group17_thespark:JITTER

JITTER

Jitter is a variation or dislocation in the pulses of a digital transmission; it may be thought of, in a way, as irregular pulses. Jitter can manifest through variations in amplitude, signal strength, and other elements of such waves. The usual causes include connection timeouts, connection time lags, data traffic congestion, and interference. Simply put, this jitter is an undesirable output of system flaws and interruptions.

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To understand jitter, one must remember that data (whether audio, video, pictures or text) is seldom sent out wholly. Data is split up into manageable ‘packets’ with headers and footers that indicate the correct order of the data packets when it’s the client computer’s turn to organize them for playback. When a jitter occurs, some data packets may be lost in transit or the code for data packet assembly in the receiving machine may be wiped out.

Thus when jitters occur,computer monitors and computer processors may malfunction, files may get lost, downloaded audio files may acquire noise, Internet phone calls may get interrupted, suffer time lags or get disconnected. Due to its undesirable consequences, jitter is an important consideration in the design of all communications links.

GROUP17_THESPARK:TYPES OF LINE CODING SCHEMES

MANCHESTER ENCODING

Manchester encoding (first published in 1949) is a synchronous clock encoding technique used by the physical layer to encode the clock and data of a synchronous bit stream. In this technique, the actual binary data to be transmitted over the cable are not sent as a sequence of logic 1′s and 0′s (known technically as Non return to zero(NRZ). Instead, the bits are translated into a slightly different format that has a number of advantages over using straight binary encoding (i.e. NRZ).

In the Manchester encoding shown, a logic 0 is indicated by a 0 to 1 transition at the centre of the bit and a logic 1 is indicated by a 1 to 0 transition at the centre of the bit. Note that signal transitions do not always occur at the ‘bit boundaries’ (the division between one bit and another), but that there is always a transition at the centre of each bit. The Manchester encoding rules are summarised below:

Original Data	Value Sent
Logic 0	0 to 1 (upward transition at bit centre)
Logic 1	1 to 0 (downward transition at bit centre)

Note that in some cases you will see the encoding reversed, with 0 being represented as a 0 to 1 transition. The two definitions have co-existed for many years. The Ethernet Blue-Book and IEEE standards (10 Mbps) describe the method in whih a Logic 0 is sent as 0 to 1 transition, and a Logic 1 as a one to zero transition (where a zero is represented by a less negative voltage on the cable). Note that because many physical layers employ an inverting line to convert the binary digits into an electrical signal, the signal on the wire is the exact opposite of that output by the encoder. Differential physical layer transmission, does not suffer this inversion.

The following diagram shows a typical Manchester encoded signal with the corresponding binary representation of the data (1,1,0,1,0,0) being sent.

The waveform for a Manchester encoded bit stream carrying the sequence of bits 110100.

Note that signal transitions do not always occur at the ‘bit boundaries’ (the division between one bit and another), but that there is always a transition at the centre of each bit.The encoding may be alternatively viewed as a phase encoding where each bit is encoded by a postive 90 degree phase transition, or a negative 90 degree phase transition. The Manchester code is therefore sometimes known as a Biphase Code.

A Manchester encoded signal contains frequent level transitions which allow the receiver to extract the clock signal using a DIGITAL PHASE LOCKED LOOP(DPLL)and correctly decode the value and timing of each bit. To allow reliable operation using a DPLL, the transmitted bit stream must contain a high density of bit transitions. Manchester encoding ensures this, allowing the receiving DPLL to correctly extract the clock signal.

The bi-phase Manchester encoding can consume up to approximately twice the bandwidth of the original signal (20 MHz). This is the penalty for introducing frequent transitions. For a 10 Mbps LAN, the signal spectrum lies between the 5 and 20 MHz. Manchester encoding is used as the physical layer of an Ethernet , where the additional bandwidth is not a significant issue for coaxial cable transmission, the limited bandwidth of CAT5e cable necessitated a more efficient encoding method for 100 Mbps transmission using a4b/5b code. This uses three signal levels (instead of the two levels used in Manchester encoding) and therfore allows a 100 Mbps signal to occupy only 31 MHz of bandwidth. Gigabit Ethernet utilises five levels and  encoding, to provide even more efficient use of the limited cable bandwidth, sending 1 Gbps within 100 MHz of bandwidth.

