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# Question on Modulation and Multiplexing – 25 marks

Modulation refers to process of addition extra information to an electrical signal. This information can be added by varying the amplitude, polarization, frequency, or phase. The most common modulation method is the Amplitude modulation (AM), this is done by varying the intensity and strength of the signal carrier so that it represents the data that is being included to the signal. Another example is the Frequency modulation (FM) whereby the variation is done on the frequency of the carrier waveform so that it includes the frequency of the data.

• Modulation of an electrical signal

Modulation and shift keys differs on the types of signal that they use. While shift keying is applied to digital signals whose values are discrete in nature, Modulation on the other hand, is used in analog signals and they vary in accordance to the data or information being transmitted.

• 4 signal levels of (15V, 7V, -7V -15V) has the following bits:
• N = log2 M
• Where N = number of bits necessary M = number of conditions, levels, or combinations possible with N bits
• Rearranging the above equation gives:
• 2 N=M; Therefore the bits are as follows
• Level of 15V, bits = [log 2 15] = 3.90689059561
• Level 7V, bits = [log 2 7] = 2.80735492206
• Level -7V, bits = [log 2 -7] = 2.80735492 + 4.53236014 i
• Level -15V, bits = [log 2 -15] = 3.9068906 + 4.53236014 i
• The answers above indicate that the bits at 15V and -15V are equal but differ in the direction. The same is the same with 7V and -7V levels.
• Shift keying and modulation

WDM Systems

• The basic principles behind Wave Division Multiplexing

Transceivers – transmitting data as light

Transceivers are lasers with specific wavelength which are used to convert data signals from SAN and IP switches to optical signals so that they can be transmitted into the fiber. Each data stream is converted into a signal with a light wavelength that has a unique color. Given the physical properties of light, channels cannot interfere with each other. All WDM wavelengths are independent therefore reducing the number of fibers required. Furthermore, it allows the creation and connection of new channels without disruption of the existing traffic services.

Multiplexers optimizing the use of fiber channels

The WDM multiplexer is a vital component in the optimization the use of the fiber. The multiplexer gathers all data streams and simultaneously transports them from one end to another over a single fiber. Demultiplexing of the streams occurs at the other end of the fiber. The WDM multiplexer is solely responsible for the gathering of all data streams and combining them together for simultaneously transportation over a single fiber.

Patch cord connecting the transceiver and the mux

Protocol data at high speed are transmitted by the transceiver on narrow wavelengths band while the multiplexer is the core of the entire operation. The patch cable is the joins the transceiver and the multiplexer together. LC connector cords are used to connect the transceiver output to the multiplexer input.

Dark fiber: Fiber pair or single fiber strand

WDM uses a dark fiber network to transport optical traffic over an architecture. Dark fibers are preferred because they allow for the maximum transportation of traffic as one end receives data while the other is transmitting the data. When a single fiber is used WDMsystems use one wavelengths to send data and another to receive.

# Question on UNIX – 25 marks

• A packet refers to a unit of data being transmitted across a packet-switched network. Networks that are interconnected by a router is referred to as a packet-switched network. Examples of such networks are the internet, and the TCP/IP. Packet switched networks apply Statistical multiplexing whereby each stream is divided into packets that will be delivered asynchronously in a first-come first-served basis.

The difference between a metarcharacter and regex is in their usage by the programmer. Metacharacters such as wildcards are special characters that provide additional information about other characters. For example a programmer can use [*] to specify how to treat the subsequent characters. Regex or regular expressions on the other hand are used by programmers to express how a program is to search for a specified pattern in a text and what to do with that pattern once it is identified. For example you can use regex to direct a computer to find the words “met acharacte” and replace it with “metarcharacter.”

• …………………………………. TO DO HIMSELF
• …………………………………… TO DO HIMSELF
• Shell metacharacters and regular expressions

The use of quotes in grep is necessary when the parameters is not a simple string, the quotes allows you to specify range from the overall parameter.

