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Friday, December 17, 2010

Semiconductor Fundamental Problems

Calculate the packing density of the body centered cubic, the face centered cubic and the diamond lattice, listed in example 2.1
At what temperature does the energy bandgap of silicon equal exactly 1 eV?
Prove that the probability of occupying an energy level below the Fermi energy equals the probability that an energy level above the Fermi energy and equally far away from the Fermi energy is not occupied.
At what energy (in units of kT) is the Fermi function within 1 % of the Maxwell-Boltzmann distribution function? What is the corresponding probability of occupancy?
Calculate the Fermi function at 6.5 eV if E_F = 6.25 eV and T = 300 K. Repeat at T = 950 K assuming that the Fermi energy does not change. At what temperature does the probability that an energy level at E = 5.95 eV is empty equal 1 %.
Calculate the effective density of states for electrons and holes in germanium, silicon and gallium arsenide at room temperature and at 100 °C. Use the effective masses for density of states calculations.
Calculate the intrinsic carrier density in germanium, silicon and gallium arsenide at room temperature (300 K). Repeat at 100 °C. Assume that the energy bandgap is independent of temperature and use the room temperature values.
Calculate the position of the intrinsic energy level relative to the midgap energy

E_midgap = (E_c + E_v)/2

Calculate the electron and hole density in germanium, silicon and gallium arsenide if the Fermi energy is 0.3 eV above the intrinsic energy level. Repeat if the Fermi energy is 0.3 eV below the conduction band edge. Assume that T = 300 K.
The equations (2.6.37) and (2.6.38) derived in section 2.6 are only valid for non-degenerate semiconductors (i.e. E_v + 3kT < E_F < E_c - 3kT). Where exactly in the derivation was the assumption made that the semiconductor is non-degenerate?
A silicon wafer contains 10¹⁶ cm^-3 electrons. Calculate the hole density and the position of the intrinsic energy and the Fermi energy at 300 K. Draw the corresponding band diagram to scale, indicating the conduction and valence band edge, the intrinsic energy level and the Fermi energy level. Use n_i = 10¹⁰ cm^-3.
A silicon wafer is doped with 10¹³ cm^-3 shallow donors and 9 x 10¹² cm^-3 shallow acceptors. Calculate the electron and hole density at 300 K. Use n_i = 10¹⁰ cm^-3.
The resistivity of a silicon wafer at room temperature is 5 Wcm. What is the doping density? Find all possible solutions.
How many phosphorus atoms must be added to decrease the resistivity of n-type silicon at room temperature from 1 Wcm to 0.1 Wcm. Make sure you include the doping dependence of the mobility. State your assumptions.
A piece of n-type silicon (N_d = 10¹⁷ cm^-3) is uniformly illuminated with green light (l = 550 nm) so that the power density in the material equals 1 mW/cm². a) Calculate the generation rate of electron-hole pairs using an absorption coefficient of 10⁴ cm^-1.b) Calculate the excess electron and hole density using the generation rate obtained in (a) and a minority carrier lifetime due to Shockley-Read-Hall recombination of 0.1 ms. c) Calculate the electron and hole quasi-Fermi energies (relative to E_i) based on the excess densities obtained in (b).
A piece of intrinsic silicon is instantaneously heated from 0 K to room temperature (300 K). The minority carrier lifetime due to Shockley-Read-Hall recombination in the material is 1 ms. Calculate the generation rate of electron-hole pairs immediately after reaching room temperature. (E_t = E_i).If the generation rate is constant, how long does it take to reach thermal equilibrium?
Calculate the conductivity and resistivity of intrinsic silicon. Use n_i = 10¹⁰ cm^-3, m_n = 1400 cm²/V-sec and m_p = 450 cm²/V-sec.
Consider the problem of finding the doping density, which results the maximum possible resistivity of silicon at room temperature. (n_i = 10¹⁰, m_n = 1400 cm²/V-sec and m_p = 450 cm²V-sec.)
The electron density in silicon at room temperature is twice the intrinsic density. Calculate the hole density, the donor density and the Fermi energy relative to the intrinsic energy. Repeat for n = 5 n_i and n = 10 n_i. Also repeat for p = 2 n_i, p = 5 n_i and p = 10 n_i, calculating the electron and acceptor density as well as the Fermi energy relative to the intrinsic energy level.
The expression for the Bohr radius can also be applied to the hydrogen-like atom consisting of an ionized donor and the electron provided by the donor. Modify the expression for the Bohr radius so that it applies to this hydrogen-like atom. Calculate the Bohr radius of an electron orbiting around the ionized donor in silicon. ( e_r = 11.9 and m_e^* = 0.26 m₀)
Calculate the density of electrons per unit energy (in electron volt) and per unit area (per cubic centimeter) at 1 eV above the band minimum. Assume that m_e^* = 1.08 m₀.
Calculate the probability that an electron occupies an energy level, which is 3kT below the Fermi energy. Repeat for an energy level which is 3kT above the Fermi energy.
Calculate and plot as a function of energy the product of the probability that an energy level is occupied with the probability that that same energy level is not occupied. Assume that the Fermi energy is zero and that kT = 1 eV
The effective mass of electrons in silicon is 0.26 m₀ and the effective mass of holes is 0.36 m₀. If the scattering time is the same for both carrier types, what is the ratio of the electron mobility and the hole mobility.
Electrons in silicon carbide have a mobility of 1000 cm²/V-sec. At what value of the electric field do the electrons reach a velocity of 3 x 10⁷ cm/s? Assume that the mobility is constant and independent of the electric field. What voltage is required to obtain this field in a 5-micron thick region? How much time do the electrons need to cross the 5 micron thick region?
A piece of silicon has a resistivity which is specified by the manufacturer to be between 2 and 5 Ohm cm. Assuming that the mobility of electrons is 1400 cm²/V-sec and that of holes is 450 cm²/V-sec, what is the minimum possible carrier density and what is the corresponding carrier type? Repeat for the maximum possible carrier density.
A silicon wafer has a 2-inch diameter and contains 10¹⁴ cm^-3 electrons with a mobility of 1400 cm²/V-sec. How thick should the wafer be so that the resistance between the front and back surface equals 0.1 Ohm?
The electron mobility is germanium is 1000 cm²/V-sec. If this mobility is due to impurity and lattice scattering and the mobility due to lattice scattering only is 1900 cm²/V-sec, what is the mobility due to impurity scattering only?
An atomic system consists of three energy levels with energy 0, 10 and 20 meV, which can contain a maximum of 1000, 2000 and 1000 electrons, respectively. The total energy of the system in thermal equilibrium is 25 eV and the total number of electrons is 2000. Calculate the Fermi energy and the temperature. (Challenge problem)
A 20 mm thin piece of gallium arsenide consists of two regions with a different carrier life time, namely t = 20 ns for 0 £ x £ 10 mm and t = ¥ for 10 £ x £ 20 mm. The material is illuminated with light so that 10²² cm^-2s^-1 electron-hole pairs are created at x = 20 mm. Calculate the steady state electron and hole density at x = 0, 10 and 20 mm. (Assume m_n = m_p = 1000 cm²/V-s and T = 300 K)
A 10-micron thin piece of n-type silicon (N_d = 10¹⁷ cm^-3) is uniformly illuminated with light, resulting in an electron-hole pair generation rate of 10²³ cm^-3s^-1. The silicon is contacted only on one side with an ideal Ohmic contact. Assume there is no recombination in the semiconductor. (Use m_n = 1000 cm²/V-s and m_p = 300 cm²/V-s)
Consider a semiconductor with two parabolic conduction bands having conduction band minima E_c1 and E_c2 with effective masses m_e1 = 0.06 m₀ and m_e2 = 0.4 m₀ and mobility m_n1 = 8000 cm²/V-s and m_n2 = 1000 cm²/V-s. The conduction band minimum E_c2 is 30 meV higher than E_c1.

