The layer is responsible for communication between machines connected in the same network for example home network such as WIFI, Satellites, WAN. The layer interface between Network layer and physical layer. It is responsible for exchanging data between locally connected devices.
They are two parts of Data Link Layer:
It provides a connection to the network. The following devices have network interface cards include:
Network Card have 48 bit MAC address which is unique globally. Data link exchanges data using source and destination MAC address. Mac address consist of two parts include:
Network interface Cards the Smart Devices
NIC inspects every packets in the network therefore able to identify the packet and discard those which does not belong to the host machine.
Processing performance. Since the NIC is able to identify packets belonging to the specific host hence discard packets belonging to other host reducing the computer workload which could have been used to process every packet.
Error checking. With the help of Cyclic Redundancy Check CRC which is appended by the source of a packet, the destination computer will be able to crosscheck the checksum of packet. If the packet checksum does not the same as checksum in the packet header the packet will be dropped.
The frame which contains the destination address must be process by all the networks machines.
Media Access Control
It concerns with arbitrating with network access.
This is Media Access Protocol used by the Ethernet. Ethernet is widely used to connect devices to the network. The media access allow multiple devices to use a common network device. When two machines in the same network sends packets simultaneously collision will occur.
How CSMA/CD Works
NIC listen to the network media to see if another device is transmitting, if it is then it wait for the network media to be available so that it transmit. If collision occurs both the stations transmitting will detect and stop transmitting. They both wait for some period of time before retransmission.
CSMA/CA
The station reserves the network media to avoid packet collisions.
How CSMA/CA Works
Before station transmitting a data frame it send a request to send a frame to the wireless access point. The wireless will send a response of a Clear To Send (CTS) frame to the wireless network. The CTS will reserve the network for the requesting station for a specific period of time. Then the requesting stations sends the frame without fear of collision without fear of collision.
Address Resolution Protocol is used to determine the MAC Address of specific machine on the network.
Network operating system maintains a table in RAM that contains the IP address and Mac address of machines that have been contacted recently. In RAM the IP address and Mac Address are store in ARP cache. These data expires after some period of time. When machine want to contact another machine in the network it look ARP cache. If the entry is not found it sends a broadcast to the network asking who has the IP in the network. The machine with that IP address will respond with its MAC address. Therefore the Ethernet has all the information need to send the frame through network.
Transport Layer
Role of Transport Layer
Segmentation and Reassembly
Application sends large streams of data to be delivered. The transport layer break this data into manageable segments so that the network layer are able to handle it. At the destination, the segmented data will be reassembled by the transport layer back to the format sent by the sending application.
Identifications of Application (Sockets)
In a computer several applications run concurrently and each of the application. Transport layer assigned each application a port number which distinguished from another application. Each host is identified by the IP address and the client application running on host is identified by port number. The combination of the IP address and port number is called socket. Each IP Addresses is associated with a port number. Port number ranges from numbered 0 – 65535. To view active sockets on windows we use netstat command.
User Application Port Numbers
User applications dynamically choose port above 1023 when they connect to the network therefore each application will have a different port numbers and sessions.
Transport Services with UDP
It provides basic transport services to the network applications. UDP header have the following include:
The cons of this UDP is that they do not support check for checksum errors hence its ideal for video and audio streaming since it is very fast and efficient. It is a stateless protocol.
A stateless protocol is a communications protocol that treats each request as an independent transaction that is unrelated to any previous request. The request and responses are independent.
UDP requests must be fully specified to what they want the destination to do.
Transport Services with TCP
TCP header contains the following headers include:
Connection Oriented protocol. Guarantee reliable delivery of data between sender and receiver.
Connection set up. Prior to sending and receiving data, connection must be set it up to allow exchange of sequence number which tracks data transfer throughout the session.
Acknowledgements. Throughout the sessions the acknowledgements are used to confirm successfully delivery of data. The sequence numbers and associated acknowledgements are the basis of reliable communications.
The cons of the TCP protocol are slow as compared to UDP protocol since the connection must be established and pass acknowledgement messages between sender and receiver.
TCP Timers
TCP uses timers to detect lost Transport Layer Segments. Sender starts the timer each time it sends a TCP segment. If acknowledgement is not receive by the sender within a specified timeout period TCP will count a segment as a lost and it resend the segment again. This can lead to duplicate segments in the network.
TCP Flags
TCP header contains numerous flags that indicate the function of segments. TCP Flags include:
TCP Connection establishment and Termination
Connection between client and server can last duration of the session until it get terminated. TCP connection are set-up using 3 handshakes:
TCP Flow Control
Flow control is required to throttle flow of data from fast sender. Windows size implements the flow control. The value here specifies the maximum number of outstanding Bytes a sender can send without receiving a corresponding acknowledgement. A Receiver can slow or stop a sender from sending data by delaying acknowledgements. This mechanism is called sliding windows flow control.
Networking and Security
Cloud computing. The process of using remote resources through internet through subscription. These resources includes network, hardware, storage etc.
Fed Uni Cloud Services
Amazon
Azure
Cloud Service Pros
Cloud Service Cons
Cloud Virtualization relies on Virtualization
Mobile Computing
Mobile technologies have undergone several revolution since its introduction in 1980. Different technologies have been added in every generations.
TOPIC 3
Campus-sized networks have centralized roles and permissions which are managed in a central point and each user cannot allocate roles on his own. Also in campus resources are shared to specific users through a dedicated server which is available throughout.
The Internet Ping program works much like a sonar echo-location, sending a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return [7].
TOPIC 5
(i) With reference to the Routing table describe what the interface entry contain?
(ii) What does the Next Hop entry contain?
(iii) Complete the associated routing table for Router 1.
(iii) Create the routing table for Router 2.
The local DNS will send a query to internet service providers asking the remote DNS servers.
TOPIC 7
References
[1]”The difference between half-duplex and full-duplex”, SearchNetworking, 2018. [Online]. Available: https://searchnetworking.techtarget.com/answer/The-difference-between-half-duplex-and-full-duplex. [Accessed: 17- May- 2018].
[2]”PERFORMANCE ANALYSIS OF STATISTICAL TIME DIVISION MULTIPLEXING (STDM)SYSTEMS”, Lejpt.academicdirect.org, 2018. [Online]. Available: https://lejpt.academicdirect.org/A16/get_htm.php?htm=151_166. [Accessed: 17- May- 2018].
[3]”Microwave Link Networks – Engineering and Technology History Wiki”, Ethw.org, 2018. [Online]. Available: https://ethw.org/Microwave_Link_Networks. [Accessed: 17- May- 2018].
[4]”Terrestrial Microwave”, Vclab.tu-sofia.bg, 2018. [Online]. Available: https://vclab.tu-sofia.bg/AVC%20Network%20Lessons/MCSE-NE/5a6bc9c.htm. [Accessed: 17- May- 2018].
[5]”Terrestrial Microwave”, Vclab.tu-sofia.bg, 2018. [Online]. Available: https://vclab.tu-sofia.bg/AVC%20Network%20Lessons/MCSE-NE/5a6bc9c.htm. [Accessed: 17- May- 2018].
[6]”What is Manchester encoding? – Definition from WhatIs.com”, SearchNetworking, 2018. [Online]. Available: https://searchnetworking.techtarget.com/definition/Manchester-encoding. [Accessed: 17- May- 2018].
[7]”Internet Ping Command, Test”, Livinginternet.com, 2018. [Online]. Available: https://www.livinginternet.com/i/ia_tools_ping.htm. [Accessed: 17- May- 2018].
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