Optical Fiber

Optical Fiber

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An optical fiber transmitting data in one direction only. Also for two entities communicate in full duplex, an optical cable must contain two optical fibers, one for transmission and one for reception. Cable can contain from 2 to 48 fibers. Fibers joined together in a cable does not create noise, because they do not carry electrical impulses that could induce electromagnetic interference. Therefore they do not need a protective shield, such as copper son.

A fiber optic cable is supported with son plastic reinforcement, such as Kevlar. This makes a stronger cable, ensuring that optical fibers do not deteriorate when folded.

Light is guided in the center of the fiber, called heart. The heart is composed mainly of silicon dioxide (silica), enriched with other elements. It is surrounded by the cladding. The sheath is made ​​of silica, but its refractive index is much lower than the heart. This allows to precisely reflect the light.
The cladding is protected by a jacket, made of plastic frequently.

The road is a ray is also called a mode. When optical fiber transmits a single ray, it is called single-mode fiber. Fiber that transmits several rays, it is called multimode fiber. For transmitting a plurality of spokes with different paths, the core of the multimode fiber to be greater than that of the single mode fiber.

Sources that scatter light in the fiber are not the same for singlemode and multimode fiber. Indeed, a multimode fiber uses LED (Light Emitting Diode), French "LED" Light Emitting Diode, while a single-mode fiber using the laser, which is generally more expensive. A laser emits rays of longer length than that of the rays emitted by an LED. Therefore, the maximum length of the multimode fiber is 2000 m. While the maximum length of the single mode fiber 3000 m. Single-mode fibers are more expensive and their use is frequently intended for WAN links between different buildings. Multimode fibers are cheaper and more used in business.

The fiber diameters have different sizes. In the diagram below you can see the types multimode and singlemode aligned, showing the relative sizes of different diameters.

Most of the equipment for local area networks transmit data in electrical form. In order to integrate the optical fiber in such a network, the electrical signals must be converted into light pulses. To do so, there are transmitters that convert, encode and send light signals. As already stated, there are two types of light source:

• LED: light emitting diode generates light infrared length of 850 nm or 1310 nm.
• LASER: (English: Light Amplification by Stimulated Emission Radiation) light amplification by stimulated emission of radiation produced narrow beams infrared light with high intensity and wavelength of 1310 nm or 1550 nm.

At the other end of the fiber is the receiver. It converts the light pulses into electrical impulses which are then transferred to other network devices.

The fiber ends are attached to the connectors that plug into the sockets of transmitters and receivers. SC-type connectors (Subscriber Connect) are most often used for multimode fibers and connectors type ST (Straight Tip) most frequently used for singlemode fiber. The diagram below shows the ST and SC, respectively.

A pair of connectors attached in a nest is called a duplex connector. A simplex connector is a simple connector that connects a fiber only.

Optical cables that exceed the maximum length is extended by repeater equipment for amplifying light signals.

Signals and noise in optical fibers

Despite the fact that the optical fiber is the best transmission media, signals passing through them can be mitigated by several factors. The most important factor is the decrease in signal caused by dispersion. It happens when the fiber is bent or too tight. The incident angle of a ray may then be less than the critical angle and a portion forming the beam is refracted. Absorption is another form of mitigation. It happens when a ray encounters impurities on its way.

To counter the problems of attenuation, we test the fiber optic links with tools that measure the energy loss and the travel time of the signals.

IMAP Protocol

IMAP Protocol :

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For over 25 years now, he is presented as the successor to the POP protocol, which is supposed to have the skin. But IMAP is still struggling to emerge. The fault of the suppliers of e-mail that the expected widespread mobile Internet offer to their users. Now that IMAP is widespread, you have no excuse not to adopt this protocol more efficient and practical. Follow the guide!

What is IMAP?

IMAP (Interactive Mail Access Protocol) is a protocol that allows you, from a program installed on your computer or smartphone access to messages in your mailbox.