Differential Manchester encoding ( Conditioned Diphase)

Conditioned Diphase is a method of digital baseband transmission. Conditioned diphase uses a line encoding technique that encodes the digital data to be transmitted with a clock signal….
encoding) is a method of encoding data in which data

DATA

Debt, AIDS, Trade in Africa is a multinational Non-governmental organization founded in January 2002 in London by U2′s Bono along with Robert Sargent Shriver III and activists from the Jubilee 2000 Drop the Debt campaign….
and clock signal

Clock signal

In electronics and especially Synchronous logic digital circuits, a clock signal is a Signalling used to coordinate the actions of two or more Electronic circuit….
s are combined to form a single self-synchronising

Synchronizing

In telecommunication, the term synchronizing has the following meanings:# Achieving and maintaining synchronism.# In fax, achieving and maintaining predetermined speed relations between the scanning spot and the recording spot within each scanning line….
data stream

Data stream

In telecommunications and computing, a data stream is a sequence of encoder coherent Signalling s used to Transmission or receive information that is in transmission ….
. It is a differential encoding, using the presence or absence of transitions to indicate logical value. This gives it several advantages over vanilla Manchester encoding:

• Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment.
• Because only the presence of a transition is important, polarity is not. differential coding
• .Differential coding
  In digital communications, differential coding is a technique used to provide unambiguous signal reception when using some types of modulation….
  schemes will work exactly the same if the signal is inverted (wires swapped).

   

  Line code

  In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes. Line coding is often used for digital data transport….
  s with this property include NRZI, bipolar encoding
  

  Bipolar encoding

  In telecommunication, bipolar encoding is a type of line code . A duobinary signal is such an encoding….
  

  Biphase Mark Code

  The biphase mark code is a type of encoding for binary data streams. When a binary data stream is sent without modification via a channel, there can be long series of logical ones or zeros without any transitions which makes clock recovery and synchronization difficult….
  

  Coded mark inversion

  In telecommunication, coded mark inversion is a non-return-to-zero line code. It encodes zero bits as a half bit time of zero followed by a half bit time of one, and while one bits are encoded as a full bit time of a constant level….
  

  MLT-3 encoding

  MLT-3 encoding is a line code that uses three voltage levels. An MLT-3 interface emits less electromagnetic interference and requires less Bandwidth than most other Boolean logic or Ternary logic interfaces that operate at the same bit rate , such as Manchester code or Bipolar encoding….
  ).

A ’1′ bit is indicated by making the first half of the signal equal to the last half of the previous bit’s signal i.e. no transition at the start of the bit-time. A ’0′ bit is indicated by making the first half of the signal opposite to the last half of the previous bit’s signal i.e. a zero bit is indicated by a transition at the beginning of the bit-time. In the middle of the bit-time there is always a transition, whether from high to low, or low to high. A reversed scheme is possible, and no advantage is given by using either scheme.

A related method is Manchester encoding in which the meaningful transitions are the mid-bit ones, and these encode data by their direction (positive-negative is one value, negative-positive is the other).

Differential Manchester is specified in the IEEE 802.5

IBM token ring

Token ring local area network technology is a local area network network protocol which resides at the data link layer of the OSI model. It uses a special three-byte frame called a token that travels around the ring….
standard for token ring LANs, and is used for many other applications, including magnetic and optical storage.

Note: In differential Manchester encoding, if a “1″ is represented by one transition, then a “0″ is represented by two transitions and vice versa.

