In the following network, two hosts Alice and Bob are connected by three links and two routers. There are two addresses (N: IP address and L: Link layer address) for each interface of hosts and routers. Host Alice sends a datagram to host Bob. The datagram passes through the routers and links to host Bob. Before sending, Alice makes a frame from the datagram, then sends to the router-R1.
Explain how host Alice encapsulates the frame from the datagram, and then explain how Router-R1 and Router-R2 prepare and forward those frames on link-2 and link-3 towards host Bob. Mention how Alice gets R1’s link layer address, how routers get next destination and link layer addresses in your explanation. Mention all link layer addresses (source and destination) for all three frames in the links.
Explain how PPP enables users to exchange data in WAN by using sub-protocols such as LCP, PAP, CHAP, NCP, IPCP.
Explain how PPP enables users to exchange data in WAN by using sub-protocols such as LCP, PAP, CHAP, NCP, IPCP.
In the following network diagram. Different network devices have been used including switches, hub, bridges and router.
How many collision domains are found in the above network diagram? Explain your answer.
Use correct devices with their label for each collision domain in your explanation.
Structure of components
Explanation of every step
When a computing device transmits information through a network to another computing device, the data undergoes via a process named as encapsulation and is enfolded with protocol information at every layer of OSI. Every layer converses with peer layer only. To exchange and communicate, every layer uses PDUs. PDUs hold and control data attached to information at every OSI layer.
- Client information is transformed to data for transmission on a network.
- Data is then converted to sections, a reliable connection is established amongst transmitting and receiving nodes.
- The sections are changed to packets, and the logical address is put in the header so that every packet is able to be routed via an internetwork.
- Datagrams/Packets are changed to frames for communication on local network. MAC addresses are deployed to identify nodes uniquely on the local network section.
- Frames are changed to bits, the digital clocking and encoding scheme is deployed.
Decapsulation refers to the opposite method of encapsulation and it takes place when information is received on the destination computing device. As data goes up from the lower layer to the upper layers of Transmission Control Protocol/IP protocol, every layer unravels the identical header and uses info enclosed in header to convey the datagram to the next network application in the waiting for the info (Alexander-Webber et al, 2016).
Diagram after host F sends a frame
When host F sends out a frame (A) a hub (hub2), every device attached to the hub is forced to listen to the frame. In case it is not sending, the frame will be forwarded to the devices. Every device will receive the frame since devices connecting the same hub belong to the same domain and collision domain. Hence, from the diagram above, every node has received frame A.
At the switches level the MAC address of the source device is looked up from the MAC address table of the switch Ge, and Han, (2015), if present, the switch will push the frame out the switch via every switch-port excluding the port which the frame came from. In case the MAC address is absent in MAC address table, the MAC address of the source device (MAC address of computer F in this case) will be flooded in MAC address table of the switch.
At hub 1, once the frame is forwarded to the hub, the hub will forward the frame outside its ports without flooding the MAC address table. Note the hub has no capability of using MAC address table.
From the above diagram, the switched network with the redundant topology will include switching loops. Since there is no any type of layer 2 technique to stop network loop, this network has a vulnerability of a nasty issue of broadcast storm, MAC table thrashing and multiple frame copies (Van den Hurk, Brogaard, Lember, Helby Petersen, and Witz, 2016, p.4). This can be avoided by loop avoidance mechanism to prevent network loops while still giving us redundancy service.
In a computing environment, thrashing takes place when a computing device’s resources become saturated resulting to continuous and rapid exchange of data in memory. The speed at which these events take place, the computing device cannot sustain. Therefore, it causes the computing device to collapse or its performance to degrade. Similarly, this is what is experienced by switches and hubs in our case. The continued loop within our network will utilize CPU and memory resources of the switch leading to MAC table thrashing. This eventually will force the network down to its knees.
Steps taken by information from Alice to Bob will be the following:
Alice sends HTTP GET inquiry which is prepared in application layer. The inquiry is forwarded in Transport layer. TCP and UDP protocols encapsulates the request into packet. The packet is conveyed to IP layer. IP layer adds more information in relation to destination and source address together with other information such as fragmentation, TTL. Once this has taken place, the packet is forwarded to link-layer. Here, info associated with MAC address is attached and packet directed to physical layer. In physical layer, streams of 1’s and 0’s are sent out of Alice’s NIC onto the physical media.
Once the packet leaves Alice’s computer, the router will be used as the next node. MAC address of the router will be deployed as the destination address.
When this packet arrives at the router, the following actions will be taken by the router:
It searches the routing info table for node address as directed by packet’s destination Internet Protocol address. In case there is no entry that is discovered, the table is examined for network address derivative of destination IP. On other hand, in case it is found, the router will forward packets that specified network (Wang et al., 2014, p.123). For our case, it will be Frame Link 2. During this process, destination MAC address of packet will changed to MAC address of the Frame 2 Link. However, the IP address info in the packet will remain. The packet will leave the router to Frame 2 Link.
