Remote Access Devices For Networking.

Modem Technology

Modem Technology:

The easiest way to connect computers over long distances is over telephone lines; you can reach almost anywhere by telephone; You need a modem. Modems convert communications signals from a form the computer can understand to a form the phone system can convey and vice versa. Modulation is the process of converting a digital signal from a computer into an analog signal the telephone system will accept. At the other end of the connection, another modem interprets those analog signals the telephone system has conveyed and converts them back into digital form so the receiving computer can understand them.

Modem Properties:

• Transfer Speed: Modem speed is often discussed in baud rates or BPS. Baud rate refers to the oscillation of a sound wave on which a single bit of data is carried. BPS means the amount of data transferred in a second. By applying special techniques that manipulate analog signals so that more than 1 bit of data can be transferred in a single cycle of the signal, a modem can encode data and achieve higher throughput.
• Compression: Compression can greatly speed up the transfer of some types of information, such as text and graphics. When establishing connection with another modem, compress the data flowing between the two modems. This compression happens without the intervention of the two computers involved. Compression makes modem communication faster.
• Error Detection and Correction: The simplest form of error detection, which all asynchronous modems can perform, is parity checking. In parity checking, there must be an odd number of 1 bits in a data word transmitted, in which case the modem is using odd parity, or there must be an even number of 1 bits in data word transmitted, in which case the modem is using even parity. Parity checking does not correct data errors. It simply signals that an error has occurred. It is up to the receiving computer to request that the transmitting computer resend the data. Another commonly used method to make sure data is sent intact is counting the number of data word sent and received, making sure they are the same and calculating a checksum, on both the sent and received data to make sure the checksums match.

Types of Modem:

The types of modems can be divided into two broad categories, depending on how they coordinate their data transmission:

• Asynchronous
• Synchronous

Asynchronous modems were developed specially for use with telephone lines and they are the most common type of modem. When your computer sends data to another computer using an asynchronous modem connection, the data is first divided into bytes. The bytes are sent 1 bit at a time and the byte is preceded by a start bit and followed by a stop bit.

The computers do not coordinate in terms of when data will be sent. The receiving computer uses the start and stop bits to recognize when it has an entire byte of data. The start and stop bits and other coordination mechanisms in asynchronous communications can take up as much as 25% of data traffic.

Synchronous modems can transmit more data than asynchronous modems because they use careful timing and coordination between modems to send large block of data called frames, having multiple bytes.

Synchronous modems also do some things that asynchronous modems do not always do, including:

• Arrange data into block
• Add control information
• Check information to provide error control

How To Design A Wide Area Network

WAN Overview:

WANs are usually required for high volume, long-distance data traffic.

To implement WANs, you can use the following transmission media:

• PSTN (Public switched telephone network)
• High-speed, high-bandwidth dedicated leased circuits
• High speed fiber-optic cable
• Microwave transmission links
• Satellite links
• Wireless radiated media (radio frequency)
• The internet

There are two types of WANs:

• Enterprise networks: When a network connects a company’s branch offices and divisions, it becomes an enterprise- wide network. For example, a corporation may have sites on every continent, all of which are interconnected to form one wide area network.
• Global networks: When a network spans several countries and continents and includes many types of organizations and individuals, it can be labeled  global. These networks serve multinational corporations and scientific, academic and military establishments. The internet, often called the “network of networks” fits that definition.

Public Network Services:

Two of the most popular methods are the PSTN and the internet.

PSTN:

•  PSTNs were originally designed exclusively for telephones but have become highly sophisticated, able to handle different kinds of data transmission, including digital data transmission.
• The PSTN provides a number of options for data transmissions, including services that route packets between different sites.

Available services and some possible transmission rates include the following:

             Service                                            Transmission Rate

              Switched 56                                    56Kbps

              X.25                                               56Kbps

              T1 circuits                                       1.544Mbps

              T3 circuits                                       44.736Mbps

              Frame relay                                     1.544Mbps

              SMDS                                            1.544Mbps

              ISDN                                              1.544Mbps

              ATM                                              44.736Mbps

The Internet:

It is a shared network of government agencies, educational institutions, private organizations and individuals from over a hundred nations.

No one owns the Internet; anyone can have access to the transmission media.

SLIP and PPP:

• Serial Line Internet Protocol (SLIP) and Point-To-Point (PPP) are two very common protocols used to transmit IP packets over serial line and telephone connections, most often as part of a dial-up internet connection.
• SLIP an PPP are similar protocols. But some differences are there:
• Basically PPP is a multistage protocol signal transport mechanism. SLIP is designed for maintaining one type of traffic protocol  (TCP/IP traffic) at a moment.
• With SLIP, you must know both the IP address assigned to you by an ISP and the IP address of the remote system your computer will be dialing into. PPP deals with these problems by negotiating configuration parameters at the beginning of your connection.
• It also said that PPP can be  negotiate header compression. SLIP does not offer this type of compression.
• Another things about PPP that it offers IP enhanced security  protection.

Switching:

• Switching is an important technique that can determine how connections are made and how data movement is handled on a WAN.
• Data sent across the PSTN or other internetworks can travel along different paths from sender to receiver.

Three major switching techniques are:

• Circuit switching
• Message switching
• Packet switching

Circuit switching:

• In circuit switching, a dedicated physical connection is established between the sender and the receiver and maintained for the entire conversation.
• PSTN uses a circuit switching system.
• Before any two computers can transfer data, a dedicated circuit must be established between the two.
• The sending machine requests a connection to the destination, after which the destination machine signals that it is ready to accept data.
• The data is then sent from the source to the destination and the destination sends acknowledgements back to the source.
• When the conversation is finished, the source sends a signal to the destination, indicating that the connection is no longer needed and disconnect itself.
• A advantage of circuit switching is that the dedicated transmission channel the machine establish provides a guaranteed data rate.
• Since the channel is always available, it does not need to be requested again.
• Disadvantages: It is often an inefficient use of the transmission media.
• Dedicated channels require more bandwidth than non-dedicated channels, so transmission media can be expensive.
• Also, this method can be subject to long connection delays; it may take several seconds to establish the connection.

Message switching:

• Message switching is unlike circuit switching in that it does not establish a dedicated path between two communicating devices.
• Instead, each message is treated as an independent unit and includes its own destination and source addresses.
• Each complete message is then transmitted from device to device through the internetwork.
• Each intermediate device receives the message, stores it until the next device is ready to receive it and then forwards it to the next device.
• For this reason, a message switching network is sometimes referred to as a store-and-forward network.

Advantages:

• It provides efficient traffic management.
• It reduces network traffic congestion. The intermediate devices are able to store messages until a communications channel becomes available.
• Its use of data channels is more efficient than circuit switching.
• It provides asynchronous communication across time zones.

Disadvantages:

• The delay introduced by storing and forwarding complete messages make message switching unsuitable for real-time applications.
• It can be costly to equip intermediate devices with enough storage capacity to store potentially long message.

Packet switching:

• In packet switching, message are broken up into packets, each of which includes a header with source, destination and intermediate node address information.
• Individual packets don’t always follow the same route; this is called independent routing.

Independent routing offers two advantages:

Bandwidth can be managed by splitting data onto different routes in a busy circuit

If a certain link in the network goes down during the transmission, the remaining packets can be sent through another route.

Methods of packet switching:

1. Datagram packet switching
2. Virtual-circuit packet switching

Datagram packet switching: In a datagram packet-switched network, a message is divided into a stream of packets.

Each packet is separately addressed and treated as an independent unit with its own control instructions

The switching devices route each packet independently through the network, with each intermediate node determining the packet’s next route segment.