Unlike the POP protocol that transfers messages in your mailbox on your computer and then delete them from the server, IMAP synchronizes messages and folders (inbox, sent messages, drafts, archives, etc.). between server and your device. Your email is stored in its entirety on the server, so you can access them from different terminals, you will have access to the same data. Any action that you perform in a terminal will be automatically transferred to the server.

IMAP protocol is ideal for those who have the habit of using their mail in different ways from their desktop, their laptop, their touch pad or smartphone.

The strength of IMAP is that the synchronization can be selective, you can download only the headers of messages or the mail folders that you use and save time loading and storage space on your terminal.

 

 

 

POP Protocol

POP Protocol :

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What is POP ?

Today, two different techniques are used to check for new mail: POP and IMAP protocols. POP (Post Office Protocol) is still commonly used today.

POP, the latest version is version 3 (POP3 and speaking), has been designed to allow you to get your mail on a remote machine when you are not permanently connected to the Internet.

POP is your servant: you give him the key to your mailbox, it will check your mail and bring it to you on a platter, that is to say in your email software.

 The POP protocol handles authentication, that is to say, to verify your username and password. It blocks also, your mailbox while you go, not allowing another connection at the same time access to your mail.
POP also supports sending and receiving messages (ERR) or acquittal (OK).

Concretely ...

POP supports the following commands:

 

• LIST : gives the number of files on the server with their number
• RETR numéro : number fetch mail waiting on your server
• DELE numéro : Number destroyed mail
• NOOP : verifies the connection
• LAST : retrieves the last message arrived at the server
• QUIT : leaves the session and allows another

Other messages such as TOP (to see the titles of posts) or APOP (which does not send multiple passwords in the case of periodic connections) are outside the protocol. Sending messages (extensions XTND XMIT) is not supported by the POP3 protocol basis.

That's why in your e-mail address you need to give your POP server, usually the type pop.bidule.fr

The evolution of the e-mail to nomadism and the lack of flexibility of POPs however favored the emergence of another protocol: IMAP

 

SMTP and POP3

SMTP and POP3:

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What is SMTP ?

 

 

The protocol SMTP (Simple Mail Transfer Protocol) is the standard protocol that allows the transfer of mail from one server to another using a point to point connection.This is a protocol that runs on-line, encapsulated in a TCP / IP frame. Mail is sent directly to the recipient's mail server. The SMTP protocol works with text commands sent to the SMTP server (port 25 by default). Each command sent by the client (validated by the ASCII string CR / LF, which is equivalent to pressing the Enter key) followed by an SMTP server response consists of a number and a descriptive message.The following describes a situation in which a request is made for sending mail to an SMTP server:· When opening the SMTP session, the first command is sent HELO command followed by a space (written <SP>) and the domain name of your computer (to say "Hi, this team"), and then validated by Enter (written <CRLF>). Since April 2001, the specifications for the SMTP protocol, defined in RFC 2821, indicate that the HELO command is replaced by the EHLO command.· The second command "MAIL FROM:" followed by the email address of the sender. If the command is accepted, the server responds with a "250 OK".· The next command is "RCPT TO:" followed by the email address of the  

o   Date

o   Subject

o   Cc

o   Bcc

                o   From 

If the command is accepted, the server responds with a "250 OK".

The following describes one example of transaction between a customer (C) and a SMTP server (S):

S: 220 Ready smtp.commentcamarche.net SMTP C: EHLO machine1.commentcamarche.net S: 250 smtp.commentcamarche.net C: MAIL FROM: <webmaster@commentcamarche.net> S: 250 OK C: RCPT TO: <meandus @ meandus.net> S: 250 C: RCPT TO: <tittom@tittom.fr> S: 550 No such user here C: DATA S: 354 Start mail input; End With <CRLF>. <CRLF> C: Subject: Hello C: Hello Meandus: C: How are your things? C: C: See you soon! C: <CRLF>. <CRLF> S: 250 C: QUIT R: 221 smtp.commentcamarche.net closing transmission

The basic specifications of the SMTP protocol indicate that all characters are sent encoded in 7-bit ASCII code and the 8th bit is explicitly zero. Therefore, to send accented characters is necessary to use algorithms that are within specifications MIME:


  base64 for attachments
  Quoted-printable (abbreviated QP) for special characters used in the message body.