N_c^*

E_g

E_c1

E_v

m_n^*

s_n

m_n^*

An intrinsic piece of GaAs (n_i = 2 x 10⁶ cm^-3 ) of length L (= 1 mm) is uniformly illuminated with light yielding an electron-hole pair generation rate of 10²² cm^-3 s^-1. On both ends (at x = 0 and x = L) the material is contacted with “ideal” ohmic contacts. Calculate the maximum value of the steady-state excess hole density in the material under illumination. The hole diffusion length, L_p, equals L/ln(2) and the hole mobility is 500 cm²/V-s. Assume that there is no electric field in the semiconductor and T = 300 K.
An intrinsic piece of GaAs (n_i = 2 x 10⁶ cm^-3 ) of length L (= 1 mm) is uniformly illuminated with light yielding an electron-hole pair generation rate of 10²² cm^-3 s^-1. On one end (at x = 0) the material is contacted with an “ideal” ohmic contact, while on the other end (at x = L) the current is limited by a finite recombination velocity, s, (s = 10⁵ cm/s), as described by the following relation between the excess hole density, d_p, and the hole current density, J_p:
Calculate the maximum value of the steady-state excess hole density in the material under illumination. The hole diffusion length, L_p, equals L/ln(2) and the hole mobility is 500 cm²/V-s. Assume that there is no electric field in the semiconductor and T = 300 K.