• Circumstances to use quotes with grep

The choice between single or double quotes is only important if the search string contains variables or other items that you expect to be evaluated. With single quotes, the string is taken literally and no expansion takes place. When the Pattern parameter is not a simple string, you usually must enclose the entire pattern in single quotation marks. In an expression such as [a-z], the – (minus sign) cml specifies a range, according to the current collating sequence. With double quotes, variables are expanded.

• When one uses single quotes and double quotes

# Question on Satellite Communication – 25 marks

Triangulation is the determination of the location of a radio transmission radially or through signals from different points usually two to three. Triangulation can be applied in cellphone communications in determining the geographic position of the user. With respect to satellites communication, satellites are placed in an orbit above the earth surface. These satellites spaced strategically such that at any particular point in time they are always above the horizon. Furthermore, each satellite’s computer is equipped with an atomic clock and radio. As the satellites rotate the earth they broadcast their changing positions and time. Meanwhile on earth any GPS receiver will have a computer that will triangulate its position by receiving coordinates from the three or four satellites above the earth surface. The outcome is in form of latitudes and longitudes hence its geographic position.

• Triangulation and GPS satellites

The angular separation of the satellites that are orbiting the earth is the angular distance, or the apparent distance between the two point objects, that is, earth and satellite, as is observed from a particular location that is different from the two objects. It is the angle size between 2 directions that are coming from the observer and towards the two selected objects.

• Angular separation

Angular Separation distance

From the figure above, the angular separation distance is the distance between L4 and L5.

Conceptually, the angular distance or separation is similar to an angle, it can be measured using units similar to degrees or radians, using goniometers or optical instruments that have been developed and designed for the in a way that it can point in particular directions and record the angles correctly. The angular distance should be able to give suggestions of linear distances that exist between satellite objects as they are observed from the earth surface. At any particular time satellites should maintain a minimal angle to give correct estimates and be above the horizon.

• Reason for a minimum angular separation between satellites
• Form(s) of multiplexing
• Frequency Division Multiplexing
• Frequency Division Multiplexing (FDM) works by transmitting all of the signals along the same high speed link simultaneously with each signal set at a different frequency. For FDM to work properly frequency overlap must be avoided. Therefore, the link must have sufficient bandwidth to be able to carry the wide range of frequencies required. The demultiplexor at the receiving end works by dividing the signals by tuning into the appropriate frequency.
• FDM operates in a similar way to radio broadcasting where a number of different stations will broadcast simultaneously but on different frequencies. Listeners can then “tune” their radio so that it captures the frequency or station they want. FDM gives a total bandwidth greater than the combined bandwidth of the signals to be transmitted. In order to prevent signal overlap there are strips of frequency that separate the signals. These are called guard bands
• Time-Division Multiplexing
• Time division multiplexing is a technique used to transmit a signal over a single communication channel by dividing the time frame into slots – one slot for each message signal. Time-division multiplexing is primarily applied to digital signals as well as analog signals, wherein several low speed channels are multiplexed into high-speed channels for transmission. Based on the time, each low-speed channel is allocated to a specific position, where it works in synchronized mode. At both the ends, i.e., the multiplexer and demultiplexer are timely synchronized and simultaneously switched to the next channel.
• A limit to the range of frequencies that satellites can use
• For satellites to be useful and spacecraft must communicate, sometimes to relay communications between two points, sometimes to transmit data they have collected. Although there have been some experiments in optical communications using lasers, most satellite communication is accomplished by radio, one part of the electromagnetic spectrum. Radio frequencies must be shared with terrestrial radio services, and international frequency assignment is essential to avoid interference between all the different uses made of the radio spectrum.

A diagram showing satellite communication with LAN

• Satellite communication

# Question on Local Area Networks – 25 marks

Twisted pair cables are more susceptible to electrical interference that can be caused by having electrical equipment in the area. Since the casing of this type of cable is relatively thin compared to that of coaxial cable, it does not have a second insulator like the coaxial cable. Shielded Twisted Pair also has a layer of metal foil or mesh shielding around all the pairs or each individual pair to further shield the pairs from electromagnetic interference. Shielded twisted pair is typically deployed in situations where the cabling is subjected to higher than normal levels of interference.