At frame 2 link, the MAC address of the destination will be changed to MAC address of router 2 will the IP address remaining. The Frame 2 Link will be unwrapped, checked whether this is the destination. Of course it is not, the packet is the wrapped again and forwarded to router 2.
At router 2, similar process occurs – destination IP address remains, destination MAC address changes to that of Bob’s computer. The packet is pushed to Bob’s computer.
Now at destination, packets are received at physical layer. Data is directed to the data link layer checking whether this packet is indeed for Bob’s computer. In case this check is approved, the packet is forwarded to IP layer where verification is done then passed to relevant transport layer. After this has been accomplished, with the know-how of ports, the information is passed to application layer listening port. Bob is able to read the request at application layer.
Point-to-Point Protocol is pretty important in WAN aspect due its industry-standard protocol feature-it can be deployed to create a link between different vendor’s equipment (it is not proprietary like HDLC) (Academy, 2014). It uses NCP a section in Data Link header to recognise Network level protocol carried and permits multilink and authentication networks to be executed over synchronous and asynchronous links.
PPP deploys Link Control Protocol (LCP) to establish and sustain data-link connections. LCP is technique of creating, configuring, retaining and ending the point-to-point connection. This method also offers features for example authentication, compression and error detection.
MAC table thrashing
NCP (Network Control Protocol) is a technique of creating and configuring diverse Network layer protocols for the transport across Point-to-Point Protocol link. Network Control Protocol is premeditated to allow the concurrent use of numerous network layer protocols. For instance, protocols used here are (CDPCP) Cisco Discovery Protocol Control Protocol and Internet Protocol Control Protocol (IPCP) (Fu, Bertze, Da Silva, Kuivinen, and Wang, 2017, p.9)
PAP (Password Authentication Protocol) is less safe and sound of the two methods. It sends passcodes in plain text and is accomplished only upon the initial link creation. When PPP link is first created, remote computer sends the user-id and passcode back to initiating target router till authentication is approved.
CHAP (Challenge Handshake Authentication Protocol) is deployed at initial start-up of the link and at intervallic check-ups on a link to ensure that a router is still interaction with the same host. Once the PPP has finished its initial link-creation phase, resident router sends a challenge inquiry to remote computing device. Remote computing device send a value calculated using the one-way mix up function known as MD5. Resident router checks the hash value to make ensure it ties. In case it does not tie, the link is straightaway terminated.
In any telecommunication system, it’s essential to signify info-bearing signal with a wave form that is able to precisely pass via a communication medium. This process of assigning a waveform is achieved via modulation (Tehrani, Uysal, and Yanikomeroglu, 2014, p.87). This is a technique whereby some features of a carrier wave is speckled in respect with the info signal. Modulation is required for the following reasons:
- Several communication channels are featured with partial passbands-for instance, they will authorize only particular varieties of frequencies minus attenuating them. The modulation techniques must consequently be applied to information signals so that frequency translate signal to the variety of frequencies legalised by channel.
- Several channel instances a communication is shared by many users. To preclude conjoint interference, every user’s data signal is modulated on top of an assigned carrier of particular frequency.
2). It should be noted that bandpass channel never sends the digital signal directly to the channel. Therefore there is a need to translate a digital signal to the analog signal afore any transmission is done.
Three digital modulation techniques are used so that only restricted frequency range is used in a bandpass filtered channel. The three methods are
ASK (Amplitude Shift Keying)
Phase Shift Keying
Frequency Shift Keying
In Amplitude Shifting Key, amplitude of carrier is modulated with respect to message signal. When the on-off signalling binary message signal is multiplied with the high frequency sinusoidal carrier signal.
Phase Shift Keying is the digital modulation structure that relays information by changing the pattern of orientation signal.
Frequency shift keying is a means of transmitting the digital signals. The 2 binary states, logic 1 (high) and 0 (low), are characterized by a wave at a precise frequency, logic 1 is presented by the wave at a dissimilar frequency.
There are nine collision domains
Every port on a hub is in the similar collision domain. Every port on a switch, a bridge or a router is in a different or separate Collison domain. See above diagram.
Collision domain refers to the part of the network in which when a packet collisions may occur. The collision may occur when more than two computing nodes send a packet at the similar time on a joint network segment (Ge, and Han, 2015, p.130). The packets crash and both computing devices are forced to deliver the packets once more. This diminishes network productivity. Collision domains are usually in a hub setting, since every port on the hub in identical collision (Zhu, Yang, Huang, and Lu, 2015, p.340).
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