Data Structure

What is data structure?

A data structure is a way of organizing data that considers not only the items stored, but also their relationship to each other. Advance knowledge about the relationship between data items allows designing of efficient algorithms for the manipulation of data.

List out the areas in which data structures are applied extensively?

Compiler Design,
Operating System,
Database Management System,
Statistical analysis package,
Numerical Analysis,
Graphics,
Artificial Intelligence,
Simulation

What are the major data structures used in the following areas : RDBMS, Network data model & Hierarchical data model.

RDBMS – Array (i.e. Array of structures)
Network data model – Graph
Hierarchical data model – Trees

If you are using C language to implement the heterogeneous linked list, what pointer type will you use?

The heterogeneous linked list contains different data types in its nodes and we need a link, pointer to connect them. It is not possible to use ordinary pointers for this. So we go for void pointer. Void pointer is capable of storing pointer to any type as it is a generic pointer type.

Minimum number of queues needed to implement the priority queue?

Two. One queue is used for actual storing of data and another for storing priorities.

What is the data structures used to perform recursion?

Stack. Because of its LIFO (Last In First Out) property it remembers its 'caller' so knows whom to return when the function has to return. Recursion makes use of system stack for storing the return addresses of the function calls.

Every recursive function has its equivalent iterative (non-recursive) function. Even when such equivalent iterative procedures are written, explicit stack is to be used.

What are the notations used in Evaluation of Arithmetic Expressions using prefix and postfix forms?

Polish and Reverse Polish notations.

Convert the expression ((A + B) * C – (D – E) ^ (F + G)) to equivalent Prefix and Postfix notations.

Prefix Notation:

^ - * +ABC - DE + FG
Postfix Notation:

AB + C * DE - - FG + ^

Sorting is not possible by using which of the following methods?

(a) Insertion

(b) Selection

(c) Exchange

(d) Deletion

(d) Deletion.

Using insertion we can perform insertion sort, using selection we can perform selection sort, using exchange we can perform the bubble sort (and other similar sorting methods). But no sorting method can be done just using deletion.

A binary tree with 20 nodes has null branches?

Let us take a tree with 5 nodes (n=5)

It will have only 6 (ie,5+1) null branches. In general,

A binary tree with n nodes has exactly n+1 null nodes.

What are the methods available in storing sequential files ?

Straight merging,
Natural merging,
Polyphase sort,
Distribution of Initial runs.

How many different trees are possible with 10 nodes ?

1014

For example, consider a tree with 3 nodes(n=3), it will have the maximum combination of 5 different (ie, 2³- 3 = 5) trees.

i ii iii iv v

In general:

If there are n nodes, there exist 2ⁿ-n different trees.

List out few of the Application of tree data-structure?

The manipulation of Arithmetic expression,
Symbol Table construction,
Syntax analysis.

List out few of the applications that make use of Multilinked Structures?

Sparse matrix,
Index generation.

In tree construction which is the suitable efficient data structure?

(a) Array (b) Linked list (c) Stack (d) Queue (e) none

(b) Linked list

What is the type of the algorithm used in solving the 8 Queens problem?

Backtracking

In an AVL tree, at what condition the balancing is to be done?

If the 'pivotal value' (or the 'Height factor') is greater than 1 or less than –1.

What is the bucket size, when the overlapping and collision occur at same time?

One. If there is only one entry possible in the bucket, when the collision occurs, there is no way to accommodate the colliding value. This results in the overlapping of values.

Traverse the given tree using Inorder, Preorder and Postorder traversals.

Inorder : D H B E A F C I G J
Preorder: A B D H E C F G I J
Postorder: H D E B F I J G C A

There are 8, 15, 13, 14 nodes were there in 4 different trees. Which of them could have formed a full binary tree?

15.
In general:

There are 2ⁿ-1 nodes in a full binary tree.

By the method of elimination:

Full binary trees contain odd number of nodes. So there cannot be full binary trees with 8 or 14 nodes, so rejected. With 13 nodes you can form a complete binary tree but not a full binary tree. So the correct answer is 15.

Note:

Full and Complete binary trees are different. All full binary trees are complete binary trees but not vice versa.

In the given binary tree, using array you can store the node 4 at which location?

At location 6

Root

LC1

RC1

LC2

RC2

LC3

RC3

LC4

RC4

where LCn means Left Child of node n and RCn means Right Child of node n

Sort the given values using Quick Sort?

Sorting takes place from the pivot value, which is the first value of the given elements, this is marked bold. The values at the left pointer and right pointer are indicated using ^L and ^R respectively.