• Form of multiplexing on a twisted pair copper transmission cable
• Frequency-division multiplexing: One multiplexing that can be found on twisted copper is the Frequency-division multiplexing (FDM) whereby there is a shift of spectrum of each signal to various range of frequency. Though (FDM) is analog in nature, it’s capable of combining several signals in differing frequencies and sending it over a single medium. The (FDM) signals are electrical in nature and are commonly used in radio and television broadcasting. Users can select multiple channels by tuning to the appropriate frequencies. This is despite the fact that only a single cable reaches the client’s residential area. These optical communications use wavelength-division multiplexing (WDM.)
• Problem with twisted pair cables

The concept for the twist of the conductors is to prevent interference. Ideally, each conductor of the pair basically receives the same amount of interference, positive and negative, effectively cancelling the effect of the interference. Typically, most inside cabling has four pairs with each pair having a different twist rate. The different twist rates help to further reduce the chance of crosstalk by making the pairs appear electrically different in reference to each other. If the pairs all had the same twist rate, they would be electrically identical in reference to each other causing crosstalk, which is also referred to as capacitive coupling

• The twisted pair copper cables

Though they are continuously advancing, these technologies are still inferior to WAN because of their inherent limitations. Newer, diffuse or reflective IR LANs attempt to overcome these limitation by using a technology where a beam of light is mounted on the wall or a ceiling so that it can reflect its beam on an open area. Still, the performance of IR technologies in general is easily affected by weather and opaque background.

• Technologies used to extend LANs.
• Frequency-hopping spread- spectrum
• Local area networks uses the frequency-hopping spread- spectrum to spread signals by randomly hopping from one frequency to another several times a second. However, receivers have to match the frequency and time of the hop sequence from the signal being transmitted in order for them to pick up the signal. The technology is difficult to intercept because it has a high resistance to interference. Furthermore, it can cover larger distances due to its low frequencies usage.
• Infrared (IR) technology
• IR LANs operate in a similar manner as remote controllers. They do not have Bandwidth limitations and some offer speeds of 16 Mbit/sec token Ring performance hence, suitable for point-to-point file-transfer applications such as computers to printers. However they can`t penetrate opaque barriers, so most IR networks must operate on a line-of-sight basis and require that the laser diodes or light-emitting diodes in each system be aimed directly at each other.
• Extending a LAN

Many of the LAN technologies have limited speed with only a few capable of offering more than 20 Mbits/sec when extended. Unfortunately, even the ones with high-frequency carrier are still subjected to absorption or reflection by common building material. Moreover, they are still susceptible to multipath distortion as well as interference thus limiting range. Their Bandwidth cannot be comparable to wired LANs. Signal reliability is also a major concern. And despite new technologies featuring encryption, data security remains a major issue, particularly for firms in financial and banking markets.

A LAN host creates a physical connection then the network protocols define the standards which will be used to communicate to other computers whereas a protocol establishes the rules and encoding specifications for sending data. This will then determine how the computers in the network will identify each other, how the data is to be transferred, and how the information is to be processed once it reaches its intended destination.

• The mechanism used by Ethernet wired LAN network hosts

Protocols are useful in defining the procedures that will determine the error checking criteria to be used, the method of compressing the data, how the computer sending the data will give notice of completion of the process, how the recipient computer will notify the other upon successfully receiving the data, and how to handle transmission that are broken or damaged

Even though there are different network protocols, a shared physical cable is used in order to allow multiple protocols to coexist peacefully over the network media therefore allowing the use of a common hardware by the network builder. A mechanism normally referred to as “protocol independence” as compatible devices at the layer of data and physical link enables the users to use many protocols over the same medium.

