IT133 – Week 3 Assignment
IT133 – Week 3 Assignment
4B/5B is a block encoding system that breaks up long strings of zeroes and ones but does not increase the frequency bandwidth by breaking them up in four-bit sequence. Table 1 shows each block of four bits replaced by a block of five bits. Due to this, there cannot be more than one leading zero and two trailing zeros. This means that the resulting bit stream has no more than three consecutive zeros (Figure 1). This bit stream is transmitted using non-return to zero inverted (NRZI) method and is used in 100BaseTX (Fast Ethernet) networks in conjunction with multi-level encoding scheme MLT-3 as only half the five-bit codes are used.
Figure 2 represents the bits 0 and 1 in Manchester encoding with the signal transition occurring in the middle of the transmission (Figure 3). It finds use in 10 Mbs 10BaseT Ethernet network. However, the level of the signal alternates every clock signal. This can lead to a broader frequency spectrum. Due to this, the spectrum of this signal goes beyond the high-frequency limit of the unshielded twisted pair Ethernet cables. Therefore, it is not fit for 100 Mbs or higher Ethernet networks. For 4B/5B encoding, the baud rate is equal to the bps, but since it involves a conversion before transmission, there is an overhead of 20 percent. In the case of the Manchester encoding, baud rate is twice the bps, which means there is a 100 percent overhead. Therefore, 4B/5B encoding is preferred for faster transmission.
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Iridium Satellite constellation is a Low Earth Orbiting (LEO) satellite constellation that provides voice and data communications over the entire earth. It consists of 66 satellites to provide coverage over the entire earth. The satellites are at a height of about 780 kilometers from the earth. The number of satellites depends on mainly six factors, one of which is the altitude of the orbit. Generally, 40 to 80 LEO satellites are required for continuous coverage. Figure 4 shows the coverage over a part of the earth. Iridium will launch 82 satellites costing 2.9 billion USD for the satellites and 600 million USD to launch them. This is costlier than INMARSAT, which launched four GEO satellites at 1.5 billion USD to provide global coverage using GEO satellites at 36,000 km orbit.
Figure 4: Iridium LEO satellite constellation
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Figure 5: Geostationary Orbit satellites
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Cost of the satellite varies based on the requirements, to illustrate, Motorola made satellites costing only 5 million. With those satellites, the total cost for LEO with 80 satellites would be about 400 million for satellites, 600 million for launching, and 300 million for maintaining the infrastructure. A minimum of three GEO satellites are required for global coverage, so the cost would be less.
The Universal Serial Bus (USB) is a digital interface that uses a standard connector for connecting various parallel and serial peripherals to the computer. This obviates the necessity of converting the signals from digital to analog and back, reducing the noise. It is thin, takes up less space, and is plug-and-play so that the computer recognizes the peripheral dynamically on connection and establishes the connection. USB allows daisy chaining, provides charging capabilities, and supports half-duplex since the data transfer is bi-directional with only one device being able to transmit at one time. USB 1.1 had a maximum transfer speed of 12 Mbps, USB 2.0 had a speed of 480 Mbps, while USB 3.0 supports speeds up to 4.8 Gbps. USB 2.0 can support slow devices such as mice and keyboards (10 – 100 Kbps). It can support full-speed devices such as audio and compressed video (500 Kbps to 10 Mbps). It can also support high-speed devices such as video and broadband (greater than 10 Mbps).
The USB transfers both data and electricity using four components, using a four-wire cable. The VBUS and GND can carry 5-volts that can power the device. The other two wires, D+ and D- can carry signals and data. USB connectors can be four types: Connectors A and B, Mini Connectors A and B. Both the connectors have four pins while the mini connectors have five pins, the fifth of which is a signal pin and generally connected to the VBUS or GND pin. Connector A is the most commonly used connector. Figure 6 shows the various types of connectors.
Figure 6: The four types of USB connectors
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The USB uses a tiered star topology and can operate up to 127devices. The bus in the USB denotes it as a high-speed connection to which devices can attach. The computer polls the peripheral to find out if it has any data to transmit. The host controller, which is the USB interface to the computer, initiates all data transfers, which can be control transfers (used for device configuration), bulk-data transfers, interrupt data transfers (reliable data transfer over time), and isochronous transfers (continuous real-time transfer of data such as streaming video or audio).
References
Benthien, G. W. (2007). Digital encoding and decoding.
Blank, S. (2010, November 1). No business plan survives first contact with a customer: The 5.2 billion dollar mistake. Retrieved from steveblank.com: https://steveblank.com/2010/11/01/no-business-plan-survives-first-contact-with-a-customer-%E2%80%93-the-5-2-billion-dollar-mistake/
Fitchard, K. (2012, August 12). How Iridium took a chance on SpaceX and won. Retrieved from gigaom.com: https://gigaom.com/2012/08/27/how-iridium-took-a-chance-on-spacex-and-won/
Poole, I. (2017, January 17). Geostationary satellite orbit, GEO: details of the basics of the geostationary earth orbit, GEO, used by satellites. Retrieved from radio-electronics.com: http://www.radio-electronics.com/info/satellite/satellite-orbits/geostationary-earth-orbit.php
Singh, V. (2014, May 31). What is the difference between GEO, MEO, and LEO satellites? Retrieved from tech.queryhome.com: http://tech.queryhome.com/45871/what-is-the-difference-between-geo-meo-and-leo-satellites
White, C. M. (2016). Data communications and computer networks: A business user’s approach (8th ed.). Boston, MA: Course Technology.