70^L

45^R

Since pivot is not yet changed the same process is continued after interchanging the values at ^L and ^R positions

75^L

50^R

80^L

55^R

85^L

60^R

85^L

When the L and R pointers cross each other the pivot value is interchanged with the value at right pointer. If the pivot is changed it means that the pivot has occupied its original position in the sorted order (shown in bold italics) and hence two different arrays are formed, one from start of the original array to the pivot position-1 and the other from pivot position+1 to end.

60^L

55^R

85^L

70^R

55^L

50^R

70^R

80^L

50^L

45^R

80^L

75^R

In the next pass we get the sorted form of the array.

For the given graph, draw the DFS and BFS?

BFS: A X G H P E M Y J
DFS: A X H P E Y M J G

Classify the Hashing Functions based on the various methods by which the key value is found.

Direct method,
Subtraction method,
Modulo-Division method,
Digit-Extraction method,
Mid-Square method,
Folding method,
Pseudo-random method.

What are the types of Collision Resolution Techniques and the methods used in each of the type?

Open addressing (closed hashing),
The methods used include:
Overflow block,
Closed addressing (open hashing)
The methods used include:
Linked list,
Binary tree…

In RDBMS, what is the efficient data structure used in the internal storage representation?

B+ tree. Because in B+ tree, all the data is stored only in leaf nodes, that makes searching easier. This corresponds to the records that shall be stored in leaf nodes.

Draw the B-tree of order 3 created by inserting the following data arriving in sequence – 92 24 6 7 11 8 22 4 5 16 19 20 78

Of the following tree structure, which is, efficient considering space and time complexities?
1. Incomplete Binary Tree
2. Complete Binary Tree
3. Full Binary Tree

(b) Complete Binary Tree.

By the method of elimination:

Full binary tree loses its nature when operations of insertions and deletions are done. For incomplete binary trees, extra storage is required and overhead of NULL node checking takes place. So complete binary tree is the better one since the property of complete binary tree is maintained even after operations like additions and deletions are done on it.

What is a spanning Tree?

A spanning tree is a tree associated with a network. All the nodes of the graph appear on the tree once. A minimum spanning tree is a spanning tree organized so that the total edge weight between nodes is minimized.

Does the minimum spanning tree of a graph give the shortest distance between any 2 specified nodes?

No.

Minimal spanning tree assures that the total weight of the tree is kept at its minimum. But it doesn't mean that the distance between any two nodes involved in the minimum-spanning tree is minimum.

Convert the given graph with weighted edges to minimal spanning tree.

the equivalent minimal spanning tree is:

Which is the simplest file structure?
1. Sequential
2. Indexed
3. Random

(a) Sequential

Whether Linked List is linear or Non-linear data structure?

According to Access strategies Linked list is a linear one.

According to Storage Linked List is a Non-linear one.

Draw a binary Tree for the expression :

A * B - (C + D) * (P / Q)

For the following COBOL code, draw the Binary tree?

01 STUDENT_REC.

02 NAME.

03 FIRST_NAME PIC X(10).

03 LAST_NAME PIC X(10).

02 YEAR_OF_STUDY.

03 FIRST_SEM PIC XX.

03 SECOND_SEM PIC XX.

Networking concepts

What are the two types of transmission technology available?

(i) Broadcast and (ii) point-to-point

What is subnet?

A generic term for section of a large networks usually separated by a bridge or router.

Difference between the communication and transmission.

Transmission is a physical movement of information and concern issues like bit polarity, synchronisation, clock etc.

Communication means the meaning full exchange of information between two communication media.

What are the possible ways of data exchange?

(i) Simplex (ii) Half-duplex (iii) Full-duplex.

What is SAP?

Series of interface points that allow other computers to communicate with the other layers of network protocol stack.

What do you meant by "triple X" in Networks?

The function of PAD (Packet Assembler Disassembler) is described in a document known as X.3. The standard protocol has been defined between the terminal and the PAD, called X.28; another standard protocol exists between hte PAD and the network, called X.29. Together, these three recommendations are often called "triple X"

What is frame relay, in which layer it comes?

Frame relay is a packet switching technology. It will operate in the data link layer.

What is terminal emulation, in which layer it comes?

Telnet is also called as terminal emulation. It belongs to application layer.

What is Beaconing?

The process that allows a network to self-repair networks problems. The stations on the network notify the other stations on the ring when they are not receiving the transmissions. Beaconing is used in Token ring and FDDI networks.

What is redirector?

Redirector is software that intercepts file or prints I/O requests and translates them into network requests. This comes under presentation layer.

What is NETBIOS and NETBEUI?