# Question on Wide Area Networks – 25 marks

ICMP Datagrams showing encapsulation with the IP

• IPV4 packets
• IPv4 packages the segment layers of the transportation of the information to assist the network in its delivery to the designated host. This encapsulation of the IPv4 stays with the packet as from the time it exits the layer network of the initial host until reaching the final host. The process of packaging data by layers ensures that more layers are developed and scaled without interfering with other layers. As a result, segment layers of the transport can be encapsulated easily by the laid existing layer network protocols like IPv6 or IPv4, and other protocols yet to be developed.
• Routers assist in the implementation of diverse protocols of network layers to function at the same time in a particular network to and from the designated hosts. From the time a router receives a packet, it performs generic steps of routing, forwarding/switching, and encapsulation prior to routing the packets.
• The routing process involves the routers controlling the plane by recording and listing the packets in a table that is used to forward the packed data in given network routes.
• The forwarding process of the data packet is done once the router has found the outgoing interface while using adjacent tables on the processor of the router by involving the device’s CPU.
• Meanwhile, the encapsulation process is dependent on the type of media information that is to be transmitted. While the header remains intact apart from media such as nating or vpn, the second layer of the header will keep alternating by hopping depending on the media. Therefore, the transmission of a packet of data on the router necessitates it to find other information regarding the media.
• The network layers have IP addresses that routers use in forwarding of data packets to the designated hosts. After the frame has been received by the final host, the router first checks the Frame Check Sequence (FCS) field to determine whether the data has errors. When the frame has no errors, what follows is the router deleting the old data header and trailer links before viewing the packed data. A comparison of the designated final address host’s IP and the packet and the address encapsulated with the packed data in a new link of the headers and trailer after which it is forwarded to the accurate interface.
• As the encapsulation happens at initial source host, the network layer constructs an IP packet for the transportation and differentiates the destination hosts by distinguishing their networks via routers. During this, the router examines the header and forwards it to the hosts repeatedly until the IP packet is eventually received by the final router within the web server that sends an APR message that the server responds by creating an address to the router before being forwarded to the web server.
• Error reporting mechanism used by IP
• IP is integrated with an ICMP protocol that it uses as an error reporting mechanism. This protocol tells the sender why its data packets failed to reach their set final destination by exploiting the information on the internet.

From the datagrams above, it indicates the encapsulation that exists between the ICMP protocol and IP. Thus, the packaging assists routers to send the messages that entails the errors of the IP to the sender.

# Question on Distributed Applications – 25 marks

• DHCP’s role and working
• The distribution and updates of IP addresses on a network are automated by DHCP. DHCP’s server provides the information and data about the configuration of the network to its client via multifold messages referred to as DHCP transaction/conversation. Moreover, it uses relay agents to facilitate transactions with clients located on the various subnets.
• DHCP’s structure consists of clients, servers, and relay agents within a particular network. The DHCP clients interact the servers using messages contained in the conversation to assist them in obtaining new IP leases for the addresses.
• Additionally, DHCPs provide support to client computers with the various Microsoft operating systems. The client achieves this by contacting the servers to find the configuration of the network before allowing it to attach itself automatically to the network. Typically, the servers give a response and the IP address leases are renewed and will continuously try sending the same message of renewal until the lease expires again when it’s renewed.
• However, if the DHCP client is successful in renewing the lease before expiry of the IP address, the device mutes and goes offline.
• DHCP operation of the leasing of IP addresses and circumstances of expiry of the lease
• DHCP servers are entitled with the mandate of maintaining and renewing of the IP addresses as set by the administrator of the device.
• The administrator defines the scope and activates it for the DHCP clients and DHCP server’s automatic configuration. The scope entails parameters for the lease like the name, IP address, subnet mask, and duration values. The durations of the lease of an IP address is always set to 8 days by the default mode but the administrator is able to change the values.
• However, the set of the duration value may sometimes be varied and leases expire when the affected versions of the network system such iOS fail to renew before the expiry of the lease even though iOS continuous use of the IP address after the duration. These may rise due to the following:
• Failure of the user to attach the device to a power source like a computer or adapter.
• The device is in sleep mode.
• Failure of the application to assists the operating systems’ API multitasking application on the background.
• The lack of a cellular interface signal or disabling of that signal on devices that have the interface.
• The choice by the device to remain attached in a Wi-Fi network despite the device being in sleep mode.

