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This article covers another hi- tech ways of preventing the railway train collision .

The system consists of the following parts GPS device, a transmitter (may be GSM based),and a decision maker  an, display etc.

Now how the system installed? Each train consists of GPS device
Each GPS device and systems associated with that have database  for rail line tracks and their locations. They have some Gsm/GPRS link also so after a particular time intervals the GPRS device sends the location information of that train to the main server which is situated  to some central portion under some management. You can consider a central server which have all the location information of railway tracks. So when ever the GPS +GPRS device sending the information (location information) to the central server then from that location information it can be found that whether 2 trains locations approached to a collisionable    distance or not ,if so then a command or signal may be issued from the central server after deciding that and send to the corresponding Trains via GPRS link. So by using this a train can remotely be stopped also. Note trains also send their Ids to central server so that the server can identity which train is at what location now. Next few situations need to be considered. First when two trains passing side by side as tracks are not far so that distance may also be considered as collisionable  distance  and wrong alarm may be generated.
So to avoid this each train path is need to be prefixed and tracks need to have unique ID. For example say “from station :Hariana” to next 200 km if  tracks are uninterrupted then they are given a track no and unique track ID for each track.

Then to build the database of the tracks I mean which track has what GPS location info ,just a trolley or test engine which has GPS device installed run on a track and GPS location information are send via the GPRS link to the central server, this way server   may have created the data bank for that track and can uniquely identify the track through which the   test engine is moving.

Then whenever the actual Train is moving on that track and when ever this train’s engine sending the GPS info of location to the central server via GPRS link then server can easily search the data base and can identify through which track this train is currently moving. Is that track is the allocated track of the train or by some signalling mistake this train has gone into some wrong track ,so that can immediately be identified and can be tracked. So this is the simple idea ,now rest the things i mean the implementation details involved is not easy and many test runs are required as well as the collaboration with the Govt. Is required. Please NOTE:GPS devices available give a little bit error always like 10 meter to 100 meter (can vary).So we  need to take account that also, may be more than one GPS device can be used and from their data actual location may be extracted.

For more information on this type of project:

See More here

So to prepare for the IIT JEE following are some suggestions.
But take it from me that only the last night study is never sufficient to get a good marks/Rank in the JEE examination as for that you need to prepare for a long time .
So what are the ways to prepare for that very examination that can change your life -i mean change your life means- can have a good track in your life and in future based on that you may get your favourable subject to study for—in the engineering and technology.
Before that keep in mind you need to prepare   for the board examination as well before you appear for the JEE or the AIEEE.
So need to know the basic differences for that.Board examinations are for the basic concepts , the examinations   and the syllabus are arranged in such a way that you gain the basic concepts about the subjects and then appear for the examination. In JEE examination are mainly application oriented i.e what ever knowledge on basics on different subjects like physics ,chemistry and biology or mathematics you got need to be applied in problems.
So must give special stress on problem solving based on basic concepts
So need to understand that JEE is just the extension of the board examinations.
So basic task for the preparation is that read a chapter ,try to understand the chapter very well then try to grab a good concept on the chapter you just read then ask yourself about what ever portions are not clear to you. Then read those portions again and again to get a good grab on those blurred topics.
Then after you think you have  gained a good concept on that chapter you just read-go on solving few problems on that chapter and after you do a little bit of problems on that book ,move to a different writer’s book and on same chapter try to solve the problems given on that book’s (same chapter).Then make your target as that you will try to solve all the problems ,after that move to questions etc and try to answer the questions by yourself.
If you have correctly answered all  the questions then you can think that your conception is good.
To know whether your answer is OK or not try y to consult your teacher.

Now talking about the mocks tests: MOCK tests are they helpfull ?
My answer is yes, MOCK test are helpful only if you have prepared for the MOCK test each time ,So each time before appearing for the MOCK test just prepare by thinking that you are going to sit for a real test or JEE examination. Then sit for the MOCK test and then after that see how you performed on that test. Now  sit and think what are the problems, what mistakes  you made and how to cope with those mistakes you made so that next time you should not do any mistakes on the same.
So the purpose of the MOCK is to improve yourself not for just to sit in MOCK. Monitor your performance each time, your performance should increase.
I suggest not to take  more than one test per week.
.
So basic thing is take the test, analyse the results and then correct yourself and prepare for the next examination, this way you will be advanced to more better performance each time.
Now i think study on previous years questions papers is a good idea, then you will be able to judge yourself that how much you can answer for that examination.
I prefer not to waste your time by discussing the things which you can’t solve with your friends rather try yourself with the help of your teacher.

Perhaps the most important service provided by X.25 is multiplexing. A DTE is allowed to establish up to 4095 simultaneous virtual circuits with other DTEs over a single DTE-DCE link. The DTE can internally assign these circuits in anyway it pleases. Individually virtual circuits could correspond to applications, process, or terminals for example. The DTE-DCE link provides full duplex multiplexing, i.e., at anytime, a packet associated with a given virtual circuit can be transmitted in either direction.