NETBIOS is a programming interface that allows I/O requests to be sent to and received from a remote computer and it hides the networking hardware from applications.

NETBEUI is NetBIOS extended user interface. A transport protocol designed by microsoft and IBM for the use on small subnets.

What is RAID?

A method for providing fault tolerance by using multiple hard disk drives.

What is passive topology?

When the computers on the network simply listen and receive the signal, they are referred to as passive because they don't amplify the signal in any way. Example for passive topology - linear bus.

What is Brouter?

Hybrid devices that combine the features of both bridges and routers.

What is cladding?

A layer of a glass surrounding the center fiber of glass inside a fiber-optic cable.

What is point-to-point protocol

A communications protocol used to connect computers to remote networking services including Internet service providers.

How Gateway is different from Routers?

A gateway operates at the upper levels of the OSI model and translates information between two completely different network architectures or data formats

What is attenuation?

The degeneration of a signal over distance on a network cable is called attenuation.

What is MAC address?

The address for a device as it is identified at the Media Access Control (MAC) layer in the network architecture. MAC address is usually stored in ROM on the network adapter card and is unique.

Difference between bit rate and baud rate.

Bit rate is the number of bits transmitted during one second whereas baud rate refers to the number of signal units per second that are required to represent those bits.

baud rate = bit rate / N

where N is no-of-bits represented by each signal shift.

What is Bandwidth?

Every line has an upper limit and a lower limit on the frequency of signals it can carry. This limited range is called the bandwidth.

What are the types of Transmission media?

Signals are usually transmitted over some transmission media that are broadly classified in to two categories.

Guided Media:

These are those that provide a conduit from one device to another that include twisted-pair, coaxial cable and fiber-optic cable. A signal traveling along any of these media is directed and is contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic that accept and transport signals in the form of electrical current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.

b) Unguided Media:

This is the wireless media that transport electromagnetic waves without using a physical conductor. Signals are broadcast either through air. This is done through radio communication, satellite communication and cellular telephony.

What is Project 802?

It is a project started by IEEE to set standards to enable intercommunication between equipment from a variety of manufacturers. It is a way for specifying functions of the physical layer, the data link layer and to some extent the network layer to allow for interconnectivity of major LAN

protocols.

It consists of the following:

802.1 is an internetworking standard for compatibility of different LANs and MANs across protocols.
802.2 Logical link control (LLC) is the upper sublayer of the data link layer which is non-architecture-specific, that is remains the same for all IEEE-defined LANs.
Media access control (MAC) is the lower sublayer of the data link layer that contains some distinct modules each carrying proprietary information specific to the LAN product being used. The modules are Ethernet LAN (802.3), Token ring LAN (802.4), Token bus LAN (802.5).
802.6 is distributed queue dual bus (DQDB) designed to be used in MANs.

What is Protocol Data Unit?

The data unit in the LLC level is called the protocol data unit (PDU). The PDU contains of four fields a destination service access point (DSAP), a source service access point (SSAP), a control field and an information field. DSAP, SSAP are addresses used by the LLC to identify the protocol stacks on the receiving and sending machines that are generating and using the data. The control field specifies whether the PDU frame is a information frame (I - frame) or a supervisory frame (S - frame) or a unnumbered frame (U - frame).

What are the different type of networking / internetworking devices?

Repeater:

Also called a regenerator, it is an electronic device that operates only at physical layer. It receives the signal in the network before it becomes weak, regenerates the original bit pattern and puts the refreshed copy back in to the link.

Bridges:

These operate both in the physical and data link layers of LANs of same type. They divide a larger network in to smaller segments. They contain logic that allow them to keep the traffic for each segment separate and thus are repeaters that relay a frame only the side of the segment containing the intended recipent and control congestion.

Routers:

They relay packets among multiple interconnected networks (i.e. LANs of different type). They operate in the physical, data link and network layers. They contain software that enable them to determine which of the several possible paths is the best for a particular transmission.

Gateways:

They relay packets among networks that have different protocols (e.g. between a LAN and a WAN). They accept a packet formatted for one protocol and convert it to a packet formatted for another protocol before forwarding it. They operate in all seven layers of the OSI model.

What is ICMP?

ICMP is Internet Control Message Protocol, a network layer protocol of the TCP/IP suite used by hosts and gateways to send notification of datagram problems back to the sender. It uses the echo test / reply to test whether a destination is reachable and responding. It also handles both control and error messages.

What are the data units at different layers of the TCP / IP protocol suite?

The data unit created at the application layer is called a message, at the transport layer the data unit created is called either a segment or an user datagram, at the network layer the data unit created is called the datagram, at the data link layer the datagram is encapsulated in to a frame and finally transmitted as signals along the transmission media.

