WORKING OF MODEM

Modem is an abbreviation for Modulator Demodulator. A modem converts data from digital computer signals to analog signals that can be sent over a phone line (modulation). The analog signals are then converted back into digital data by the receiving modem (demodulation). A modem is given digital information in the form of ones and zeros by the computer. The modem converts it to analog signals and sends over the phone line. Another modem then receives these signals, converts them back into digital data and sends the data to the receiving computer.The actual process is much more complicated then it seems.Here we discuss some internal functions of modem that helps in the modulation and demodulation process.1. Data CompressionComputers are capable of transmitting information to modems much faster than the modems are able to transmit the same information over a phone line. However, in order to transmit data at a speed greater than 600 bits per second (bps), it is necessary for modems to collect bits of information together and transmit them via a more complicated sound. This allows the transmission of many bits of data at the same time. This gives the modem time to group bits together and apply compression algorithms to them. Modem compresses them and sends over.2. Error CorrectionError correction is the method by which modems verify if the information sent to them has been undamaged during the transfer. Error correcting modems break up information into small packets, called frames and send over after adding a checksum to each of these frames. The receiving modem checks whether the checksum matches the information sent. If not, the entire frame is resent. Though error correction data transfer integrity is preserved.3. Flow ControlIf one modem in a dial up connection is capable of sending data much faster than the other can receive then flow control allows the receiving modem to tell the other to pause while it catches up. Flow control exists as either software or hardware flow control. With software flow control, when a modem needs to tell the other to pause, it sends a certain character signaling pause. When it is ready to resume, it sends a different character. Since software flow control regulates transmissions by sending certain characters, line noise could generate the character commanding a pause, thus hanging the transfer until the proper character is sent. Hardware flow control uses wires in the modem cable. This is faster and much more reliable than software flow control.4. Data BufferingData buffering is done using a UART. A UART (Universal Asynchronous Receiver/Transmitters) is an integrated circuit that converts parallel input into serial output. UART is used by computers to send information to a serial device such as a modem. The computer communicates with the serial device by writing in the UART's registers. UARTs have buffers through which this communication occurs on First in First out basis. It means that the first data to enter the buffer is the first to leave. Without the FIFO, information would be scrambled when sent by a modem. This basically helps the CPU to catch up if it has been busy dealing with other tasks.

Microsoft's Glimpse of Future.

Microsoft's glimpse of the future from BlackTale on Vimeo.

PROTOCOLS

VARIOUS PROTOCOLS OF APPLICATION LAYER ARE:

9P : Plan 9 from Bell Labs distributed file system protocol
AFP : Qaisar Javeed
APPC : Advanced Program-to-Program Communication
AMQP : Advanced Message Queuing Protocol
BitTorrent
Atom Publishing Protocol
BOOTP : Bootstrap Protocol
CFDP : Coherent File Distribution Protocol
DDS : Data Distribution Service
DHCP : Dynamic Host Configuration Protocol
DeviceNet
DNS : Domain Name System (Service) Protocol
eDonkey
ENRP : Endpoint Handlespace Redundancy Protocol
FastTrack (KaZaa, Grokster, iMesh)
Finger : User Information Protocol
Freenet
FTAM : File Transfer Access and Management
FTP : File Transfer Protocol
Gopher : Gopher protocol
HL7 : Health Level Seven
HTTP : HyperText Transfer Protocol
H.323 : Packet-Based Multimedia Communications System
IMAP : IMAP4, Internet Message Access Protocol (version 4)
IRCP : Internet Relay Chat Protocol
Kademlia
LDAP : Lightweight Directory Access Protocol
LPD : Line Printer Daemon Protocol
MIME (S-MIME): Multipurpose Internet Mail Extensions and Secure MIME
Modbus
Netconf
NFS : Network File System
NIS : Network Information Service
NNTP : Network News Transfer Protocol
NTCIP : National Transportation Communications for Intelligent Transportation System Protocol
NTP : Network Time Protocol
OSCAR : AOL Instant Messenger Protocol
PNRP : Peer Name Resolution Protocol
POP : POP3, Post Office Protocol (version 3)
RDP : Remote Desktop Protocol
Rlogin : Remote Login in UNIX Systems
RPC : Remote Procedure Call
RTP : Real-time Transport Protocol
RTPS : Real Time Publish Subscribe
RTSP : Real Time Streaming Protocol
SAP : Session Announcement Protocol
SDP : Session Description Protocol
SIP : Session Initiation Protocol
SLP : Service Location Protocol
SMB : Server Message Block
SMTP : Simple Mail Transfer Protocol
SNMP : Simple Network Management Protocol
SNTP : Simple Network Time Protocol
SPTP : Secure Parallel Transfer Protocol
SSH : Secure Shell
SSMS : Secure SMS Messaging Protocol
TCAP : Transaction Capabilities Application Part
TDS : Tabular Data Stream
TELNET : Terminal Emulation Protocol of TCP/IP
TFTP : Trivial File Transfer Protocol
TSP : Time Stamp Protocol
VTP : Virtual Terminal Protocol
Waka : an HTTP replacement protocol
Whois (and RWhois) : Remote Directory Access Protocol
WebDAV
X.400 : Message Handling Service Protocol
X.500 : Directory Access Protocol (DAP)
XMPP : Extensible Messaging and Presence Protocol

WHY OSI MODEL STARTED??

