Media Access Control Method

A way to permit PC to transmit data over network cabling as ensure that just one PC transmits at a time. If two PC at once place signals on the wire. Then collision can take place and data may be corrupted except a method is used to resolve the collision gracefully. Media access control method makes the smooth flow of traffic on a network, and they prevent or deal with collisions. Media access control methods are implemented at the data-link layer of the Open Systems Interconnection (OSI) reference model

There are four main media access control methods in Networking:

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  Carrier Sense Multiple Access with Collision Detection (CSMA/CD), used in Ethernet networking

  
  


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  Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), used in AppleTalk networking

  
  


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  Demand priority, which is used in 100BaseVG networking

  
  


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  Token passing, which is used in Token Ring and Fiber Distributed Data Interface (FDDI) networking

  
  

Layer 2 protocols specify the encapsulation of a packet into a frame as well as the method for getting the encapsulated packet on and off each medium. The method used for getting the frame on and off the media is called the media access control method.

As packets travel from the source to the destination, they usually traverse over different physical networks. These physical networks can consist of different types of physical media such as copper wires, optical fibers, and wireless consisting of electromagnetic signals, microwave and radio frequencies, and satellite links.

Without the data link layer, network layer protocols such as IP would have to make provisions for connecting to every type of media that could exist along a delivery path. Moreover, IP would have to adapt every time a new network technology; or medium was developed. This process would slow down protocol and network media innovation and development. This is a key reason for using a layered approach to networking

Providing Access to Media

During a single communication, different media access control methods may be required. Every network environment has different characteristics. For example, Ethernet LAN, WLAN, and serial links have different characteristics.

Router interfaces encapsulate the packet into the suitable frame, and a proper media access control method is used to access each link. In any given exchange of network layer packets, there may be several data link layers and media transitions.At each hop along the path, a router does the following:

• Accepts a frame from a medium


• De-encapsulates the frame


• Re-encapsulates the packet into a new frame


• Forwards the new frame appropriate to the medium of that segment of the physical network

Controlling Access to The Media

Media access control layer (data link sub-layer) standardize the placement of data frames onto the media. Media access control is the same of traffic rules that control the entry of vehicles onto a roadway. The lack of any media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles. On the other hand, not all roads and entrances are the same. Traffic can enter the road by merging, by waiting for its turn at a stop sign, or by obeying signal lights. A driver follows a different set of rules for each type of entrance.

In the same manner, there are different methods to control placing frames onto the media. The protocols at the data link layer define the rules for access to different media. These media access control techniques describe if and how the nodes share the media. The actual media access control method used depends on:

Topology

How the connection between the nodes appears to the data link layer.

Media sharing

 How the nodes share the media. The media sharing can be point-to-point, such as in WAN connections, or shared such as in LAN networks.

Topologies and media sharing will be discussed briefly in coming articles.

Data Link Layer

The data link layer(Layer 2) of the OSI model is the protocol layer that handles the moving of data in and out across a physical link in a network. This layer is responsible for the following:-

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  The data link layer is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination.

  
  


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  Allowing the upper layers to access the media

  
  


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  Responsible for logical link control, media access control, hardware addressing,

  
  


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  Handling and defining physical layer standards.

  
  


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  Preparing network data for the physical network

  
  


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  Controlling how data is placed and received on the media

  
  


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  Exchanging frames between nodes over a physical network media, such as UTP or fiber-optic

  
  


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  Receiving and directing packets to an upper layer protocol

  
  


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  Performing error detection

  
  

The Layer 2 notation for network devices connected to a common media is called a node. Nodes build and forward frames. The OSI data link layer is responsible for the exchange of Ethernet frames between source and destination nodes over a physical network media.

The data link layer effectively separates the media transitions that occur as the packet is forwarded from the communication processes of the higher layers. The data link layer receives packets from and directs packets to an upper layer protocol, in this case, IPv4 or IPv6. This upper layer protocol does not need to be aware of which media the communication will use.