What is difference between ARP and RARP?

The address resolution protocol (ARP) is used to associate the 32 bit IP address with the 48 bit physical address, used by a host or a router to find the physical address of another host on its network by sending a ARP query packet that includes the IP address of the receiver.

The reverse address resolution protocol (RARP) allows a host to discover its Internet address when it knows only its physical address.

What is the minimum and maximum length of the header in the TCP segment and IP datagram?

The header should have a minimum length of 20 bytes and can have a maximum length of 60 bytes.

What is the range of addresses in the classes of internet addresses?

Class A 0.0.0.0 - 127.255.255.255

Class B 128.0.0.0 - 191.255.255.255

Class C 192.0.0.0 - 223.255.255.255

Class D 224.0.0.0 - 239.255.255.255

Class E 240.0.0.0 - 247.255.255.255

What is the difference between TFTP and FTP application layer protocols?

The Trivial File Transfer Protocol (TFTP) allows a local host to obtain files from a remote host but does not provide reliability or security. It uses the fundamental packet delivery services offered by UDP.

The File Transfer Protocol (FTP) is the standard mechanism provided by TCP / IP for copying a file from one host to another. It uses the services offer by TCP and so is reliable and secure. It establishes two connections (virtual circuits) between the hosts, one for data transfer and another for control information.

What are major types of networks and explain?

Server-based network
Peer-to-peer network

Peer-to-peer network, computers can act as both servers sharing resources and as clients using the resources.

Server-based networks provide centralized control of network resources and rely on server computers to provide security and network administration

What are the important topologies for networks?

BUS topology:

In this each computer is directly connected to primary network cable in a single line.

Advantages:

Inexpensive, easy to install, simple to understand, easy to extend.

STAR topology:

In this all computers are connected using a central hub.

Advantages:

Can be inexpensive, easy to install and reconfigure and easy to trouble shoot physical problems.

RING topology:

In this all computers are connected in loop.

Advantages:

All computers have equal access to network media, installation can be simple, and signal does not degrade as much as in other topologies because each computer regenerates it.

What is mesh network?

A network in which there are multiple network links between computers to provide multiple paths for data to travel.

What is difference between baseband and broadband transmission?

In a baseband transmission, the entire bandwidth of the cable is consumed by a single signal. In broadband transmission, signals are sent on multiple frequencies, allowing multiple signals to be sent simultaneously.

Explain 5-4-3 rule?

In a Ethernet network, between any two points on the network ,there can be no more than five network segments or four repeaters, and of those five segments only three of segments can be populated.

What MAU?

In token Ring , hub is called Multistation Access Unit(MAU).

What is the difference between routable and non- routable protocols?

Routable protocols can work with a router and can be used to build large networks. Non-Routable protocols are designed to work on small, local networks and cannot be used with a router

Why should you care about the OSI Reference Model?

It provides a framework for discussing network operations and design.

What is logical link control?

One of two sublayers of the data link layer of OSI reference model, as defined by the IEEE 802 standard. This sublayer is responsible for maintaining the link between computers when they are sending data across the physical network connection.

What is virtual channel?

Virtual channel is normally a connection from one source to one destination, although multicast connections are also permitted. The other name for virtual channel is virtual circuit.

What is virtual path?

Along any transmission path from a given source to a given destination, a group of virtual circuits can be grouped together into what is called path.

What is packet filter?

Packet filter is a standard router equipped with some extra functionality. The extra functionality allows every incoming or outgoing packet to be inspected. Packets meeting some criterion are forwarded normally. Those that fail the test are dropped.

What is traffic shaping?

One of the main causes of congestion is that traffic is often busy. If hosts could be made to transmit at a uniform rate, congestion would be less common. Another open loop method to help manage congestion is forcing the packet to be transmitted at a more predictable rate. This is called traffic shaping.

What is multicast routing?

Sending a message to a group is called multicasting, and its routing algorithm is called multicast routing.

What is region?

When hierarchical routing is used, the routers are divided into what we will call regions, with each router knowing all the details about how to route packets to destinations within its own region, but knowing nothing about the internal structure of other regions.

What is silly window syndrome?

It is a problem that can ruin TCP performance. This problem occurs when data are passed to the sending TCP entity in large blocks, but an interactive application on the receiving side reads 1 byte at a time.

What are Digrams and Trigrams?

The most common two letter combinations are called as digrams. e.g. th, in, er, re and an. The most common three letter combinations are called as trigrams. e.g. the, ing, and, and ion.

Expand IDEA.