HISTORY OF OSI-MODEL::-

The idea behind the creation of networking standards is to define widely-accepted ways of setting up networks and connecting them together. The OSI Reference Model represented an early attempt to get all of the various hardware and software manufacturers to agree on a framework for developing various networking technologies

In the late 1970s, two projects began independently, with the same goal: to define a unifying standard for the architecture of networking systems. One was administered by the International Organization for Standardization (ISO), while the other was undertaken by the International Telegraph and Telephone Consultative Committee, or CCITT(the abbreviation is from the French version of the name). These two international standard bodies each developed a document that defined similar networking models.

In 1983, these two documents were merged together to form a standard called The Basic Reference Model for Open Systems Interconnection. That's a mouthful, so the standard is usually referred to as the Open Systems Interconnection Reference Model, the OSI Reference Model, or even just the OSI Model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200.

OSI Model
	Data unit	Layer	Function
Host layers	Data	7. Application	Network process to application
		6. Presentation	Data representation and encryption
		5. Session	Interhost communication
	Segment	4. Transport	End-to-end connections and reliability
Media layers	Packet	3. Network	Path determination andlogical addressing
	Frame	2. Data Link	Physical addressing
	Bit	1. Physical	Media, signal and binary transmission

APPLE TALK

Definition::-

AppleTalk is a set of local area network communication PROTOCOLS originally created for Apple computers. An AppleTalk network can support up to 32 devices and data can be exchanged at a speed of 230.4 kilobits per second (Kbps). Devices can be as much as 1,000 feet apart. AppleTalk's Datagram Delivery Protocol corresponds closely to the network layerof the Open Systems Interconnection (OSI) communication model.

NETWORKING MODEL::-

OSI Model	Corresponding AppleTalk layers
Application	Apple Filing Protocol (AFP)
Presentation	Apple Filing Protocol (AFP)
Session	Zone Information Protocol (ZIP) AppleTalk Session Protocol (ASP) AppleTalk Data Stream Protocol (ADSP)
Transport	AppleTalk Transaction Protocol (ATP) AppleTalk Echo Protocol (AEP) Name Binding Protocol (NBP) Routing Table Maintenance Protocol (RTMP)
Network	Datagram Delivery Protocol (DDP)
Data link	EtherTalk Link Access Protocol (ELAP) LocalTalk Link Access Protocol (LLAP) TokenTalk Link Access Protocol (TLAP) Fiber Distributed Data Interface (FDDI)
Physical	LocalTalk driver Ethernet driver Token Ring driver FDDI driver

DESIGN::-

The AppleTalk design rigorously followed the OSI model of protocol layering. Unlike most of the early LAN systems, AppleTalk was not built using the archetypal Xerox XNS system. The intended target was not Ethernet, and it did not have 48-bit addresses to route. Nevertheless, many portions of the AppleTalk system have direct analogs in XNS.

One key differentiation for AppleTalk was it contained three protocols aimed at making the system completely self-configuring. The AppleTalk address resolution protocol (AARP) allowed AppleTalk hosts to automatically generate their own network addresses, and the Name Binding Protocol (NBP) was dynamic system for mapping network addresses to user-readable names. Although systems similar to AARP existed in other systems, Banyan VINES for instance, nothing like NBP has existed until recently.

Both AARP and NBP had defined ways to allow "controller" devices to override the default mechanisms. The concept was to allow routers to provide the information or "hardwire" the system to known addresses and names. On larger networks where AARP could cause problems as new nodes searched for free addresses, the addition of a router could reduce "chattiness." Together AARP and NBP made AppleTalk an easy-to-use networking system. New machines were added to the network by plugging them and optionally giving them a name. The NBP lists were examined and displayed by a program known as the Chooser which would display a list of machines on the local network, divided into classes such as file-servers and printers.

DYNAMIC DNS

DNS? What's that???.........

HERE'S ANSWER::-

You got to this Web page by looking up http://www.sahilarora.org/dynamic/. DNS, the Domain Name Service, is responsible for the big part in the middle: www.sahilarora.org. The Internet is divided into literally millions of domains; each one has its own name.To a human, names like that (or ibm.com, or yahoo.com, or any of the other four million domain names registered) make perfect sense. IP addresses consist of four numbers, each between 0 and 255. More or less. (Some blocks of numbers are reserved for a variety of special purposes.) But not to the computer.

