B.Sc. IT BT0086 (Semester 5, Mobile Computing) Assignment

Fall 2013 Assignment

Bachelor of Science in Information Technology (BSc IT) – Semester 5

BT0086 – Mobile Computing – 4 Credits

(Book ID: B1181)

Assignment Set (60 Marks)

1.      Write notes on FDMA and CDMA techniques.

Ans. -  Frequency Division Multiple Access (FDMA):- FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations. Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

Code Division Multiple Access (CDMA):- CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire bandwidth it has available. Multiple calls are overlaid on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.

All the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code is used to recover the signal. Because CDMA systems need to put an accurate time stamp on each piece of a signal, it refers the GPS system for this information. Between eight and 10, separate calls can be carried in the same channel space as one analog AMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800 MHz and 1900 MHz frequency bands. Ideally, TDMA and CDMA are transparent to each other. In practice, high power CDMA signals will raise the noise floor for TDMA receivers, and high power TDMA signals can cause overloading and jamming of CDMA receivers.

2.      Explain the three basic communication modes.

Ans. -  Three Modes of Communication

1. Interpersonal Mode: - The Interpersonal Mode is characterized by active negotiation of meaning among individuals. Participants observe and monitor one another to see how their meaning and intentions are being communicated. Adjustments and clarifications can be made accordingly. As a result, there is a higher probability of ultimately achieving the goal of successful communication in this mode than in the other two modes. The Interpersonal Mode is most obvious in conversation, but both the interpersonal and negotiated dimensions can be realized through reading and writing, such as the exchange of personal letters or electronic mail messages.

Interpersonal examples:-

1.      Exchange information via letters, e-mail/video mail, notes, conversations or interviews on familiar topics (e.g., school events, weekend activities, memorable experiences, family life).

2.      Express and compare opinions and preferences about information gathered regarding events, experiences and other school subjects.

3.      Clarify meaning (e.g., paraphrasing, questioning).

4.      Give and follow directions, instructions and requests (e.g., installing software, dance steps).

5.      Demonstrate the ability to acquire goods, services or information (e.g., using public transportation, making a hotel reservation, buying food).

2. Interpretive Mode: - The Interpretive Mode is focused on the appropriate cultural interpretation of meanings that occur in written and spoken form where there is no recourse to the active negotiation of meaning with the writer or the speaker. Such instances of “one-way” reading or listening include the cultural interpretation of texts, oral or written, must be distinguished from the notion of reading and listening “comprehension,” where the term could refer to understanding a text with an American mindset. Put another way, interpretation differs from comprehension in that the former implies the ability to “read (or listen) between the lines.”
Since the Interpretive Mode does not allow for active negotiation between the reader and the writer or the listener and the speaker, it requires a much more profound knowledge of culture from the outset. The more one knows about the other language and culture, the greater the chances of creating the appropriate cultural interpretations of a written or spoken text. It must be noted, however, that cultural literacy and the ability to read or listen between the lines are developed over time and through exposure to the language and culture.

Interpretive examples:-

1.      Follow directions, instructions and requests (e.g., recipes, travel directions, prompts on ATMs).

2.      Use listening and reading strategies (e.g., skimming and scanning techniques) to determine main ideas and purpose.

3. Presentational Mode: - The Presentational Mode refers to the creation of messages in a manner that facilities interpretation by members of the other culture where no direct opportunity for active negotiation of meaning between members of the two cultures exists. Examples of the “one-way” writing and speaking require a substantial knowledge of language and culture from the outset, since the goal is to make sure that members of the other culture, the audience, will be successful in reading and listening between the lines.

Presentational examples:-

1.      Summarize information from authentic language materials and artifacts (e.g., TV programs, articles from youth magazines, Internet, videos, currency) and give personal reactions.

2.      Use information acquired from target language sources to solve everyday problems and situations (e.g., using a newspaper to make plans to see a movie, perusing a catalog to shop for a birthday gift, watching a weather forecast to help plan an activity).

3.      Create and present a narrative (e.g., current events, personal experiences, school happenings).

4.      Present differences in products and practices (e.g., sports, celebrations, school life) found in the target culture.

5.      Prepare and deliver a summary of characters and plot in selected pieces of literature.

Apply age-appropriate writing process strategies (prewriting, drafting, and revising, editing, publishing).

1.      Differentiate circuit switching and packet switching techniques.

Ans. -  Packet Switching: - Packet-switched networks move data in separate, small blocks -- packets -- based on the destination address in each packet. When received, packets are reassembled in the proper sequence to make up the message. Circuit-switched networks require dedicated point-to-point connections during calls.

In packet-based networks, however, the message gets broken into small data packets that seek out the most efficient route as circuits become available. Each packet may go a different route; its header address tells it where to go and describes the sequence for reassembly at the destination computer.