IDEA stands for International Data Encryption Algorithm.

What is wide-mouth frog?

Wide-mouth frog is the simplest known key distribution center (KDC) authentication protocol.

What is Mail Gateway?

It is a system that performs a protocol translation between different electronic mail delivery protocols.

What is IGP (Interior Gateway Protocol)?

It is any routing protocol used within an autonomous system.

What is EGP (Exterior Gateway Protocol)?

It is the protocol the routers in neighboring autonomous systems use to identify the set of networks that can be reached within or via each autonomous system.

What is autonomous system?

It is a collection of routers under the control of a single administrative authority and that uses a common Interior Gateway Protocol.

What is BGP (Border Gateway Protocol)?

It is a protocol used to advertise the set of networks that can be reached with in an autonomous system. BGP enables this information to be shared with the autonomous system. This is newer than EGP (Exterior Gateway Protocol).

What is Gateway-to-Gateway protocol?

It is a protocol formerly used to exchange routing information between Internet core routers.

What is NVT (Network Virtual Terminal)?

It is a set of rules defining a very simple virtual terminal interaction. The NVT is used in the start of a Telnet session.

What is a Multi-homed Host?

It is a host that has a multiple network interfaces and that requires multiple IP addresses is called as a Multi-homed Host.

What is Kerberos?

It is an authentication service developed at the Massachusetts Institute of Technology. Kerberos uses encryption to prevent intruders from discovering passwords and gaining unauthorized access to files.

What is OSPF?

It is an Internet routing protocol that scales well, can route traffic along multiple paths, and uses knowledge of an Internet's topology to make accurate routing decisions.

What is Proxy ARP?

It is using a router to answer ARP requests. This will be done when the originating host believes that a destination is local, when in fact is lies beyond router.

What is SLIP (Serial Line Interface Protocol)?

It is a very simple protocol used for transmission of IP datagrams across a serial line.

What is RIP (Routing Information Protocol)?

It is a simple protocol used to exchange information between the routers.

What is source route?

It is a sequence of IP addresses identifying the route a datagram must follow. A source route may optionally be included in an IP datagram header.

Operating Systems

Following are a few basic questions that cover the essentials of OS:

Explain the concept of Reentrancy.

It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.

Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.

Explain Belady's Anomaly.

Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process' virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.

What is a binary semaphore? What is its use?

A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.

What is thrashing?

It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.

List the Coffman's conditions that lead to a deadlock.

Mutual Exclusion: Only one process may use a critical resource at a time.
Hold & Wait: A process may be allocated some resources while waiting for others.
No Pre-emption: No resource can be forcible removed from a process holding it.
Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.

What are short-, long- and medium-term scheduling?

Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.

Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of real-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.

Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.

What are turnaround time and response time?

Turnaround time is the interval between the submission of a job and its completion. Response time is the interval between submission of a request, and the first response to that request.

What are the typical elements of a process image?

User data: Modifiable part of user space. May include program data, user stack area, and programs that may be modified.
User program: The instructions to be executed.
System Stack: Each process has one or more LIFO stacks associated with it. Used to store parameters and calling addresses for procedure and system calls.
Process control Block (PCB): Info needed by the OS to control processes.

What is the Translation Lookaside Buffer (TLB)?

In a cached system, the base addresses of the last few referenced pages is maintained in registers called the TLB that aids in faster lookup. TLB contains those page-table entries that have been most recently used. Normally, each virtual memory reference causes 2 physical memory accesses-- one to fetch appropriate page-table entry, and one to fetch the desired data. Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.

What is the resident set and working set of a process?

Resident set is that portion of the process image that is actually in real-memory at a particular instant. Working set is that subset of resident set that is actually needed for execution. (Relate this to the variable-window size method for swapping techniques.)

When is a system in safe state?

The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.

What is cycle stealing?

We encounter cycle stealing in the context of Direct Memory Access (DMA). Either the DMA controller can use the data bus when the CPU does not need it, or it may force the CPU to temporarily suspend operation. The latter technique is called cycle stealing. Note that cycle stealing can be done only at specific break points in an instruction cycle.

What is meant by arm-stickiness?

If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc). High-density multisurface disks are more likely to be affected by this than low density ones.

What are the stipulations of C2 level security?

C2 level security provides for:

Discretionary Access Control
Identification and Authentication
Auditing
Resource reuse

What is busy waiting?

The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting. The CPU is not engaged in any real productive activity during this period, and the process does not progress toward completion.

Explain the popular multiprocessor thread-scheduling strategies.

Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing refers to a scheme where work is allocated to processors on a more permanent basis.
Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to processors. For the duration of program execution, each program is allocated a set of processors equal in number to the number of threads in the program. Processors are chosen from the available pool.
Dynamic scheduling: The number of thread in a program can be altered during the course of execution.

