Wk 3: Short Answer Paper Charlene Quinones Network and Telecommunications Concept/NTC-360 June 19, 2011 Fernando Casafranca Here are the short answer responses for the following terms: • Synchronous and asynchronous 1 SYNCHRONOUS Synchronous systems negotiate the communication parameters at the data link layer before communication begins. Basic synchronous systems will synchronize both clocks before transmission begins, and reset their numeric counters for errors etc. More advanced systems may negotiate things like error correction and compression. 2 ASYNCHRONOUS

Asynchronous communication utilizes a transmitter, a receiver and a wire without coordination about the timing of individual bits. There is no coordination between the two end points on just how long the transmiter leaves the signal at a certain level to represent a single digital bit. Each device uses a clock to measure out the ‘length’ of a bit. The transmitting device simply transmits. The receiving device has to look at the incoming signal and figure out what it is receiving and coordinate and retime its clock to match the incoming signal. • Analog and digital Analog A string tied to a doorknob is a practical example of an ‘analog’ system. The string, when shaken, creates waves that travel down the length of the string until they hit the doorknob. Shake the string harder and you get bigger waves that travel faster down the string and hit the doorknob harder. Shake the string more gently and you get smaller waves. Shake the string up and down and the waves are oriented vertically, shake the string sideways and the waves are oriented horizontally. One thing is constant–the shape created by the string is wave-like. 5 Digital

A digital system would be more like flipping a light switch on and off. Speaking in general terms, there is no ‘in between’ values, unlike our doorknob and string example above. If the switch you are using is not a dimmer switch, then the light is either on, or off. Our light-switch example uses the light bulb as our transmitter, the transmission media is the space between the lightbulb and your eyes, and the receiver of the signal is your eye. This would be a digital system. A graph showing the properties of a digital transmission would look something like this: • XON and XOFF

Xon/Xoff (X-on/X-off or XON/XOFF) Xon/Xoff (sometimes written “X-on/X-off” or “XON/XOFF” and pronounced eks-AWN eks-AWF ) is a protocol for controlling the flow of data between computers and other device s on an asynchronous serial connection. For example, a computer typically sends data to a printer faster than the printer can print. The printer contains a buffer where data is stored until the printer catches up with the computer. If the buffer becomes full before the printer catches up, a small microprocessor in the printer sends back an X/off signal to stop sending data.

When enough data is printed and buffer storage becomes free, the printer sends an X/on signal telling the computer to resume sending data. The “X” stands for “transmitter” so the X/on and X/off are signals to turn a transmitter on or off. The actual signal for X/on is the same bit configuration as the ASCII Ctrl-Q keyboard combination. The X/off signal is the Ctrl-S character. When you define your modem to your computer’s operating system, you may need to specify the use of flow control with X/on/Xoff or with CTS/RTS (Clear to Send/Ready to Send).

When sending binary data, Xon/Xoff may not be recognized because it is character-encoded. o http://whatis. techtarget. com/definition/0,,sid9_gci213406,00. html • Simplex and duplex 1 SIMPLEX Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the printer was available or if the document printed properly since that was a human job.

Simplex links are built so that the transmitter (the one talking) sends a signal and it’s up to the receiving device (the listener) to figure out what was sent and to correctly do what it was told. No traffic is possible in the other direction across the same connection. 2 HALF DUPLEX A half duplex link can communicate in only one direction, at a time. Two way communication is possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic this behavior in that you cannot hear the other person if you are talking.

Half-duplex connections are more common over electrical links. Since electricity won’t flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit, the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex. 3 FULL DUPLEX

Full duplex communication is two-way communication achieved over a physical link that has the ability to communicate in both directions simultaneously. With most electrical, fiber optic, two-way radio and satellite links, this is usually achieved with more than one physical connection. Your telephone line contains two wires, one for transmit, the other for receive. This means you and your friend can both talk and listen at the same time. • Serial and parallel transmission o Serial Serial communications is the transfer of data 1 bit at a time in sequence across a communications link.

Learning how serial links work is useful for troubleshooting a wide variety of problems such as desktop computer communications problems, modems and T1’s. Below are links to several tutorials regarding the standard serial protocols. o Parallel SERIAL When information is sent across one wire, one data bit at a time, its called serial. Every computer on the face of the earth has some form of serial communications connector on it, whether internally or externally. Most people are familliar with the ‘D’ shaped 9-pin connector on the back of thier computer. This is a serial connector.

The typical 9-pin ‘D’ shaped connector on the back of your computer uses 2 loops of wire (1 in each direction) for data communication, plus additional wires to control the flow of information. However, in any given direction, data is still flowing over a single wire. PARALLEL Instead of squishing bits together, bits are sent over more wires simultaneously. In the case of an 25-pin parallel port, you have eight data-carrying wires so that eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being transferred eight times faster than a serial connection. http://www. inetdaemon. com/tutorials/basic_concepts/parallel_vs_serial. shtml o • Baseband and broadband o Baseband Baseband communication is unmodulated communication using a simple transmitter and a stream of information that uses all of the available communications capacity. Baseband transmitter systems transmit a single signal. The signal could be electrical, radio frequency or light. The signal is not modulated–however some other device may accept this commuinication as input and create a second modulated signal from this baseband signal. o Broadband

When communication occurs over a medium (copper wire, fiber optic cable, radio waves) whose transmitter is simultaneously transmitting multiple carrier signals or frequencies and employs some sort of modulation technique to expand the capacity of the individual frequencies, the communication is referred to as ‘broadband’. • Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) o PPP runs at the Datalink layer (ISO layer 2), providing symmetric, peer-to-peer connections utilizing encapsulation, transmission and link management services for the upper layer network protocols.

Modems, routers and even workstations utilize PPP for various serial connections. o • Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP) o Hypertext Transport Protocol (HTTP) is used to transfer web pages from a web server to a client web browser using TCP, usually on port 80, though this is not required. HTTP utilizes Multipart Internet Message Protocol (MIME) headers sent by the HTTP server to inform the client web browser what type of data it is receiving. o File Transfer Protocol (FTP) is one of the oldest applications in use on the Internet.

First proposed in April of 1971, it predates TCP/IP, the a pair of protocols FTP needs in order to operate. File Transfer Protocol is designed to do exactly that, transfer files between a server and a client. There are many applications which use FTP to transfer files between computers. To use FTP, you can start up the FTP program on your machine and connect to a server. The FTP application you use to connect to the server is a client, and your client software can connect to an FTP server running on port 21 on a remote machine.

FTP is included on most of today’s operating systems. • Transmission Control Protocol (TCP) and Internet Protocol (IP) o Transmission Control Protocol is a Transport Layer host-to-host protocol that provides reliable, connection-oriented communication over IP networks. o Internet Protocol is part of the Internet suite of communications protocols that provides globally unique addresses in dotted quad notation, transmits data in packets and performs routing between IP based networks. References • http://www. inetdaemon. com/