Data Link Sub-Layers

As we know that data link layer(Layer 2) of the OSI model is the protocol layer as well as handles moving data in and out across a physical link in a network.The data link layer is theoretically divided into two sublayers. which is logical link control (LLC) and media access control (MAC) layers. This division is based on the architecture used in the IEEE 802 Project; which is the IEEE working group responsible for creating the values that describe many networking technologies.

• Logical Link Control (LLC)


• Media Access Control (MAC)

Logical Link Control (LLC)

This upper sublayer is called Logical Link Control(LLC) which is communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols; such as IPv4 and IPv6, to utilize the same network interface and media. it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.

Media Access Control (MAC)

This lower sublayer defines the media access processes performed by the hardware. It also provides data link layer addressing and access to various network technologies.

The figure 3.17 illustrates how the data link layer is divided into the LLC and MAC sublayers. The LLC communicates with the network layer while the MAC sublayer allows various network access technologies. For instance, the MAC sublayer communicates with Ethernet LAN technology to send and receive frames over copper or fiber-optic cable. The MAC sublayer also communicates with wireless technologies such as Wi-Fi and Bluetooth to send and receive frames wirelessly.

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Wireless Media

Wireless media carry data in the form of electromagnetic signals using radio or microwave frequencies.

Wireless media provides the best mobility options, and the number of wireless-enabled devices continues to increase. As network bandwidth options increase, wireless is quickly gaining in popularity in enterprise networks. Wireless does have some important point to consider before planning:-

• Coverage area: Wireless data communication technologies work well in open environments. However, certain construction materials used in buildings and structures, and the local terrain, will limit the effective coverage.


• Interference: Wireless is at risk to intrusion and can be disrupted by such common devices as household cordless phones, some types of fluorescent lights, microwave ovens, and other wireless communications.


• Security: Wireless communication coverage requires no access to a physical strand of media. thus, devices and users, not authorized for access to the network, can gain access to the transmission. Network security is the main component of wireless network administration.


• Shared medium: WLANs work in half-duplex, which means just one device can send or receive at a time. The wireless medium is shared amongst all wireless users. The more users need to access the WLAN simultaneously, results in less bandwidth for each user.

Types of Wireless Media

The IEEE and telecommunications industry standards for wireless data communications cover both the data link and physical layers. cellular and satellite communications can also provide data network connectivity. But, we are not discussing these wireless technologies here in this chapter. In each of these standards, physical layer specifications are applied to areas that include:

• Transmission Frequency


• Transmission power of transmission


• Data to radio signal encoding


• Signal reception and decoding requirements


• Antenna design and construction

Wi-Fi is a trademark of the Wi-Fi Alliance. Wi-Fi is used with certified products that belong to WLAN devices that are based on the IEEE 802.11 standards. Different standards is following:-

WI-FI standard IEEE 802.11

WLAN technology commonly referred to as Wi-Fi. WLAN uses a protocol known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The wireless NIC must first listen before transmitting to determine if the radio channel is clear. If another wireless device is transmitting, then the NIC must wait until the channel is clear. CSMA/CA is will be discussed later.

Bluetooth standard IEEE 802.15

Wireless Personal Area Network (WPAN) standard, commonly known as "Bluetooth", uses a device pairing process to communicate over distances from 1 to 100 meters.

WI-MAX Standard IEEE 802.16

Usually known as Worldwide Interoperability for Microwave Access (WiMAX), uses a point-to-multipoint topology to provide wireless broadband access.

Wireless LAN (WLAN)

General wireless data implementation wireless LAN requires the following network devices:

• Wireless Access Point (AP): In a wireless local area network (WLAN), an access point (AP) is a station that transmits and receives data. An access point connects users to other users within the network and also can serve as the point of interconnection between the WLAN and a fixed wire network. Each access point can serve multiple users within a defined network area; as people move beyond the range of one access point, they are automatically handed over to the next one. A small WLAN may only require a single access point; the number required increases as a function of the number of network users and the physical size of the network.