The computer doesn't have a clue. Computers work with numbers. Computers use IP addresses ("dotted quad" numbers like 10.20.30.255) to talk with each other on the Internet.

DNS is the middleman, translating domain names into numbers (and, occasionally, the other way around).

DYNAMIC DNS? What's that???.........

HERE'S ANSWER::-

In theory, there are 2³² (about 4.29e9, 4 billion or so) possible numeric addresses for the Internet. In practice, though, many of them were allocated in an inefficient manner a long time ago, in a way that can't easily be undone today. Some groups, like MIT, were given literally millions of addresses, more than they can ever use, but it's not really practical for them to give them back now. Over the next few years, IPv6 will be phased in, increasing the number of addresses to 2¹²⁸ (3.40e38, give or take), enough for everyone and all their major appliances to have an address. But until then...

There's only so many numbers out there, at least as far as the computer is concerned. (Basically, each of the four parts in the "dotted quad" address can only be between 0 and 255.) Silly technical limitations eat up a lot of those addresses; historical design decisions eat up some more; and of course a LOT of them are already in use.

This means that Internet IP addresses are a finite, scarce resource, and have to be treated somewhat carefully.

EXAMPLE::=

Suppose you have a normal, $20 per month, Internet dialup account from "Some Local ISP, Inc." They have three thousand customers, but it's rare that all of them are online at the same time. (In fact, if they follow industry practice, they probably only have 500 or so phone lines anyway.) So that ISP may only have 600 or 700 IP addresses -- enough to provide one for each phone line, a few for internal use, a few for future growth, but nowhere near one for each of those 3000 customers.

Or maybe you have a cable modem, though "Big CableCo Inc." Whenever your cable modem goes online (when you first plug it in and turn it on), it broadcasts a request for an open address, and some computer in their office eventually answers. Cable modem addresses are usually assigned with "leases," which work just like the lease on an apartment - you're guaranteed to have that address for a certain time, but after that all bets are off. Your landlord (the cable company) might evict you, forcing you to move (get a new IP address) at the end of the lease. (These 'leases' usually only last for a few days, and sometimes only a few hours.) At the end of the lease, you may be able to negotiate a new lease, but you can't be sure of it.

So not everyone can have their own IP address. Your ISP, cable company, or whoever, might let you have a dedicated IP, but they'll probably charge you extra for it. It's more likely, though, that they can't or won't help you...

FUNCTION:::-

Dynamic DNS providers provide a software client program that automates the discovery and registration of client's public IP addresses. The client program is executed on a computer or device in the private network. It connects to the service provider's systems and causes those systems to link the discovered public IP address of the home network with a hostname in the domain name system. Depending on the provider, the hostname is registered within a domain owned by the provider or the customer's own domain name. These services canfunction by a number of mechanisms This group of services is commonly also referred to by the term Dynamic DNS, although it is not the standards-based DNS Update method. However, the latter might be involved in the providers systems.

MODES OF COMMUNICATION

Basic Communication Modes of Operation

Let's begin with a look at the three basic modes of operation that can exist for any network connection, communications channel, or interface.

1.Simplex Operation

In simplex operation, a network cable or communications channel can only send information in one direction; it's a “one-way street”. This may seem counter-intuitive: what's the point of communications that only travel in one direction? In fact, there are at least two different places where simplex operation is encountered in modern networking.

The first is when two distinct channels are used for communication: one transmits from A to B and the other from B to A. This is surprisingly common, even though not always obvious. For example, most if not all fiber optic communication is simplex, using one strand to send data in each direction. But this may not be obvious if the pair of fiber strands are combined into one cable.

Simplex operation is also used in special types of technologies, especially ones that are asymmetric. For example, one type of satellite Internet access sends data over the satellite only for downloads, while a regular dial-up modem is used for upload to the service provider. In this case, both the satellite link and the dial-up connection are operating in a simplex mode.

2. Half-Duplex Operation

Technologies that employ half-duplex operation are capable of sending information in both directions between two nodes, but only one direction or the other can be utilized at a time. This is a fairly common mode of operation when there is only a single network medium (cable, radio frequency and so forth) between devices.

While this term is often used to describe the behavior of a pair of devices, it can more generally refer to any number of connected devices that take turns transmitting. For example, in conventional Ethernet networks, any device can transmit, but only one may do so at a time. For this reason, regular (unswitched) Ethernet networks are often said to be “half-duplex”, even though it may seem strange to describe a LAN that way.

3. Full-Duplex Operation

In full-duplex operation, a connection between two devices is capable of sending data in both directions simultaneously. Full-duplex channels can be constructed either as a pair of simplex links (as described above) or using one channel designed to permit bidirectional simultaneous transmissions. A full-duplex link can only connect two devices, so many such links are required if multiple devices are to be connected together.