Additionally, a signal is sent to the server by the client in the following steps:

• Steps in FTP’s communication between the client and the server
• File Transfer Protocol (FTP) entails protocols that an internet uses to transmit files between the computer’s internet and the connections of TCP/IP. The client uses FTP to download, upload, remove, copy or delete files on a particular server.
• The following steps are used to communicate:
• User starts an FTP program for the client on a computer used for sending.
• Selection of the address and the port to be used in the server.
• Indication to the program that a connection to the server is needed.
• Sending of information regarding the application layer via presenting of the layer and sending it a command to open a connection to the other computer’s address and port.
• Negotiation of the session layer via the FTP’s server connected. After this, synchronized signals are sent between the client and server computers in a bid of creating a connection.

A chart diagram showing recursive query flow

• The client’s session layer sends a data packet to the layer of transportation.
• On reaching the layer, a TCP header is added to the packet to indicate the source port and destinations.
• A source IP address is then added by the network layer as well as its destination’s IP address with other useful information.
• The determination of the hardware address by the data link layer will then take place in addition to the final host computer. Also, an ethernet is added to create the hardware address that will receive the message.
• Transmission of the information via hardware layer until it arrives at the network server of the computer.
• The layer 4 protocol used by the FTP operating at the fifth layer to send data is the DOD model. The model originally designed by the defense team, has the link layer that the driver of the device and its interface card uses to map to the link of the OSI model. Further, it has a network with ICMP, IP, AND IGMP protocols while its transport layer includes the TCP protocols. Additionally, its application corresponds to the DNS, FTP, and other useful layers. Besides the DOD model contains FTP sessions that diversely operate in both the passive and active modes.
• In correspondence with network systems, a directory service is used by users to locate resources and other various services within a particular network. It entails information that is customizable and stores the packed data while creating single points that the administrators use to manage the devices objects and various attributes.
• The DNS queries include inverse queries, recursive query, and iterative / non-recursive queries.
• Recursive queries requires the DNS server that received the query and does all the task of fetching the answer and giving it back. Also, during this process, the DNS server may query other servers with DNS on the behalf of the user.
• A diagram example of wanting to browse www.example.com is as follows:

However, an iterative query’s name server does not fetch a complete answer for the user but rather gives a referral to other DNS servers that may have its solution or answer.

In the diagram representing the flow in a recursive query, the DNS server 172.16.200.30 fetched the result on behalf of the user and gave a final answer. However, supposing the DNS server was an iterative query it only gives an answer available in its records or give a referral of the serer that has the solution / answer and requires the user to query the root to find an answer.

A diagram chart showing an iterative DNS query

# Question on Unix – 25 marks

• Types of services offered by Cloud computing
• IaaS that means Infrastructure as a Service. This provides access to businesses on the essential web structure like servers, storage space, and connections minus them buying to manage the internet infrastructure. It is associated with economies of scale that are beneficial to the businesses and enables them to grow. Moreover, PaaS and SaaS clouds are rooted and has a foundation of the IaaS cloud.
• An example is the Rackspace Cloud.
• PaaS (Platform as a Service) clouds. These are created severally inside IaaS by specialists to assist them in achieving scalability or deployment of the application’s expenses.
• An example is the Google App Engine
• SaaS (Software as a Service) clouds. These are relatively mature and contains phrases, which use predates of cloud computing. The cloud applications necessitates the software structure to be leveraged to minimize the challenges of maintenance, operations, and support.
• An example is Gmail.