Circuit Switching: - Circuit-switched networks and packet-switched networks have traditionally occupied different spaces within corporations. Circuit-switched networks were used for phone calls and packet-switched networks handled data. But because of the reach of phone lines and the efficiency and low cost of data networks, the two technologies have shared chores for years. 

The primary hardware for a circuit-switched network is the private branch exchange (PBX) system. Computer servers power packet-switched networks.

In modern circuit-switched networks, electronic signals pass through several switches before a connection is established. And during a call, no other network traffic can use those switches.

A basic technical distinction between mobile data networks is whether they are circuit-switched or packet-switched. As a rule of thumb, all analogue and early 2G digital PCS networks provide circuit-switched data services. Newer technologies, such as 2.5G and 3G networks will also offer packet-switched service.

2.      Explain any two tables driven routing protocol.

Ans. -  Fisheye state routing: - Fisheye State Routing (FSR) is an improvement of GSR. The large size of update messages in GSR wastes a considerable amount of network bandwidth. In FSR, each update message does not contain information about all nodes. Instead, it exchanges information about closer nodes more frequently than it does about farther nodes, thus reducing the update message size. Each node receives accurate information about neighbours, and the detail and accuracy of information decreases as the distance from the node increases. The scope is defined in terms of the nodes that can be reached in a certain number of hops. The center node has the most accurate information about the other nodes. Even though a node does not have accurate information about distant nodes, the packets are routed correctly because the route information becomes more and more accurate as the packet moves closer to the destination. FSR scales well to large networks as the overhead is controlled in this scheme.

Hierarchical state routing: - The characteristic feature of Hierarchical State Routing (HSR) is multilevel clustering and logical partitioning of mobile nodes. The network is partitioned into clusters and a cluster head is elected as in a cluster-based algorithm. In HSR, the cluster heads again organize themselves into clusters and so on. The nodes of a physical cluster broadcast their link information to each other. The cluster head summarizes its cluster’s information and sends it to the neighbouring cluster heads via the gateway. These cluster heads are members of the cluster on a level higher and they exchange their link information as well as the summarized lower-level information among themselves and so on. A node at each level floods to its lower level the information that it obtains after the algorithm has run at that level. The lower level has hierarchical topology information. Each node has a hierarchical address. One way to assign hierarchical address is to use the cluster numbers on the way from the root to the node. A gateway can be reached from the root via more than one path. Thus, a gateway can have more than one hierarchical address. A hierarchical address is enough to ensure delivery from anywhere in the network to the host.

In addition, nodes are also partitioned into logical subnetworks and each node is assigned a logical address <subnet, host>. Each subnetwork has a Location Management Server (LMS). All the nodes of that subnet register their logical address with the LMS. The LMS advertise their hierarchical address to the top levels and the information is sent down to all LMS too. The transport layer sends a packet to the network layer with the logical address of the destination. The network layer finds the destination’s LMS from its LMS and then sends the packet to it. The destination’s LMS forwards the packet to the destination. Once the source and destination know each other’s hierarchical addresses, they can bypass the LMS and communicate directly. Since logical address/hierarchical address is used for routing, it is adaptable to network changes.

3.      Explain BCH, CCCH, DCCH control channels.

Ans. -  Broadcast Control Channel: - The Broadcast Control Channel is transmitted by the BTS at all times. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. The MS monitors the information carried on the BCCH periodically (at least every 30 secs), when it is switched on and not in a call.

The BCCH Consists of:

A.    Broadcast Control Channel (BCCH): Carries the following information:

·         Location Area Identity (LAI).

·         List of neighboring cells that should be monitored by the MS.

·         List of frequencies used in the cell.

·         Cell identity.

·         Power control indicator.

·         DTX permitted.

·         Access control (i.e., emergency calls, call barring ... etc.).

·         CBCH description.

The BCCH is transmitted at constant power at all times, and all MS that may seek to use it to measure its signal strength. “Dummy” bursts are transmitted to ensure continuity when there is no BCCH carrier traffic.

B.     Frequency Correction Channel (FCCH): This is transmitted frequently on the BCCH timeslot and allows the mobile to synchronize its own frequency to that of the transmitting base site. The FCCH may only be sent during timeslot 0 on the BCCH carrier frequency and therefore it acts as a flag to the mobile to identify Timeslot 0.

C.     Synchronization Channel (SCH) The SCH carries the information to enable the MS to synchronize to the TDMA frame structure and know the timing of the individual timeslots. The following parameters are sent:

·         Frame number.

·         Base Site Identity Code (BSIC).

The MS will monitor BCCH information from surrounding cells and store the information from the best six cells. The SCH information on these cells is also stored so that the MS may quickly resynchronize when it enters a new cell.