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group17_thespark:internet checksum

IP Checksum

A checksum is a simple error-detection scheme in which each transmitted message that results in a numerical value based on the value of the bytes in a message. The sender places the calculated value in the message (usually in the message header) and sends the value with the message. The receiver applies the same formula to each received message and checks to make sure the accompanying numerical value is the same. If not, the receiver can assume that the message has been corrupted in transmission.

The simplest form of checksum, which adds up the bytes in the data to form a sum value, cannot detect a number of types of errors. In particular, such the checksum value is not changed by:

• Reordering of the bytes in the message in the message block
• Inserting or deleting zero-valued bytes
• Multiple errors that cancel

More sophisticated types of redundancy check, including cyclic redundancy checks (CRCs) are typically used at the link layer. These are designed to address these weaknesses by considering not only the value of each byte but also the order of the values. The cost of the ability to detect more types of error is the increased cost of computing the checksum. Packet corruption is not only caused by errors introduced by the physical layer. It may be, and is, also (on occassions) caused by bugs in host and router hardware and software. Even if every link implemented strong error detection in the form of frame CRCs, it is still essential that end-to-end checksums at and above the IP level are used at the receiving end host

The internet protocol(IP) and most higher-layer protocols of the Internet Protocol Suite (ICMP, IGMP, UDP, UDP-Lite, TCP) use a common checksum algorithm to validate the integrity of the packets that they exchange. The IP (IPv4) header checksum protects only the IPv4 header, while the TCP, DCCP, ICMP, IGMP, and UDP checksums provide end-to-end error detection for both the transport header (including network and transport layer information) and the transport payload data. Protection of the data is optional for applications using UDP [RFC768] for IPv4, but is required for IPv6.

Transport Checksums

When used above the IP-level (e.g. in the UDP, TCP, and DCCP transport protocols), the checksum algorithm includes both the data bytes in the protocol data unit and some additional bytes, known as a pseudo-header (built from the information present in the IP network layer header).

The 16-bit checksum field in the header is zeroed prior to checksum calculation. The calculation is made 16 bits at a time (e.g. 2 octets). If the datagram is odd-numbered in length, a zero octet is virtually added at the end, so that 16-bit maths can be used throughout. If the computed checksum is 0, it is transmitted as all ones (the equivalent in one’s complement arithmetic).

To ease implementation, the addition operations in the checksum can be performed using 32-bit maths (see below for an example for UDP processing). This is a natural size of word in many modern processors. However, most processors only provide a 32-bit add instruction, and do not provide an instruction that independently adds two 16-bit quantities contained in one 32-bit word. This therefore requires the algorithm to be modified slightly to take into consideration the carry that may result at bit 15.

 

 

 

Group17_TheSpark : Ppt # 2

group17_thespark:cyclic redundancy check

Cyclic Redundancy Checks

A CRC is an error detecting code. Its computation resembles a long division operation in which the quotient  is discarded and the remainder becomes the result, with the important distinction that the arithmetic used is the carry-less arithmetic of a finite field. The length of the remainder is always less than or equal to the length of the divisor, which therefore determines how long the result can be. The definition of a particular CRC specifies the divisor to be used, among other things.

An important reason for the popularity of CRCs for detecting the accidental alteration of data is their efficiency guarantee. Typically, an n-bit CRC, applied to a data block of arbitrary length, will detect any single error burst not longer than n bits (in other words, any single alteration that spans no more than n bits of the data), and will detect a fraction 1 − 2 ^{− n} of all longer error bursts. Errors in both data transmission channels and magnetic storage media tend to be distributed non-randomly (i.e. are “bursty”), making CRCs’ properties more useful than alternative schemes such as multiple parity checks.

k is the length of the message we want to send, ie the number of information bits.

• n is the total length of the message we will end up sending: the information bits followed by the check bits. Peterson and Brown call this a code polynomial.
• n-k is the number of check bits. It is also the degree of the generating polynomial. The basic (mathematical) idea is that we’re going to pick the n-k check digits in such a way that the code polynomial is divisible by the generating polynomial. Then we send the data, and at the other end we look to see whether it’s still divisible by the generating polynomial; if it’s not then we know we have a error, if it is we hope there was no error.