# Question on Disks and Filesystems – 25 marks

• Unix commands functionality
• Ipr: lpr filename — print. Use the -P option to specify the printer name if you want to use a printer other than your default printer. For example, if you want to print double-sided, use ‘lpr -Pvalkyr-d’, or if you’re at CSLI, you may want to use ‘lpr -Pcord115-d’.
• ls – la
• ls: lists your files
• ls –l: lists your files in ‘long format’, which contains lots of useful information, e.g. the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified.
• ls –a: lists all files, including the ones whose filenames begin in a dot, which you do not always want to see.
• chmod go-wx myFile
• chmod options filename — lets you change the read, write, and execute permissions on your files. The default is that only you can look at them and change them, but you may sometimes want to change these permissions. For example, chmod o+r filename will make the file readable for everyone, and chmod o-r filename will make it unreadable for others again. Note that for someone to be able to actually look at the file the directories it is in need to be at least executable.
• head logfile – name of the file to be processed
• sftp [email protected]
• lynx – lets you browse the web from an ordinary terminal. Of course you can see only the text, not the pictures. You can type any URL as an argument to the G command. When you’re doing this from any Stanford host you can leave out the .stanford.edu part of the URL when connecting to Stanford URLs. Type H at any time to learn more about lynx, and Q to exit.
• Relative and Absolute Paths
• A relative path assumes that the file is on the current server and their use enables the user to construct personal sites offline and gives room for fully testing it before its uploaded.
• e.g.
• php/webct/itr/index.php
• On the other hand, an absolute path directs a file on the Internet using its full URL and tells the browser precisely where to go.
• e.g.
• http://www.uvsc.edu/disted/php/webct/itr/index.php
• Moreover, absolute paths are easier to understand than relative paths. Nevertheless, the ability of constructing your own site offline gives an upper hand to relative paths as one is able to test all the domains before uploading them.
• Unix commands to do the following:
• Copying file CHI.doc to current directory
• First a new directory dir1 is created as:
• mkdir dir1
• Then making a duplicate of a file CHI.doc to directory is as:
• cp – CHI.doc dir1
• Listing of the contents of directory
• The cp command is also used to create a copy of a directory and its contents.
• Assuming that the contents of the directory are project1 the copying it to directory proj1copy is as follows:
• cp -r project1 proj1copy
• Changing the current directory to a directory called teaching
• cp -r dir1 dir teaching
• Piping Concept in Unix
• In Unix-like OS, a pipe is a form of redirection used to send the output of a program to the other to be processed further.
• An example is the standard output when it’s abbreviated as stdout to redirect the initial output to the final destination of the output.
• Types of Files in Unix
• Ordinary files: These store information of the user that he/she usually work with.
• Directories
• Special files: They represent real physical devices like a printer, tapes, CDs.
• Pipes
• Shell Metacharacters and Regular Expressions
• Shell metacharacters like wildcards are genetic terms that can be substituted for all possibilities and can match characters. However, a regular expression entails the functionality but is a more powerful character matcher and provides the user with the ability of restricting the type of characters while giving more flexible specification on the number of characters to match.
• An example of a UNIX command that uses wildcards is the Regex parsing, while *.txt is used by a regular expression.

a. Inode in location of files

An Inode is a data structure that the operating system uses to keep track of all the information about a particular file. These contains numbers referred to as inode numbers that the operating system first searches in an inode table. The inode numbers are unique and exact corresponding to the file being located in the table. Also, reaching a certain file using its “name” requires an inode number but reaching the inode number doesn’t need a file name.

The mode keeps information and data about only two things: permission information, and the type of inode for a file or directory. The Owner Info has access details of the file such as the file’s owner. Size is the location store for the size of the file in terms of bytes while time stamps stores the inode time for modification, creation, and other activities.

Additionally, the block size are the size of chunks in which the data will be spread. However, they are set to the default to enable the block size return to its original size whenever a partition will be formatted. Moreover, the direct block pointers points to the addresses of the blocks that the file’s data are contained. Direct block pointers carry a limited number bytes and can address only 48K. Therefore, data sizes that are above 48K adopt indirect block pointers that provides the provision for copying the blocks and can address up to 4MB of data and an increase in the data sizes necessitates the double indirect block pointers, which addresses up to 4GB of data.

b. Purpose of RAID arrays

The Redundant Array of Independent / Inexpensive Disks also known as RAID arrays involves a virtualization technology for the storage of data by combining various components of the disk drive into one or more logical units to achieve redundancy or improve its performance.