Therefore, you can send an email using a simple telnet to port 25 of the SMTP server:

telnet smtp.commentcamarche.net 25

(The above server does not exist. Commentcamarche.net Try replacing the domain name of your Internet service provider. 


Below is a summary of the main SMTP commands:

A continuación se brinda un resumen de los principales comandos SMTP:

Vommand	Example	Description
HELO (EHLO now)	EHLO 193.56.47.125	Identification using the IP address or domain name of the sending computer
MAIL FROM:	MAIL FROM: originator@domain.com	Identification of the sender's address
RCPT TO:	RCPT TO: recipient@domain.com	Identification of the recipient's address
DATA	DATA message	Email body
QUIT	QUIT	SMTP server output
HELP	HELP	List of commands supported by the server SMTP

  All SMTP protocol specifications are defined in RFC 821 (since April 2001, the SMTP protocol specifications are defined in RFC 2821).

What is POP3?

 

The POP (Post Office Protocol), as its name suggests, allows you to collect email on a remote server (POP server). It is necessary for people who are not permanently connected to the Internet, as they can check their e-mails received without them being connected.

There are two main versions of this protocol, POP2 and POP3, which is assigned the ports 109 and 110 respectively, and operate using text commands radically different.

As with SMTP, POP protocol (POP2 and POP3) works with text commands sent to the POP server. Each of these commands sent by the client (validated by the chain CR / LF) consists of a keyword, possibly accompanied by one or more arguments, and is followed by a response from the POP server composed of a number and a descriptive message . 

Below is a summary of the main commands POP2:

Commands POP2
Command	Description
HELLO	Identification using the IP address of the sender
FOLDER	Name inbox to be consulted
READ	Message number to be read
RETRIEVE	Message number to be collected
SAVE	Number of the message is to be stored
DELETE	Number of the message to be deleted
QUIT	POP2 server output

Below is a summary of the main command POP3:

Commands POP2
Command	Description
USER identification	This command enables authentication. Must be followed by the user name, i.e., a character string that identifies the user to the server. The USER command must precede the PASS command.
PASS password	The PASS command to specify the password for the user whose name is specified by a prior USER command.
STAT	Information about server messages
RETR	Message number to be collected
DELE	Number of the message to be deleted
LIST [msg]	Number of the message to be displayed
NOOP	Lets keep the connection open in case of inactivity
TOP <messageID> <n>	Command that displays n lines of the message, whose number is given in the argument. In the case of a positive response from the server, it will send back the message headers, then a blank line and then the first n lines of the message.
UIDL [msg]	Request to the server to send a line containing information about the message possibly given in the argument. This line contains a character string called a unique identifier listing (list of unique identifiers) that uniquely identifies the message on the server, regardless of the session. The optional argument is a number related to an existing message on POP server, ie, a message that has not been erased.
QUIT	The QUIT command requests the POP3 server output. Leads to the elimination of all messages marked as deleted and sends the status of this action

Therefore, the POP3 protocol manages authentication using the user name and password. However, this is not safe, because passwords, like emails, circulating on the network as clear text (non-encrypted). In fact, according to RFC 1939, you can encrypt the password using an MD5 algorithm and benefit from secure authentication. However, because this command is optional, few servers deployed. In addition, the protocol POP3 inboxes blocks during access, which means that it is impossible for two users simultaneously access the same inbox.
In the same way that you can send an email using telnet, you can also access your incoming mail by using a simple telnet POP server port (110 by default):