• ITU--International Telecommunications Union
• EIA--Electronic Industries Association
• IEEE--Institute of Electric and Electronic Engineers

RS232c

• Also called a serial line or tty line.
• It was used to connect terminals to computers.  It's now used to connect the mouse and keyboard.
• Specified by the EIA--simple standard for the transmission of characters.
• It's called serial because characters are sent one after the other.
• Communication is "asynchronous" because the transmitter and receiver do not co-ordinate before the transmission of a character. i.e.  -- the receiver doesn't know when a character will arrive.

• +15 volts = 0
• - 15 volts = 1
• When line is idle, the level is always -15 volts -- i.e. 1
• A start bit (zero) is used to tell the receiver that a character is on its way.

• The sender and receiver must agree on the number of bits per character (7) or (8) and the must agree on the duration of each bit (T)
• If there's disagreement in the format of the message, the receiver gets a "frame error"
• Typical speeds are 9.6Kbps, 14.4Kbps, 28.8Kbps, 56.6Kbps.
• Serial lines are usually "full duplex" meaning that both ends can send AND receive data.  (In contrast to half duplex, which is just one way--one end transmits and the other receives.

• Serial lines need at least three cables

• Limited to about 50 feet, so they are not for long distance because the resistance of the cable decreases the signal as it travels.
• Capacitance and inductance of cable distorts signal.
• Magnetic and electrical interference also distorts signal.

• RS232 deals with this problem by having a voltage threshold less than 15 volts and by taking multiple samples.
• Direct current, like the one used by RS232 can't be used for long distances.
• For long distance communication it is necessary to encode the signal in a sin wave (also called a carrier wave)
• Some of the properties of the signal are changed (modulated) to encode the information.
• This technique is also used by TV and radio

• Snoop--A way to get packets onto the network and to see what the raw packets are.  Only the root can run this.
• With our lab we can snoop packets on the private network using a wrapper

1. Generate different packets in the private network and capture them.  Then save the packets into a file.
2. Write a program to decode the packets and write the header information.  Packets are stored in a file.
• Types of ethernet packets are: IP, TCP, UDP, ARP, ICMP
• To run:  %> analyse <packetfile>
• function to decode packets:

• How do you know a packet is ARP or IP?

• How do you know if the packet is TCP, UDP, ICMP?
•    Decode IP packet.  IP packet is in the ethernet packet data area.

• How do you place a packet header in the data area

        struct ehdr{
            unsigned char sdt[6];
            unsigned char src[6];
            unsigned short type;
            unsigned char data[1];
         } // end struct

        e = (struct ehdr*) buffer;
        // you can access e->type
        // to decode IP header:
        struct iphdr* ip;
        i = (struct iphdr*e)->data;

• Use ntohl() or ntohs() to convert long and shorts from network byte order to host byte order

• Amplitude Modulation -- Amplitude or signal encodes 0 or 1

• Frequency Modulation -- Data is encoded in changes in frequency

• Phase Shift Modulation -- Phase change in the signal determines the data. Data is encoded in the phase shifts, thus pi/4 = 00, pi/2 = 01, (3/2)pi = 10, pi = 11

• A hardware device used for long distance communication.
• A modem has a MODulator and a DEModulator -- thus called a MODEM.

• Modems are most common for telephone lines but other types of modems also exist.
• Modem types
• Conventional--Electrical signals, 4 copper wires
• Optical--Light/optical fibre
• Wireless--Air/RF waves
• Dialup--Telephone system, modulated audio tones.
• Other functions for a dialup modem
• Dial
• Answer

• Propagation Delay--Time required for a signal to travel across medium
• Bandwidth -- Max times per second that a signal can change
• Throughput--Number of bits per second that can be transmitted.  It is related to the hardware bandwidth, thus higher bandwidth = higher throughput

• Relates network bandwidth and throughput
• D = 2B lgK
• Where
• D = throughput (max)
• B = bandwidth
• K = # of values that encode data
• Example

            So, D = 2B lg2 ===> 2B, so 2 times the bandwidth
 

• Example

•  Why not just make K = infinity?
•  Nyquist theorem does not consider noise, therefore in only gets a maximum theoretical limit

• C = B lg (1 + (S/N))
• Where
• C = throughput (capacity) in bits/sec
• B = hardware bandwidth
• S = power of the signal in watts
• N = power of the noise in watts
• S/N = signal to noise ratio
• If N=0 then C can grow to infinity, but N never is zero.
• Example

                    C = 3000 lg (1 + 1000)  ==> 30,000bps

• With S/N ratio = 1000, there is little hope to accomplish more than 30 Kbps
• So, why do we have 56Kbps modems?
• 56Kbps modems have this speed only with good quality telephone lines with larger S/N ratios and if they also use compression.
• S/N is usually given in decibels (dB)
• Convert S/N in dB = 10 log S/N
• Example

• Example

• Conclusion
• Nyquist Theorem implies that we could increase throughput by finding new ways to encode more bits per cycle
• Shannon's Theorem implies that no amount of clever encoding can overcome the limit of real transmission lines with noise.

• Using a signal channel to transmit separate sources of information

• Multiplexing needs to prevent interference among signals.
• Multiplexer--accepts data from different sources and combines them to send them through a shared channel.
• Demultiplexer--receives the combined data and extracts the data that corresponds to each destination.
• Two kinds of multiplexing:
• Time division multiplexing (TDM)
• Frequency division multiplexing (FDM)