The distribution of data across the drives in various ways uses RAID levels that depend on the level of redundancy or performance set to be achieved. This distribution scheme have prefix “RAID” to their names and each level gives a difference in balancing of the goals such as performance, reliability, capacity, and availability. As the levels increases over RAID 0, protection is given to recoverable sector read errors and the success of the drives.

RAID arrays operate in the following four levels of RAIDs: RAID 0, RAID 1, RAID 5, and RAID 10 also represented as RAID 1+0.
In all the diagrams mentioned below: A, B, C, D, E and F represents blocks while p1, p2, and p3 represents parity

RAID LEVEL 0

Level RAID 0 contains a minimum of two disks, exhibits zero redundancy while the performance is excellent. The excellence in performance is attributed to the stripping of the blocks and zero redundancy is due to the lack of a mirror or parity.

RAID LEVEL 1

The RAID LEVEL 1 has two disks minimum with excellent redundancy as blocks are mirrored and a good performance as there is no striping or parity.

RAID LEVEL 5

This level has a minimum three disks and is associated with good performance as blocks are striped and a distributed parity ensures a good redundancy. Notably, this level provides the best cost effective option of both the redundancy and performance.

RAID LEVEL 10

This is the last level and has a minimum of 4 disks, while both the performance and redundancy are excellent.

c. Filesystem

This is the way or manner that the files are given names and the location in which they are placed logically for storage or retrieval.

An example of a filesystem is the hierarchical / tree structure. This is a filesystem that is used by Windows, Macintosh, DOS, OS/2, and OSes all use to place files in a folder or directory or the subdirectory in a desired position in the hierarchy.

Also, the file systems assist the user in specify conventions while naming the files, helps in the inclusion of the highest number of characters in a name that characters can be used, and the length of the suffix of the file name.

# Question on Wireless Networking – 25 marks

• A file handle is the number that the OS assigns to a file temporarily when the file is opened and uses it internally to assist in the file’s access.
• Chugging noise when one starts a PC plays when a person disconnects or is connecting the computer. The noise can be attributed to the following faults the occasional powering of the computer’s monitor is only enough to turn on the speakers. While this is unusual, one needs to first check the power management settings before considering it a fault. When the hard disk goes stand by or resumes operations it may make a clunking sound and will hang up the system until it’s fully spun up. Verify your hard drive power management settings. Either set it so the disk never sleeps, or set it to a much higher time out value.

A diagram of a Bluetooth piconect showing communications

• Bluetooth and spread spectrum frequency hopping.
• The technology of frequency-hopping spectrum spread helps Bluetooth to avoid the problem of interference. The technology used by Bluetooth allows it to “hopes” over the channels at a rate of 1600 hops per second. Since it applies this scheme other users or devices cannot interfere with its frequency. Given the pseudo-random nature of the hopping patterns, there are limited chances that other Bluetooth devices will interfere with the flow of data. Furthermore, since the other devices only broadcast using simple frequency there impact on the data being transmitted will be minimal.
• Bluetooth piconet

Piconet represents the basic units for networking in bluetooth. The relationships are usually in such a way that either there is one master and one slave or one master and multiple slaves. The channel and phase are determined by the master. Master will determine channel and phase. The above diagrams illustrate this relationship. A piconet will have 3 bits thereby limiting devices to a maximum of eight.

A mesh network is a local area network (LAN), wireless local area network (WLAN) or virtual LAN (VLAN) that uses either the full mesh topology or partial mesh topology connection arrangements. A full mesh topology occurs when each network node is connected directly to each of the other nodes. A partial mesh topology occurs when some nodes are connected to all the others, but others are only connected to those nodes with which they exchange the most data.

• The mesh network and wireless networking

The Internet of Things (IoT) is likely to will likely prefer the use of mesh networks. One advantage of mesh network nodes over star topology is that they do not require routers to offer internet services. Furthermore, this decentralized topology cannot have a single point of failure (SPoF). Incase anode is not operational the rest can communicate with each othereither directly or through intermediate nodes.