• Wireless NIC adapters: Provide wireless communication capability to each network host.

As the technology has developed, a number of WLAN Ethernet-based standards have emerged. Care needs to be taken in purchasing wireless devices to ensure compatibility and interoperability.

The benefits of wireless data communications technologies are clear, particularly the savings on costly premises wiring and the convenience of host mobility

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Fiber versus Copper

Fiber versus copper - choose the perfect solution for your cabling infrastructure is not an easy task. but; once you be aware of the different properties of copper and fiber your solution may seem clearer.There are many advantages of fiber-optic cable compared to copper cables. Following table highlights some of these differences.

Implementation Issues	UTP Cabling	Fiber-optic-cabling
Bandwidth Supported	Fiber-optic can significantly increase your bandwidth potential and the Bandwidth is upto 10 Mbps to 10 Gbps	Copper wire infrastructure and TDM technology are limited in nature. Because it was originally designed for transmitting voice calls only, the demand for bandwidth wasn't high. So it support 10 Mbps to 100 Gbps.
Distance	The signal travel on copper wire networks degrades as the signal is carried from the central office. So the distance covered by copper wire is very Short which is from 1 meter to 100 meters.	Fiber was originally used for long haul networks. The signal travel on fiber optic cable do not degrades as the signal degrade in copper wire. Distance covered by fiber optic is very long from 1-100,000 meters.
Security	Intercepting copper cable can be performed by just connecting taps to a line to pick up the electronic signals. So it is very easy to compromise your security and dificult to trace compromise cable.	Putting a tap on a fiber-optic cable to intercept data transmissions is very difficult. It's also easy to quickly identify compromised cables, which visibly emit light from transmissions.
Immunity to EMI and RFI	Copper wire is a conductor so there is very low immunity to EMI and RFI for copper wire.	Fiber Optic Cable is non conducting meterial and electrical signal do not interfere on it So the immunity to EMI and RFI is very High (Completely immune)
Immunity to electrical hazards	Low	High (Completely immune)
Size of Cable	The speed via copper cable is directly associated with the weight of cable used. For achieving a higher speeds, more cable must be used, which requires more space in a system room.	Fiber cable's speed is not associated to its size, and it's far lighter weight than copper. This renders it easier to use, and less demanding of limited space in small rooms.
Media and connector costs	Lowest	Highest
Installation skills required	Lowest	Highest
Safety precautions	Lowest	Highest
Power over Ethernet (PoE)	Yes	No
Flexibility	High	Low
Reliability	Less reliable	More reliable

Optical Fiber Cable Testing

Installation and splicing of fiber optic cables required special training and equipment. the wrong termination of fiber-optic media will result in diminished signaling distances or complete transmission failure.

Three common types of fiber-optic termination and splicing errors are:

• Misalignment: The fiber-optic media are not exactly aligned to one another when joined.


• End gap: The media does not completely touch at the splice or connection.


• End finish: The media ends are not well polished, or dirt is present at the termination.

Testing is used to evaluate the above mention error and performance of fiber optic components; cable plants and systems. As the components like fiber, connectors, splices; LED or laser sources, detectors and receivers are being developed; testing confirms their performance specifications and helps understand how they will work together.

A quick and easy field test can be performed by shining a bright flashlight into one end of the fiber while observing the other end. If the light is visible; the fiber is capable of passing light. Although this does not ensure performance; it is a quick and inexpensive way to find a broken fiber. For quick and easy testing you need the right tools and test equipment for the job. Following is the list of fiber optic testing tool.

• Optical inspection microscope, 100-200X video scope recommended


• Source and power meter, optical loss test set (OLTS) or test kit with proper equipment adapters for the cable plant you are testing.


• optical fiber splicing machine.


• Reference test cables that match the cables to be tested and mating adapters, including hybrids if needed


• Cleaning materials - dry cleaning kits or lint free cleaning wipes and pure alcohol


• Fiber Tracer or Visual Fault Locator


• OTDR with a launch and/or receive cables for outside plant jobs and troubleshooting.