The way we calculate a CRC is we establish some predefined n-k+1 bit number P (called the Polynomial, for reasons relating to the fact that modulo-2 arithmetic is a special case of polynomial arithmetic). Now we append n-k 0′s to our message, and divide the result by P using modulo-2 arithmetic. The remainder is called the Frame Check Sequence. Now we ship off the message with the remainder appended in place of the 0′s. The receiver can either recompute the FCS and see if it gets the same answer, or it can just divide the whole message (including the FCS) by P and see if it gets a remainder of 0!

As an example,

As an example, 4-bit polynomial of 1001, and compute the CRC of a 7 bit message where divisor is 1011 and gives the remainder as 110 which is added to 4 bit data word to form a 7 bit codeword

It is not necessary to keep track of the quotient because the main goal is to get the remainder(110). CRC’s can actually be computed in hardware using a shift register and some number of exclusive-or gates .The key insight is that we can perform a subtraction any time there is a 1 in the bit that lines up with the most significant bit of the polynomial, and we can perform that subtraction by performing an exclusive-or of the bits corresponding to 1′s in all the other places of the polynomial. We can  implement the CRC calculation by using a shift register,the string to be checked is inserted from the right. Whenever a “1″ exits the left side of the shift register, it means there is a 1 in the most significant bit of the part of the dividend we’re working with; since we’re working in modulo-2 arithmetic, this means we can do a subtraction. It will be implemented by following steps.

1. The most significant bit will be xored away, so it falls off to the left.
2. For every other bit with a “1″ in the divisor, perform an exclusive-or with the corresponding bit in the number being checked.
3. For bits with a “0″ in the divisor, do nothing

Properties of Cyclic Redundancy Checks

group17_the spark:checksum

A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match, it’s assumed that the complete transmission was received. Both TCP and UDP communication layers provide a checksum count and verification as one of their services.A checksum is a value which is computed which allows you to check the validity of something. Typically, checksums are used in data transmission contexts to detect if the data has been transmitted successfully.

Checksums take on various forms, depending upon the nature of the transmission and the needed reliability. For example, the simplest checksum is to sum up all the bytes of a transmission, computing the sum in an 8-bit counter. This value is appended as the last byte of the transmission. The idea is that upon receipt of n bytes, you sum up the first n-1 bytes, and see if the answer is the same as the last byte. Since this is a bit awkward, a variant on this theme is to, on transmission, sum up all the bytes, the (treating the byte as a signed, 8-bit value) negate the checksum byte before transmitting it. This means that the sum of all n bytes should be 0. These techniques are not terribly reliable; for example, if the packet is known to be 64 bits in length, and you receive 64 ” bytes, the sum is 0, so the result must be correct. Of course, if there is a hardware failure that simply fails to transmit the data bytes (particularly easy on synchronous transmission, where no “start bit” is involved), then the fact that you receive a packet of 64 0 bytes with a checksum result of 0 is misleading; you think you’ve received a valid packet and you’ve received nothing at all. A solution to this is to do something like negate the checksum value computed, subtract 1 from it, and expect that the result of the receiver’s checksum of the n bytes is 0xFF (-1, as a signed 8-bit value). This means that the 0-lossage problem goes away.

group17_thespark:Question and answers

Question and answers for Group A project:   HTTP and FTP Proxy servers

1. what is MIME filtering ?

a:   MIME stands for Multipurpose Internet Mail Extensions . It  is an Internet standard  that extends the format of  e-mail to support:

• Text in character sets other than ASCII
• Non-text attachments
• Message bodies with multiple parts
• Header information in non-ASCII character sets

Filtering based on these MIME formats is known as MIME filtering

Asked by : Bhargavi

Answered by: Gowri

2. What is the difference between a thread and a process ?

• Process is a program in execution where as thread is a separate path of execution in a program.
• Threads  share a common address space as that of the process that created it, but process have their own address space
• process need interface of  operating system but not threads