When you have data that you want transmitted and there is only one medium which is shared by many hosts, you can choose to transmit your data regardless or pause your transmission until it’s your turn. First approach is contention-free and the probability collision occurring with another host transmitting before completion of your transmission finished is high. This may cause you to repeat the process. The second approach is contention-based. There is no risk of transmission collution as you wait for your turn.

• Contention-based and contention-free access

Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking refers to multiple methods for accessing networks whereby carrier sensing is applied. Furthermore, the nodes wait until the channel is idle to transmit in order to avoid any collusion. They do not transmit the data in bits, instead they transmit the whole packet data at once. CSMA/CA is a protocol that uses the Data Link Layer (Layer 2) of the OSI model. It is useful for wireless networks, where the alternative CSMA/CD collision detection is unreliable as a result of problem arising from a hidden node.

• Carrier Sense Multiple Access/Collision Detection and wireless networks

The use of CAMA/CD mechanism is made impossible and impractical by the physical characteristics WiFi. This is caused by CSMA/CD’s nature waiting for a medium to be free before packets of data transmitted. CSMA/CD works well for wired networks, however, in wireless networks, there is no way for the sender to detect collisions the same way CSMA/CD does. This is because the sender can transmit and receive packets but cannot sense data traversing the same medium. Therefore, CSMA/CA is used on wireless networks. CSMA/CA doesn’t detect collisions but rather it uses a control message to avoids. In the event that there is a collusion of control messages, a back-off algorithm is applied as it means that the transmission medium is unavailable.

# Question on network models – 25 marks

Protocol and Internet communication

The end points of communication technology uses a special rules to communicate. These rules determine the interactions between those ends. The special set of rules are known as protocols. Protocols are used at different levels in a telecommunication system both at hardware and application for the purpose of interchanging data. The internet uses Transmission Control Protocol (TCP) to relay and receive messages with other connection points of the internet

Standards and protocols are instructions that are complementary. While protocols give the instructions and tools for a project, the standards are the rules of the fields or projects.

• Protocols and standards relationship

A client-server model is used to define the relationship between the resources and services that a server provides its clients. Servers such as web servers and mail servers, can service multiple clients at the same time. Client usually requests for a connection to a server, in turn the server may choose to grant or reject the request for connection. Once a permission to connect is granted, the server will then establish and maintains a connection with the client using a specific protocol. The mail server has an SMTP application which will then request authentication from the client, in form of user name and password. Once verified, the server then send an email to the intended user.

• The client-server model

The amount of traffic that a physical machine is capable of handling is limited despite the fact that servers can provide multiple connections to multiple users at the same time. Therefore, multiple physical servers are used to distribute online services through a process known as distributed computing. Furthermore, all the servers are giving the same service to the users irrespective of the one the user is connected to.

Packet switching and circuit switching are two networking methods for transferring data between two nodes or hosts. However, they differ on purpose, connections, layers used, flexibility and even the technology applied. The table below illustrate this differences.

• Packet switched and circuit switched paradigms

Table showing the difference between circuit switching and packet switching

 Basis of comparison Circuit switching Packet switching orientation Its connection oriented Do not use connections flexibility Inflexible Since it follows a singular path. Flexible as each packet travels on its own path. order The message received is in the same order as the source. Information are received in different packets which are assembled upon reaching the destination technology It uses space division switching or time division switching technology approach It uses virtual circuit approach or datagram approach purpose It was intended to be used in voice communication Its initial intention was to be used in data transmission. layers It is implemented at physical layer Implemented at network layer

The major differences between a wireless network and a wired one is that the former is not connected to the router using cables, Instead, wireless radio connections known as Wireless Fidelity is applied. Wi-Fi devices only require antennas an access point and a Wi-Fi client to communicate. The speed and range used in a Wi-Fi network are determined by the Wi-Fi standards.

• Wi-Fi standards and their purposes

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