When does the condition 'rendezvous' arise?

In message passing, it is the condition in which, both, the sender and receiver are blocked until the message is delivered.

What is a trap and trapdoor?

Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods of access authentication. A trap is a software interrupt, usually the result of an error condition.

What are local and global page replacements?

Local replacement means that an incoming page is brought in only to the relevant process' address space. Global replacement policy allows any page frame from any process to be replaced. The latter is applicable to variable partitions model only.

Define latency, transfer and seek time with respect to disk I/O.

Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.

Describe the Buddy system of memory allocation.

Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k. When some memory is required by a process, the block size of next higher order is chosen, and broken into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined, and put into the original free-block linked-list.

What is time-stamping?

It is a technique proposed by Lamport, used to order events in a distributed system without the use of clocks. This scheme is intended to order events consisting of the transmission of messages. Each system 'i' in the network maintains a counter Ci. Every time a system transmits a message, it increments its counter by 1 and attaches the time-stamp Ti to the message. When a message is received, the receiving system 'j' sets its counter Cj to 1 more than the maximum of its current value and the incoming time-stamp Ti. At each site, the ordering of messages is determined by the following rules: For messages x from site i and y from site j, x precedes y if one of the following conditions holds....(a) if Ti<Tj or (b) if Ti=Tj and i<j.

How are the wait/signal operations for monitor different from those for semaphores?

If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should be perfectly balanced in the program.

In the context of memory management, what are placement and replacement algorithms?

Placement algorithms determine where in available real-memory to load a program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.

In loading programs into memory, what is the difference between load-time dynamic linking and run-time dynamic linking?

For load-time dynamic linking: Load module to be loaded is read into memory. Any reference to a target external module causes that module to be loaded and the references are updated to a relative address from the start base address of the application module.

With run-time dynamic loading: Some of the linking is postponed until actual reference during execution. Then the correct module is loaded and linked.

What are demand- and pre-paging?

With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.

Paging a memory management function, while multiprogramming a processor management function, are the two interdependent?

Yes.

What is page cannibalizing?

Page swapping or page replacements are called page cannibalizing.

What has triggered the need for multitasking in PCs?

Increased speed and memory capacity of microprocessors together with the support fir virtual memory and
Growth of client server computing

What are the four layers that Windows NT have in order to achieve independence?

Hardware abstraction layer
Kernel
Subsystems
System Services.

What is SMP?

To achieve maximum efficiency and reliability a mode of operation known as symmetric multiprocessing is used. In essence, with SMP any process or threads can be assigned to any processor.

What are the key object oriented concepts used by Windows NT?

Encapsulation
Object class and instance

Is Windows NT a full blown object oriented operating system? Give reasons.

No Windows NT is not so, because its not implemented in object oriented language and the data structures reside within one executive component and are not represented as objects and it does not support object oriented capabilities .

What is a drawback of MVT?

It does not have the features like

ability to support multiple processors
virtual storage
source level debugging

What is process spawning?

When the OS at the explicit request of another process creates a process, this action is called process spawning.

How many jobs can be run concurrently on MVT?

15 jobs

List out some reasons for process termination.

Normal completion
Time limit exceeded
Memory unavailable
Bounds violation
Protection error
Arithmetic error
Time overrun
I/O failure
Invalid instruction
Privileged instruction
Data misuse
Operator or OS intervention
Parent termination.

What are the reasons for process suspension?

swapping
interactive user request
timing
parent process request

What is process migration?

It is the transfer of sufficient amount of the state of process from one machine to the target machine

What is mutant?

In Windows NT a mutant provides kernel mode or user mode mutual exclusion with the notion of ownership.

What is an idle thread?

The special thread a dispatcher will execute when no ready thread is found.

What is FtDisk?
It is a fault tolerance disk driver for Windows NT.
What are the possible threads a thread can have?

Ready
Standby
Running
Waiting
Transition
Terminated.

What are rings in Windows NT?

Windows NT uses protection mechanism called rings provides by the process to implement separation between the user mode and kernel mode.

What is Executive in Windows NT?

In Windows NT, executive refers to the operating system code that runs in kernel mode.

What are the sub-components of I/O manager in Windows NT?

Network redirector/ Server
Cache manager.
File systems
Network driver
Device driver

What are DDks? Name an operating system that includes this feature.

DDks are device driver kits, which are equivalent to SDKs for writing device drivers. Windows NT includes DDks.

What level of security does Windows NT meets?

C2 level security.

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Friday, December 17, 2010

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