MSc Revision Notes - Networks
Exam topics
Main topics
- Asynchronous communication
- Long distance communication
- Packets, frames and error detection
- LAN wiring
- IP
- ARP
- TCP/UDP
- NAT
- DNS
- Email/FTP/HTTP
- VPN
- IPv6/VOIP
Bandwidth
- Bandwidth is the maximum rate in Hz that the hardware can change the signal.
Nyquist's theorem: D = 2B log2 K where D is data rate in bps, B is bandwidth in Hz and K is the number of signal levels.
Asynchronous communication
- Sender and receiver do not need to synchronise before each transmission.
RS-232
- +5 to +15 volts - 0 bit (SPACE)
- -5 to -15 volts - 1 bit (MARK)
- Least-significant bit transmitted first
- Bits are recognised by timing
RS232 doesn't leave a zero voltage on the line so the sender must transmit a 0 start bit before the data, then 8 data bits, then leave the line idle for the time of 1 bit. This is the stop bit.
Long distance signals
- Sin waves travel longer distance than single bit changes on a wire.
- Uses amplitude modulation and frequency modulation
ADSL
- Broadband ADSL divides bandwidth of 1100kHz into 256 channels of 4kHz.
- Channel 0 is for POTS
- Channels 1 - 5 separate voice from data
- 1 for upstream / 1 for downstream
Packets, frames and error detection
Byte stuffing uses escape characters to send special characters like soh (start of header) and eot (end of transmission).
- * Parity bits
- An additional bit is computed based on the previous data bits. The receiver does the same computation and compares the bits. A single bit alteration can be detected. Parity can be even or odd. If odd, the parity bit is set so that the total number of 1 bits is odd and vice versa.
- * Checksums
- The sender computes the sum of the data and the receiver compares their computation. But simple summing cannot detect all common errors.
- Cyclic Redundancy Check
- Uses XOR units and shift registers to detect single/double-bit errors, errors containing an odd number of bits and burst errors.
- Can do error correction but usually detection and re-transmission is preferred.
