Data Link Layer (the focus of this paper), are the Internet Layer and the Host-To-Host Transport Layer. The Internet layer includes the source address and destination address of devices that wish to communicate. Above this is the host-to-host transport layer, which sends packets back and forth from one computer to another. Together, these are commonly referred to as TCP/IP and are widely used on the Internet. At levels above these are application protocols.

Data Framing

• Framing is one of the functions of DLL. A stream of bits is framed into blocks of information that can be recognized (meaningful) to the receiving device. Data is broken into “chunks” that can be checked for corruption. If there is an error in transmission (a “glitch”), it can be detected and only the corrupted frames need to be retransmitted, rather than the whole (sometimes large) data stream. The general format for a frame is a frame header followed by a data area. Often, the data frame can be of variable length which makes more efficient use of network transport. Surrounding the frame are header and trailer bytes that allow the sender and receiver to synchronize transmissions

Data Framing

• At the transport layer, TCP encapsulates higher level data, adding a TCP header, and creating a segment. Then, at the network layer, IP encapsulates the segment, adding its own header that creates a datagram. Finally,

• DLL encapsulates the datagram into one or more frames.

• Frame structures are confined to specific network segments. Internetworking may have multiple data link segments. An example of this would be an Ethernet network communicating with a token ring network.

Addressing

• An Internet Protocol address is a unique numerical value that identifies a computer on a network. Public IP addresses are assigned by the Internet Assigned Numbers Authority (IANA), are unique, and are reachable on the Internet. Private addresses are within reserved blocks set aside by the IANA; these are cannot be used on the global Internet, as they are reserved for internal use. These are generally used within organizations and avoid costs associated with Internet Service Providers (ISPs) and for organizational control.

• The common domain names everyone is familiar with are associated with an IP address outside of the private reserved ranges. As an example, it is possible to reach google.com with http://216.58.218.110. As can be seen, the address is in the form ‘xxx.xxx.xxx.xxx’ where each xxx can be a number from 0 to 255. The first part of the address is a unique identifier for the network; the second part is a unique ID of the device in the network.  

Error Detection and Handling

• Frames on a network are susceptible to errors during transmission from one node to another. This can be caused by many factors, such as electromagnetic interference or crosstalk between wires. The data link layer determines if errors have occurred using a Frame Check Sequence (FCS). Each frame has a FSC embedded in it. This is extra error-detecting code added to the frame, and this field is checked at the destination using the same calculation by

• which it was established. If the end-point calculation does not match the FSC, then the frame is discarded and the destination node requests for it to be resent.

• There are different methods of performing frame error checks. Among the most widely used are:

• Cyclic Redundancy Checking (CRC) – Polynomial calculations that are performed on the data;

• Two-Dimensional Parity – Uses a parity bit to make sure the data has not been corrupted; the parity bits on the sending and receiving hosts must be equal if the packet was received without error.

• Checksum – Sums the data to arrive at a total value. It is similar to CRC, but uses a different algorithm.

Error Detection and Handling

• Level 2 of the OSI networking reference model provides a conceptual solution to linking network devices and assuring proper (accurate) transmission of data to and from connected nodes. To accomplish is purpose, each node in a network are assigned addresses, and communication occurs through frames. The diagram below shows a sample frame, using an Ethernet frame as an example:

Error Detection and Handling

• Using frames encapsulates all the information necessary to identify the source and target nodes of a network device (using the IP address method), transmit packets of data, and provide for error checking after transmission. It de-couples these functions from other layers in a network, allowing them to operate independently from logical data transport.

Conclusion

• Using the OSI reference model for computer networking helps to describe in easy to understand terms the complexity of today’s network systems. Each of the seven layers in the model warrants in-depth study for those who chose networking as their career field. At the same time, the model provides a framework less technical people to conceptualize key aspects of networking which has permeated the lives of so many billions of people around the world.

• Level 2, the Data Link Layer, is critical. Data is the fundamental building block of information and knowledge, and communication it the movement of data from one location to another. Without assuring the connection and moving the data reliably, what good would a network be?

References

• Sessions Summary (2009). Fixed Wireless Broadband Blog. Accel Networks: What is Layer 2, and Why Should You Care? Web.archive.org. Retrieved 25 November 2016, from https://web.archive.org/web/20100218075030/http://www.accel-networks.com/blog/2009/09/what-is-layer-2-and-why-should-you-care.html

• Rouse, M. (2016). What is OSI reference model (Open Systems Interconnection)? - Definition from WhatIs.com. Search Networking. Retrieved 29 November 2016, from http://searchnetworking.techtarget.com/definition/OSI

• The TCP/IP Guide - Data Link Layer (Layer 2). (2005). Tcpipguide.com. Retrieved 30 November 2016, from http://www.tcpipguide.com/free/t_DataLinkLayerLayer2.htm

• Brumley, D. (2016). Networking 101 - The Basics of Protocols. Cisco Networking Fundamentals [video]. Retrieved 29 November 2016, from http://cisconetworkfundamentals.weebly.com/chapters-1-4.html

• Framing in Data Transmissions. (2001). Linktionary.com. Retrieved 29 November 2016, from http://www.linktionary.com/f/framing.html

• Basic structure of an IP address. (2015). Liutilities.com. Retrieved 29 November 2016, from http://www.liutilities.com/articles/basic-structure-of-an-ip-address/#.WEJiPlyGMyI

• Detecting Frame Errors with Frame Check Sequence (FCS) - Layer 2 Switching. (2016). Internetwork Training. Retrieved 1 December 2016, from http://www.internetworktraining.com/article-fcs.aspx

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