To sort out which packets belong to which virtual circuits, each packet contains a 12-bit virtual circuit number. The assignment of virtual circuit numbers follows the convention depicted in the figure. Number zero is always reserved for diagnostic packets common to all virtual circuits. Then contiguous ranges of numbers are allocated for four categories of virtual circuits. Permanent virtual circuits are assigned numbers beginning with 1. The next category is one way, incoming virtual calla. This means that only incoming calls from the network can be assigned these numbers; the virtual circuit however, is two-way (full duplex). When a call request comes in, the DCE selects an unused number from this category.

One-way outgoing calls are those initiated by the DTE. In this case, the DTE selects an unused number from among those allocated for these calls. This separation of categories is intended to avoid the simultaneous selection of the same number for two different virtual circuits by the DTE and DCE.

The two-way virtual-call category provides an overflow for allocation shared by DTE and DCE, allowing for peak differences in traffic flow.

One interesting comparison can be made between these two algorithms. Consider first the Bellman-Ford algorithm where the calculation for node n involves knowledge of the link cost to all neighboring nodes to node n ( djn) plus the total path cost to each of those neighboring nodes from a particular source node s. Each node can maintain a set of costs and associated paths for every other node in the network and can exchange this information with its direct neighbors from time to time. Each node therefore deals with only one information from its neighbors and knowledge of its link costs,to update its costs and paths. On the other hand in  Dijkstra’s algorithm each node must have complete topological information about the network. That is each node must know the link costs of all links in the network. Thus  for this algorithm, information must be exchanged with all other nodes.
A final point : Both algorithms are known to converge under static conditions of topology and link costs and will converge to the same solution. If the link cost change over time, the algorithm will attempt to catch up with these changes. However , if the link cost depends on traffic ,which in turn depends on the routes chosen, then a feedback condition exists, and instabilities may result.

What is ASN.1? Write down the utilities for using ASN.1.

Ans. ASN.1 is the acronym of Abstract Syntax Notation One. The development of ASN.1 is the most significant development in computer communications. It has become an important universal language for defining application-level protocols. ASN.1 is a language that can be used to define data structures. A structure definition is in the form of a named module.The named module can then be used to reference the structure.

The Abstract Syntax is used for the exchange consists of application-level PDUs,which contain protocol control information and user data.within a system, the information represented using an Abstract Syntax must be mapped into some form for presentation to the human user.Thus the Abstract Syntax Notation is employed by a user to define network management information; the application must then convert this definition to a form suitable for local storage.

What is X.25 interface? Define DTE & DCE

The X.25 software and hardware allow the router to communicate over a public X.25 network. The X.25 network interface complies with CCITT 1980 and 1984 specifications for X.25 interfaces by offering multiplexed channels and reliable end-to-end data transfer across a WAN. Layer 1 X.25 defines the electrical and mechanical procedures for activating and deactivating the physical medium connecting the DTE and the DCE. This relationship is shown below:

DTE: DTE or Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.

DCE: DCE or Data circuit-terminating equipment are communications devices, such as modems and packet switches that provide the interface between DTE devices and a PSE (Packet-switching exchange), and are generally located in the carrier’s facilities.

Briefly describe multiplexing service of X.25. How flow control & error control is being achieved in X.25

Multiplexing of virtual circuits takes place at layer 3.The traditional approach to packet switching makes use of X.25, which not only determine the user network interface but also influences the internal design of the network. Today’s networks employ reliable digital transmission technology over high quality, reliable transmission links, many of which are optical fiber. Thus multiplexing of X.25 makes its more reliable & easily operable.

Flow control is generally based on the start & waits technique and is achieved by using an acknowledgement (ACK) for error free frames & a negative acknowledgement (Nak) for frames containing error. Ack0 & Ack1 are usually used alternately for detecting sequence errors, which are caused by either, duplicated or lost frames.

Error control is achieved by 16-bit CRC or FCS algorithm. In the event of an error frame, the sender retransmits the frame to the receiver.

For fixed routing, a single, permanent route is configured for each source- destination pair of nodes in the network. It determines routes using a least cost algorithm. The routes are fixed, or at least only change when there is a change in the topology of the network. A central routing matrix is created for implementing fixed routing which is shown below:

: First Generation adaptive algorithm

In this algorithm, each node maintains two vectors:

Where

Di = delay vector for node i

dij = current estimate of minimum delay from node i to node j (dij = 0)

N = number of nodes in network

Si = successor node vector for node i

sij = the next node in the current minimum delay route from i to j

Periodically each node exchanges its delay vector with all of its neighbors. On the basis of all incoming delay vectors, a node k updates both of its vectors as follows:

Delay for going k to j node, dkj = min [d _ij + l _ki ]

i € A

s _kj = i using i that minimizes the preceding expression

where

A = set of neighbor nodes for k

l _ki = current estimate of delay from k to i

Figure below provides an example of the original ARPANET algorithm:

• For network above, snapshot for node 1 shown

• Later, link costs change in the network

• Update comes in from its neighbors (2, 3, 4)

LAPB (Link Access Protocol Balanced) is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that ensures that frames are correctly ordered and error-free.

