Sam is a computer science student and working on a new micro robot design. The robot continuously sends every two seconds a status string comprising the accelerometer reading (4 bits), ultrasound obstacle detection (6 bits), motor functionality (4 bits) and battery power level (2 bits).
- Calculate the data rate required for robot to remote controller communication
- Explain 3 types of suitable encoding techniques could be used to encode the status string.
- At a certain instance, the sensors provided data as acceleration 5 m/s2, obstacle at 48 cm, all motors working (i.e. motor functionality 1111), and battery is 75%.
- Write the status string in binary for this instance
- Represent the status string on ASK, FSK, and PSK encoding techniques
- Calculate the CRC for the status string derived in c) with polynomial divisor 11001101
- Briefly explain some other error control and flow control techniques that Sam can use to ensure the accuracy of robot control
Multiplexing plays an important role in communication as it allows the combination of multiple streams together. Different multiplexing techniques are used for various applications.
- Explain TDM, FDM and CDMA multiplexing techniques with suitable diagrams
- Main air interface in the change from 3G to 4G and WiMAX. Explain how OFDM is different from above techniques and the special feature of signals used by OFDM
- IEEE 802.11ac wireless LAN standard uses 40 MHz total bandwidth in OFDM scheme.
- For 48 subscribers what should be the subcarrier bandwidth ( fb ) ?
- Propose a suitable subcarrier bit time of T to achieve orthogonality
- Explain how OFDM overcomes the issue of inter symbol interference (ISI)
Prime Living is a real estate company based in Victoria and you work form them as a consultant network engineer. The company has decided to open a new office in South Eastern suburbs and the floor plan of the building is shown below. The office consists of 5 rooms and on average 8 employees work at each room. The lounge is used by both staff and customers and on average about 25 devices connect to Wi-Fi. The clients wants you to design the network for them with at least 100 Mbps data rate for all wireless connections. As resources you’ve got one distribution system and few access points of 10 m range.
- Design the network specifying the locations of access points and distributions system showing the backbone network.
- Calculate the BSS and ESS sizes
- Calculate the throughput for the DS
- Recommend a suitable IEEE substandard for the network and give reasons for your choice
- Recommend suitable security strategies for the network
Communication and Multiplexing
- Calculate the data rate required for the robot to remote controller communication
The data rate is how far we can send data from A to B every second
From the diagram the robot sends 16bits that is 4bits accelerometer, 6bits ultrasound detection, 4 bits motor functionality and 2 bits battery level. The total is 16bits bits string
Data rate is 8bps
(Koenig et al., 2013)
For ensuring the best utilization of bandwidth the following line encoding techniques could be used in the robot set up
The unipolar is the simplest and here you have got the two voltage level. Uni means 1 so you are sending an only voltage of one polarity say 2A. And here you are not sending anything that is considered top be 0 V (Tsonev, Sinanovic, & Haas, 2012). That means when you are not sending anything that is 0V and when sending something that is 2A volts.
This is illustrated below
In Manchester encoding, there is a transition in the middle of each bit sent. A binary one represents a low to high transition and a binary zero represent a high to low transition in the middle(Lee & Pan, 2015). This is illustrated in the figure below
In differential Manchester, the presence of a transition at the beginning of the bit is represented by binary zero. This is shown in the figure below
- Write the status string in binary for this instance
The status string can be represented by forming an array of the various individual bits that form the message sent by the robot. This array includes the accelerometer reading, ultrasound, motor reading and the battery level all converted to binary
Convert all the message provided in binary
The accelerometer is 5m/s2 which when converted to binary brings 0101
Ultrasound detects an obstacle at 48cm. When converted to binary we get 110000
The motor status is given already in binary hence requires no conversion that is 1111
The battery level is 75% which in decimal is 0.75 in decimal. This gives 0.11. which is just 01
The status string, therefore, is 0101110000111101
Represent the status string on ASK, FSK, and PSK encoding techniques
In amplitude shift keying, the carrier signal is always transmitted whenever there is a logic one being sent over the media while no carrier signal is transmitted whenever logic zero is transmitted over the network(Meltzer, 2011). This is shown below
In frequency shift keying, the frequency of the carrier wave is varied or switched whenever logic one is transmitted while no change is done to the carrier wave whenever logic zero is transmitted (Manfrini et al., 2011). This is as shown below.