Common Control Channels: - The Common Control Channel (CCCH) is responsible for transferring control information between all mobiles and the BTS. This is necessary for the implementation of “call origination” and “call paging” functions. It consists of the following:

A.    Random Access Channel (RACH) Used by the mobile when it requires gaining access to the system. This occurs when the mobile initiates a call or responds to a page.

B.     Paging Channel (PCH) Used by the BTS to page MS, (paging can be performed by an IMSI, TMSI or IMEI).

C.     Access Grant Control Channel (AGCH) Used by the BTS to assign a dedicated control channel to a MS in response to an access message received on the Random Access Channel. The MS will move to the dedicated channel in order to proceed with either a call setup, response to a paging message, Location Area Update or Short Message Service.

D.    Cell Broadcast Channel (CBCH) This channel is used to transmit messages to be broadcast to all MS’s within a cell. The CBCH uses a dedicated control channel to send its messages, however it is considered a common channel because all mobiles in the cell can receive the messages.

Active MS’s must frequently monitor both BCCH and CCCH. The CCCH will be transmitted on the RF carrier with the BCCH.

Dedicated Control Channels: - The DCCH is a single timeslot on an RF carrier that is used to convey eight Stand-alone Dedicated Control Channels (SDCCH). A single MS for call setup, authentication, location updating and SMS point to point use a SDCCH. As we will see later, SDCCH can also be found on a BCCH/CCCH timeslot, this configuration only allows four SDCCHs.

A.    Slow Associated Control Channel (SACCH) Conveys power control and timing information in the downlink direction (towards the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports in the uplink direction.

B.     Fast Associated Control Channel (FACCH) The FACCH is transmitted instead of a TCH. The FACCH ‘‘steals” the TCH burst and inserts its own information. The FACCH is used to carry out user authentication, handovers and immediate assignment.

All of the control channels are required for system operation, however, in the same way that we allow different users to share the radio channel by using different timeslots to carry the conversation data, the control channels share timeslots on the radio channel at different times. This allows efficient passing of control information without wasting capacity that could be used for call traffic. To do this we must organize the timeslots between those, which will be used for traffic, and those, which will carry control signaling.

4.      Write short notes on:

a.      Wireless Routing Protocol

Ans. - Wireless Routing Protocol: - The Wireless Routing Protocol (WRP) is a table-based distance-vector routing protocol. Each node in the network maintains a Distance table, an RT, a Link-cost table, and a Message Retransmission List (MRL). The Distance table of a node x contains the distance of each destination node y via each neighbour z of x. It also contains the downstream neighbour of z through which this path is realized. The RT of node x contains the of each destination node y from node x, the predecessor and the successor of node x on this path. It also contains a tag to identify if the entry is a simple path, a loop, or invalid. Storing predecessor and successor in the table is beneficial in detecting loops and avoiding counting-to-infinity problems. The Link-cost table contains cost of link to each neighbour of the node and the number of timeouts since an error-free message was received from that neighbour. The MRL contains information to let a node know which of its neighbours has not acknowledged its update message and to retransmit the update message to that neighbour.

Nodes exchange RTs with their neighbours using update messages periodically as well as on link changes. The nodes present on the response list of update message (formed using MRL) are required to acknowledge the receipt of the update message. If there is no change in RT since the last update, the node is required to send an idle Hello message to ensure connectivity. On receiving an update message, the node modifies its distance table and looks for better paths using new information. Any new path so found is relayed back to the original nodes so that they can update their tables. The node also updates its RT if the new path is better than the existing path. On receiving an Acknowledgement ACK, the node updates its MRL. A unique feature of this algorithm is that it checks the consistency of all its neighbours every time it detects a change in link of any of its neighbours. A consistency check in this manner helps eliminate looping situations in a better way and it also has fast convergence.

b.      Global State Routing

Ans. -  Global State Routing: - Global State Routing (GSR) is similar to DSDV. It takes the idea of link state routing but improves it by avoiding flooding of routing messages. In this algorithm, each node maintains a Neighbour list, a Topology table, a Next Hop table, and a Distance table. Neighbour list of a node contains the list of its neighbours (here all nodes that can be heard by a node are assumed to be its neighbours). For each destination node, the Topology table contains the link state information as reported by the destination and the timestamp of the information. For each destination, the Next Hop table contains the next hop to which the packets for this destination must be forwarded. The Distance table contains the shortest distance to each destination node.

The routing messages are generated on a link change as in link state protocols. On receiving a routing message, the node updates its Topology table if the sequence number of the message is newer than the sequence number stored in the table. After this, the node reconstructs its RT and broadcasts the information to its neighbours.

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