LAPB uses three frame format types:

Information (I) frame- These frames carry upper-layer information and some control information (necessary for full-duplex operation). I-frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send- and receive-sequence numbers.

Supervisory (S) frames- These frames provide control information. They request and suspend transmission, report on status, and acknowledge the receipt of I frames. They do not have an information field.

Unnumbered (U) frames- These frames are not sequenced. They are used for control purposes. For example, they can initiate a connection using standard or extended windowing (modulo 8 versus 128), disconnect the link, report a protocol error, or similar functions.

Ø Flag- Delimits the LAPB frame. Bit stuffing is used to ensure that the flag pattern does not occur within the body of the frame.

Ø Address- Indicates whether the frame carries a command or a response.

Ø Control- Provides further qualifications of command and response frames, and also indicates the frame format (U, I, or S), frame function (for example, receiver ready or disconnect), and the send/receive sequence number.

Ø Data- Carries upper-layer data. Its size and format vary depending on the Layer 3 packet type. The maximum length of this field is set by agreement between a PSN administrator and the subscriber at subscription time.

Ø Frame check sequence (FCS)- Ensures the integrity of the transmitted data.

What are the differences between datagram approach virtual circuit approaches in packet switching network? Which one is advantageous & why?

The datagram service, coupled with internal datagram operation, allows for efficient use of the network; no call setup and no need to hold up packets while a packet in error is retransmitted. This latter feature is desired in some real-time applications.

The virtual-circuit service can provide end-to-end sequencing and error control; this service is attractive for supporting connection-oriented applications, such as file transfer and remote-terminal access.

In practice, the virtual-circuit service is much more common than the datagram service. The reliability and convenience of a connection-oriented service is seen as more attractive than the benefits of the datagram services

What is HDLC?

HDLC (High-Level Data Link Control) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). It provides both connection oriented & connectionless service. HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM).

Describe supervisory frame in HDLC

Supervisory Frames (S-frames) in HDLC are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.

The S-frame control field includes a leading “10″ indicating that it is an S-frame. This is followed by a 2-bit type, a poll/final bit, and a sequence number. If 7-bit sequence numbers are used, there is also a 4-bit padding field. The first 2 bits mean it is an S-frame. Poll/Final bit is called Poll when used by the primary station to obtain a response from a secondary station, and Final when used by the secondary station to indicate a response or the end of transmission.

Write the sequences for HDLC initialization

Ans: Initialization may be requested by either side by issuing one of the six set-mode commands. This command serves three purposes:

It signals the other side that initialization is requested

It specifies which of the three modes ( NRM, ABM, ARM ) is requested.

It specifies whether 3- or 7-bit sequences numbers are to be used.

If the other side accepts this request, then the HDLC module on that end transmits an unnumbered acknowledged (UA) frame back to the initiating side. If the request is rejected, then a disconnected mode (DM) frame is sent.

HDLC is the protocol which is now considered an umbrella under which many Wide Area protocols sit. ITU-T developed HDLC in 1979, and within HDLC there are three types of stations defined:

• Primary Station - this completely controls all data link operations issuing commands from secondary stations and has the ability to hold separate sessions with different stations.
• Secondary Station - this can only send responses to one primary station. Secondary stations only talk to each other via a Primary station.
• Combined Station - this can transmit and receive commands and responses from one other station.

When transferring data, stations are in one of three modes:

• Normal Response Mode (NRM) where the secondary station needs permission from the primary station before it can transmit data. Mainly used on multi-point lines.
• Asynchronous Response Mode (ARM) where the secondary station can send data without receiving permission from the primary station. This is hardly ever used.
• Asynchronous Balanced Mode (ABM) where either station can initiate transmission without permission from the other. This is the most common mode used on point-to-point links.

There are three types of HDLC frame types defined by the control field:

• Information Frames are used for the data transfer between stations. The send sequence, or next send N(S), and the receive sequence, or next receive N(R), hold the frame sequence numbers. The Poll/Final bit is called Poll when used by the primary station to obtain a response from a secondary station, and Final when used by the secondary station to indicate a response or the end of transmission.
• Supervisory Frames are used to acknowledge frames, request for retransmissions or to ask for suspension of transmission. The Supervisory code denotes the type of supervisory frame being sent.
• Unnumbered Frames are used for link initialisation or link disconnection. The Unnumbered bits indicate the type of Unnumbered frame being used.