Asked by : Bhavya

Answered by: Manasa

3.  In caching , if a document in cache is updated then how is it handled in secondary memory?

a:  There exists two mechanisms through which this situation can be handled viz:

• In a write-through cache, every write to the cache causes a synchronous write to the backing store i.e., secondary memory

• In a write-back (or write-behind) cache, writes are not immediately mirrored to the store. Instead, the cache tracks which of its locations have been written over (these locations are marked dirty). The data in these locations is written back to the backing store when those data are evicted from the cache, an effect referred to as a lazy write.

Asked by : Krish

Answered by: Supriya

4.  What do u mean by Temporal aspect of Locality of Reference with respect to caching ?

a:   Temporal locality refers to the reuse of specific data and/or resources within relatively small time durations.

If at one point in time a particular memory location is referenced, then it is likely that the same location will be referenced again in the near          future. There is a temporal proximity between the adjacent references to the same memory location. In this case it is common to make efforts to store a copy of the referenced data in special memory storage, which can be accessed faster. Temporal locality is a very special case of the spatial locality, namely when the prospective location is identical to the present location.

Asked by : Himanshu

Answered by: Malini

5. If cache is full, then on what basis documents are removed?

a: There exists many algorithms to handle this situation and the most commonly used one is Hybrid algorithm where a document in cache is marked on different factors like frequency of access, time taken to download, its avilability etc . A high watermark is set to the document which received more hits. so , the document with low watermark is removed to accumulate the new document to be inserted into cache.

Asked by : Phaneesh

Answered by: Ekta

group17_thespark:squid proxy server

Implementation of HTTP proxy server using SQUID

Brief introduction on SQUID:

Squid is a proxy server and web cache daemon,  primarily used for http and ftp.  It includes limited support for several other  protocols including TLS,  SSL, INTERNET  GOPHER and HTTPS. The development version of Squid (3.1) includes IPV6and ICAPsupport.

Features:

• Reduces latency by caching

• Content filtering

• Controls web traffic

• Anonymity

How to configure squid for http proxy content filtering?

1. Setting IP address and subnet mask on server system is done with command,

“ $sudo ifconfig eth0 192.168.1.1 netmask 255.255.255.0 up”

Where sudo refers to super user do.

2.  To verify port setting use the following command

“$netstat –l –n |grep 8080”

3.  To run the squid

“sudo/etc/init.d/squid restart”

4.   Change the permissions of the configuration file /etc/squid.conf

“$sudo chmod 644 /etc/squid/squid.conf”

5.     To block sites

“$sudo vim /etc/squid/squidblock.conf”

To block sites

• First we have to create a file say bad using the command in /etc/squid/squid.conf

http_access deny bad.

• Reload squid

“$sudo /etc/squid/squid reload”

• Now you block any site by editing

“$sudo vim /etc/squid/squid_bad.acl”

Note: we can even use regular expression

6.  Editing error loading page

• “$cd /usr/share/squid/error/English”
• “/usr/share/squid/error/English ls”
• “/usr/share/squid/error/English $

ERR_ACCESS_DENIED

• Then one can make any changes one needs, then change the directory using ‘cd’ command

group17_thespark: status report

Team Name:  THE SPARK

Team Members: 1. Ekta

2. M.S. Supriya

3. Malini. B.S

4. Manasa.R

5. Gowri.P

Group A Project: HTTP and FTP Proxy servers

Group B Project: Error correction and Detection

Presentation Date of Group A Project: 30th september 2009  (completed)

Presentation Date of Group B Project: 14th october 2009(not completed)

Group A PPT Loaded: Yes

Group B PPT Loaded: No

Group A Project report made: No

Group B Project report made: No

Problems faced during Group A Project: No

Problems faced during Group A Project: No

Proxy Server Presentation