information technology

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The Network Communications Of The Physical Layer Information Technology Essay

The Physical layer is the first and lowest layer in the seven layers of the Open System Interconnection (OSI) model. The physical layer controls how data is transmitted and retrieved on the network communication. The basic principles of the physical layer are providing hardware components needed for data transmission e.g. routers, switches and cable; data encoding, decoding and signalling.

Wikipedia (8 November 2010) Physical Layer [Online] Available at: http://en.wikipedia.org/wiki/Physical_Layer [Accessed 08/11/10]

The electronic hardware devices in the physical layer are carrier the signal to represent the bits. Encoding is carrier the converting of data bits from Data Link Layer or upper layer into a predefine code. Signalling that sent from the transmitted devices to received device is representing the data bits are 1 and 0 which these signal can be transfer on wire or wireless.

Tomax7 (n.d.) The OSI 7 Layer Model [Online]. Available at: www.tomax7.com/mcse/OSI-7Model.doc [Accessed 08/11/10]

OSI Model diagram

C:\Users\NameLess\Desktop\osi_model.jpg

Adsh2007 (07 September 2010) Physical Layer (Layer 1) [Online] Available at: http://www.adsh2007.com/network/696-physical-layer-layer-1.html [Accessed 09/11/10]

The physical layer provide the way to transport the data bits that make up the data link frame, this is done by encoding the binary digits into signal and transmitting these signal across the physical media. These individual signals will be send to the receive device and restore them to the bits representation and pass the bits to the upper layer or Data Link Layer to complete frame on the receive device. When the physical layer encodings the bits into the signal for a particular destination, it must also distinguish when one frame in and the next frame begins, this is done by present specific bits pattern. The transmitting device add signal designate the start of a frame and the end of a frame, only the bits between the header of a frame and the trailer of a frame are consider data by the receiving device.

Adsh2007 (07 September 2010) Physical Layer (Layer 1) [Online] Available at: http://www.adsh2007.com/network/696-physical-layer-layer-1.html [Accessed 09/11/10]

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

The Ethernet uses the CSMA/CD technology to send each data onto the physical medium. The hosts that connected to the Ethernet must have to wait for the data channel to be clear before sending data to the destination.

Pcmag (n.d.) Ethernet [Online] Available at: http://www.pcmag.com/encyclopedia_term/0,2542,t=Ethernet&i=42781,00.asp [Accessed 09/11/10]

For example, when one host on the network is sending data to the destination, the collision might not occur. But when two hosts on the network are sending data exactly the same time, a collision would occur. Because when the data signals transmit processes collided, both hosts would be rendered unusable. In this case a standard process like CSMA/CD had to be created, to avoid the data collision occurs over the signal transmit process all the hosts that connected to the Ethernet must follow the CSMA/CD rules.

Mcmcse (n.d.) How Ethernet CSMA/CD Works [Online] Available at: http://www.mcmcse.com/cisco/guides/csma.shtml [Accessed 09/11/10]

CSMA/CD diagram

File:CSMACD-Algorithm.svg

Wikipedia (3 November 2010) Carrier sense multiple access with collision detection [Online] Available at: http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_detection [Accessed 10/11/10]

The CSMA/CD would tell the hosts to listen to the Ethernet before sending data to the destination to ensure the data channel is not busy. However if the collision occurs the transmitter will send a jam signal over the Ethernet, this jam signal will indicates to other devices on the Ethernet that there has been a collision, After sending the jam signal, each of the senders will wait a random amount of time before beginning the entire process over. The random time helps to ensure that the two devices don't transmit simultaneously again.

Mcmcse (n.d.) How Ethernet CSMA/CD Works [Online] Available at: http://www.mcmcse.com/cisco/guides/csma.shtml [Accessed 09/11/10]

Part B – Layer 2

The Data Link Layer is the second layer of the seven layers of the Open System Interconnection (OSI) model. Data Link Layer is the protocol layer which will create data frame by encapsulated the data package from the upper layer or Network Layer and transfers the data package across the physical link in a network.

Wikipedia (8 November 2010) Data Link Layer [Online] Available at: http://en.wikipedia.org/wiki/Data_Link_Layer [Accessed 10/11/10]

The Data Link Layer is a connecting layer between the software processes of the layer above it and the physical layer below it. The upper layers above the Data Link Layer will carry the responsibility of putting and receiving the data from the network. The package from the Network Layer will be encapsulated between the header and trailer to create a frame or data link PDU (Protocol Data Units). The frame header contains the control information such as Start Frame, Source and Destination Address and Type/Length. The Trailer contained Error Detection, Frame Check Sequence (FCS) and Frame Stop. While the frame is sending across the physical network, the frame will passing through varies devices such as router or switch interface and each time the frame is transmitted to another receiver device, it will be de-capsulated, restored into a package and re-encapsulated into a new data link frame before sending it from one device to another.

Highteck (n.d.) DataLink Layer [Online] Available at: http://www.highteck.net/EN/DataLink/Data_Link_Layer.html [Accessed 10/11/10]

Data Link Frame PDU diagram

data link layer services header trailer

Highteck (n.d.) DataLink Layer [Online] Available at: http://www.highteck.net/EN/DataLink/Data_Link_Layer.html [Accessed 10/11/10]

The Data-Link layer can be divided into two sublayers, the upper sublayer is Logical Link Control (LLC) and the lower sublayer is Media Access Control (MAC).

Searchnetworking (01 Apr 2005) Data-Link layer [Online] Available at: http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci211899,00.html [Accessed 11/11/10]

LLC prepared Network Layer package for transmission by placing the 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 IP and IPX, to operate the same network interface and media. MAC controls access to the physical media by providing Data Link layer addressing and marks the beginning and ending of data frame according to the physical signalling requirements of the medium and the type of Data Link layer protocol in use.

Highteck (n.d.) DataLink Layer [Online] Available at: http://www.highteck.net/EN/DataLink/Data_Link_Layer.html [Accessed 10/11/10]

A network collision will occurs at Data Link Layer when two or more hosts simultaneously sending data package over a single data channel on the physical network and no other hosts in the network can send package for a particular time, this is called network downtime. A network collision is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. In order to reduce the collision domain’s network, switch can be used to increase the number of collision domains. Because of the advantage of a switch, a port is not shared with any other ports on the switch, each port on a switch has its own collision domain and its decreases each domain’s size.

Wikipedia (5 November 2010) Collision domain [Online] Available at: http://en.wikipedia.org/wiki/Collision_domain [Accessed 11/11/10]


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