In phase shift key, the phase of the carrier wave is change by 180o whenever there is a transition from a logic one to logic zero or from logic zero to logic one but no change in phase is done whenever there is no transition in the data bits(Chow, Talli, Ellis, & Townsend, 2008). This is as shown below.
Calculate the CRC for the status string derived in 3) with polynomial divisor 11001101
Understanding the Status String
Cyclic redundancy check is an error correcting algorithm that uses redundant codes sent along with the data to the receiver and the receiver uses the preshared code to detect whether what was sent is actually what has been received (Olson, Grohoski, & Spracklen, 2013). The process of calculating a CRC is as follows,
Get the data word bits. In this case, the data word is a 16bits string as shown below.
Get the divisor bits. In this case, it is given as an 8bits string that is 11001101
Calculate the number of additional bits to add to the data word. To get this subtract 1 from the bit length of the divisor to get the off bits to add to the data word
Lenght of divisor=8-1
Therefore the off bits to add to data word is 0000000
The new data word is 01011100001111010000000. This is now called the Augmented data word
Get the code word. To do this, we have to do the long division of the augmented data word with the divisor.
This step can be done more intuitively using the polynomial long division.
Convert the augmented data word into polynomial and again convert the divisor into polynomial then conduct the polynomial long division.
The remainder after conducting the CRC shall be used to check for errors. The quotient shall form the coded word and it is what gets transmitted over to the receiver.
At the receiver, the receiver, the receiver shall perform a polynomial long division of the code word and the pre-shared divisor. If the remainder is zero, then the receiver can conclusively make the assumption that no errors occurred during the transmission while a non-zero error would signify there are errors in the transmitted message and request for retransmission.
The steps are summarised below.
In this protocol, whenever a packet is sent to the receiver, the receiver actually replies in form of acknowledgments. After receiving the acknowledgment, the sender will get this idea that the packet has been received successfully. The sender encapsulates a timer for every packet sent. If the sender receives an acknowledgment within the duration of the timer then that's okay but if the timer expires without receiving an acknowledgment, then the sender will have to resend the packet again.(Cui, Wang, Wang, Wang, & Wang, 2015) The above is illustrated below
FEC(Forward Error Control0
FEC is a method of error control that is used to detect and correct errors over unreliable or noisy channels. The central idea is that the sender encodes the bits to be sent in a redundant way by using an error correcting bits using the Hamming code. The redundancy enable the receiver to try to correct a limited number of errors that may have occurred in the message and often corrects such errors without the need for retransmission by the senderFEC gives the receiver the ability to correct errors without the need of a reverse channel to request retransmission but at a cost of higher channel bandwidth (Reed & Chen, 2012)
Line Encoding Techniques - Unipolar, Manchester, Differential Manchester
In FDMA, the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data and it belongs to the station all the time. Suppose we have a bandwidth subdivided into four bands band1, band2, bannd3, band4 and each station is allocated a particular band and that band belongs to the station all the time. This is illustrated in the diagram below
In this method, the stations share the bandwidth of the channel in time using the round robin algorithm. Each station is allocated a time slot during which it can send. In TDMA, the bandwidth of the channel is just one I.e time-shared between different stations (Palanki & Khandekar, 2012).
In CDMA, one channel carries all transmission simultaneously.CDMA means communications with different codes and the codes must meet the following mathematical properties
- I) If codes are multiplied with each other, then the answer is zero.
- ii) If codes are multiplied by itself then we get 4[Number of Stations).
CDMA uses the principle of frequency hopping spread spectrum and direct sequence to spread the sender data with codes and send together with the coded data that only the receiver with the same unique code shall be able to receive and decode the message.
