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.

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