Modems

• Modems share these characteristics
• a serial (RS-232) interface
• an RJ-11C telephone line connector
• telephones use analog signal; computers use digital signal. A modem translates between the two
• BAUD refers to the speed of the oscillation of the sound wave on which a bit of data is carried over the telephone wire
• the BPS can be greater than the baud rate due to compression and encode data so that each modulation of sound can carry more than one bit of data is carried over the telephone line. For example, a modem that modulates at 28,000 baud can actually send at 115,200 bps => bps is the most important parameter when looking at throughput.
• There are 2 types of modems

Asynchronous Communications (Async)

• use common phone lines
• data is transmitted in a serial stream
• not synchronized, no clocking device => no timing
• both sending and receiving devices must agree on a start and stop bit sequence
• error control
• a parity bit is used in an error checking and correction scheme called parity checking
• It checks to see if the # of bits sent = # of bits received
• the receiving computer checks to make sure that the received data matches what was sent.
• 25 % of the data traffic in async communications consists of data control and coordination
• MNP (Microcom Network Protocol) has become the standard for error control
• Later LAPM (Link Access Procedure for Modems) is used in V.42 modems (57,600 baud).
• It uses  MNP Class 4.
• LAPM is used between  two modems that are V.42 compliant
• If one or the other modems is MNP 4 - compliant, the correct protocol would be MNP Class 4
• Communication performance depends on
1. signaling or channel speed - how fast the bits are encoded onto the communications channel
2. throughput - amount of useful information going across the channel
• you can double the throughput by using compression. One current data compression standard is the MNP Class 5 compression protocol
• V.42 bis is even faster because of compression.
• bis => second modification
• terbo => third, the bis standard was modified
• This is a good combination:
1. V.32 signaling
2. V.42 error control
3. V.42bis compression

Standard	BPS
V.22 bis	2400
V.32	9600
V.32bis	14,400
V.32terbo	19,200
V.FastClass (V.FC)	28,800
V.34	28,800
V.42	57,600

Synchronous Communication

• relies on a timing scheme coordinated between two devices to separate groups of bits and transmit them in blocks known as frames
• NO start and stop bits =. a continuous stream of data because both know when the data starts and stops.
• if there's error, the data is retransmitted
• some synchronous protocol perform the following that asynchronous protocols don't:
1. format data into blocks
2. add control info
3. check the info to provide error control
• the primary protocols in synchronous communication are:
1. Synchronous data link control (SDLC)
2. High-level data link control (HDLC)
3. binary synchronous communication protocol (bisync)
• Synchronous communications are used in almost all digital and network communications
• 2 types of telephone lines:
1. public dial network lines (dial-up lines) - manually dial up to make a connection
2. leased (dedicated) lines - full time connection that do not go through a series of switches, 56 Kbps to 45 Mbps