This is illustrated in the figure below
Orthogonal Frequency Division Multiplexing is quite similar to the FDMA since it majorly uses the fundamental principle of FDMA which allows simultaneous transmission of multiple messages over a single channel. OFDM differs with the traditional multiplexing techniques. It uses a number of parallel narrow-band subcarriers instead of a single wide-band carrier to transport information making it much effective and efficient in wireless communication. To achieve this, the signals used in an OFDM must be orthogonal to each other that is their dot multiplication of two adjacent signals is always zero(Ratasuk, Ghosh, & Xiao, 2012).
- For 48 subscribers what should be the subcarrier bandwidth ( fb)?
Bandwidth is Frequency/No of Subscribers
- Propose a suitable subcarrier bit time of T to achieve orthogonality
Bit time T is the reciprocal of the subcarrier bandwidth to achieve orthogonality
iii. Explain how OFDM overcomes the issue of intersymbol interference (ISI)
Inter-Symbol Interference is overcome in OFDM by ensuring that any two subcarrier signals are orthogonal to one another that is a dot multiplication of the two adjacent signals is always zero. This orthogonality property makes OFDM subcarriers to be closely packed to one another without causing ISI interference (Han & Zhang, 2009).
system showing the backbone network.
The Basic service set represents the area covered by each wifi hotspot calculated by the formulae L*W.
Lounge and reception=20*10
ESS, the extended service set represents the area covered by all the access points which are connected to the distribution system. This can be calculated by adding the individual areas covered by each access point.
ESS=set of BSS
=Room1+Room2+Room3+Room4+Room5+Lounge and reception
Throughput represents the actual amount of data passing through a communication link measured in Mbps.
For the above building, the throughput is the aggregation of the individual data rate requirements assuming this is a noiseless channel.
Representing Status String on ASK, FSK, and PSK Encoding Techniques
8 employees each wants a 100mbps connection.
The total throughput =8*100
Throughput room 2
8 employees each needs 100mbps connections
Throughput room 3
Number of employees * data rate requirement
Througput room 4
Througput room 5
Throughput through lounge
Number of people accessing lounge is 25
Data rate requirement per connection is 100mbps
Throughput is therefore 25*100
(Liew, Kai, Leung, & Wong, 2010)
802.11ac is the next generation of wifi that promises to deliver not only speed but also enhancing multiple users access to the wifi connection. This in effect improves the efficiency of the network as more devices such as mobile phones, laptops, and other wifi certified products shall be networked within the network. 802.11ac is again recommended standard since it enhances backward compatibility with the older standards. This makes it suitable for integrating with the legacy components of the network hence improving on utility. Thirdly, most new wifi certified devices come with the 802.11ac as the default standard hence it is imperative for the network to implement the 802.11ac to make it compatible with newer equipment that is 802.11ac certified. (Ong et al., 2011)
In order to ensure all the traffic through the network is secure and that all the data on transit through the network are not compromised in terms of confidentiality, integrity, and availability, the following security strategies and measures could be taken to secure the network.
First, ensure all the traffic is encrypted with the Advance encryption standards. This is the most secure data encryption algorithm going by today standards. To effect this, all the access points should be WPA2 enabled.
Second, choose a strong WPA2 key. A strong encryption system is as strong as the weakest link. Here, the encryption key which comes in form of passphrase should be very strong to reduce being compromised by dictionary and or brute force attacks which exploit vulnerabilities due to the weak password for the access points.
Third, it is best practice to always put the SSID broadcasting to off to ensure the eavesdropper does not get much information about the availability of the wifi. Such information linked to the SSID can be used by a sophisticated attacker to bypass the otherwise secure network(Stallings, Brown, Bauer, & Bhattacharjee, 2012).
Fourth, it is recommended to set up a firewall to filter all the traffic ingress and egress of the network to ensure malicious packets from the internet does not have a chance to get into the private network. The firewall should have also the Intrusion detection and prevention systems to alert the administrators of potential network intrusions attempts.
Fifth, it is recommended for the organization to set up ICT security policies and procedure to provide a legal framework that could be used to deter rogue employees from coming cyber crimes inside the organization's network. This policy should have a top-down approach in design and dissemination.
Last but not the least, all software and applications should be updated regularly and then patched and hotfixes applied promptly to reduce vulnerabilities due to software and applications configurations being exploited.
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