Modem provides Internet access via telephone. Is a continuation of MOdulate / DEModulate. The engineer creates a modem because the world's major industries see profits by providing a way for computer office / home to access the Internet using the medium of transmission of the most accessible: the phone connection. At the very least, even digital phone system not designed to support network protocols such as TCP / IP. Benefits of a modem is to download the digital transmission protocol transform from a computer into analog signals that can be transferred through the telephone system and to transform analog signals coming from the phone line into digital signals that can be read by the recipient's computer.
Point-to-Point Connections
In the discussion of Network Access Layer, LAN like Ethernet and Token Ring uses a complex strategy of access (CSMA / CD and CSMA / CA) for the network media can be used together. But this time, 2 computers on both ends of the phone line do not have to scramble to use the transmission medium with a computer-kokmputer other telephone connection is only shared by both computers. Connection is called a connection point-to-point.
Connection Point-to-point is simpler than the LAN-based connections because they do not need to provide a method for multiple computers to the network transmission medium can be used together. But on the other hand, a connection via telephone lines has several limitations. The biggest is the transmission rate via a phone connection is much slower than a LAN network such as Ethernet.
Dial-up connection can be done anywhere. Dial-up protocol must handle the hardware and software from machine communication with a wider range and varied.
Modem Protocols
Modem protocols work directly with the TCP / IP and become an integrated part of the protocol stack. 2 protocol modem TCP / IP the most famous are:
Serial Line Internet Protocol (SLIP): A protocol-based modem TCP / IP is earlier made, SLIP simple and has some limitations
Point-to-Point Protocol (PPP): Protocol modem which is currently the most popular for modem connections, PPP created as an improvement of SLIP. PPP offers many important features not found in its predecessor SLIP.
PPP replacing SLIP as the preferred method for dial-up Internet connection.
Serial Line Internet Protocol (SLIP)
SLIP is the first attempt to integrate a modem protocol with TCP / IP. SLIP preceded by UNET TCP / IP from 3Com, which can later be implemented in Berkeley Unix System, and since then has been widely used, in the world of Unix and other compatible PC.
What Do SLIP
SLIP goal is to transmit IP datagrams over modem connections. SLIP does not provide physical addressing or error control and depend fully on the upper layer protocol for error-control functions. SLIP only send data and send a signal to mark the end of data transmission.
SLIP data format can be seen in the figure below.
Unlike PPP, SLIP does not allow the computer to perform connection configuration settings dynamically. Therefore, SLIP configuration is not always compatible.
Characteristic
RFC 1055 identifies several characteristics and loss SLIP as follows:
Addressing: both computers must know the IP address of each. SLIP does not support granting an IP address dynamically. This makes the SLIP can not be used for dial-up account with an ISP.
Type identification: SLIP has no fieldType ", Because SLIP does not offer a method to determine the type of protocol, SLIP is not able to support multiple protocols simultaneously. Unlike PPP, SLIP can not me-multiplex/demultiplex-kan no other protocol systems with TCP / IP.
Error correction / detection: SLIP does not provide error correction on what happened. Error-checking is better left to the upper layer.
Compression: Because of transmission via the telephone connection is very slow, various methods used to minimize the amount of data. SLIP does not support compression.
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