Date: 9/6/2000

Network byte order and host file order and host byte order

Ntohs() do the byte swapping only in the architectures that need it (i.e. Pentiums). In architectures such as sparcs ntohs() does nothing

Network byte order is the same as Little Endian

Little Endian à least significant byte is the lowest in memory (packet)

Big Endian à least signifigant byte is the highest in memory (packet)

Ntohl() same as ntohs() but for longs (4 byte quantity)

Time Division Multiplexing (TDM)

Frequency Division Multiplexing (FDM)

Multiple items are sent simultaneously using multiple channels of different frequencies.

Example: Radio, TV, and Cable

Principle: 2 or more signals that use different carrier frequencies can be transmitted over the same medium without interference.

A signal in time has a corresponding frequency Spectrum

With Frequency Division Multiplexing each channel has a different frequency carrier.

When applied to light FDM is known as Wave Division Multiplexing (WDM).

It is also known as Color Division Multiplexing (CDM)

This is used to transmit multiple channels in optical fibers

Baseband Transmission

Use only frequency carrier to send information

Data is encoded directly

Traditional Modem

Baseband Transmission

Uses multiple frequencies to achieve higher throughput

DSL Modem

Byte Orders

In a network a 2 byte number is transmitted in such a way that the least significant byte appears first in the packet

The same for 4 byte number ether packet

IP Ethernet type 0x800 : (08 is 1 byte; 00 is 1 byte)

In Memory

However, some architectures like Pentium expect the least significant byte to be in the highest memory address

Time Division Multiplexing

The stored channel is used by only one transmitter at a time

Part II

Packets, Frames, Parity, Checksums, and CRC

• Main reasons for shared networks
• The alternative is to connect computers to one another

• This approach is too expensive in both wires, interfaces, and maintenance
• This is the reason that shared networks are needed

Problems with Shared Networks

• Throughput decreases in the presence of traffic
• Some appliances may have larger transfers
• Some applications need a fast/high response
• Access to the network has to be fair

Solution for Fairness (solve 2, 3, and 4)

• Use packet switching
• Packet switching consists of dividing the data into small units called packets
• Allow each station to send a packet before any station sends another packet
• This is a form of time division-division multiplexing
• Each packet has an identification with the destination known

Packet Switching Steps

1. Acquire shared medium
2. Send one packet
3. Allow other stations to send packets before sending more packets again

This structure of a packet depends on the hardware characteristics

• Minimum/Maximum length
• Format
• The structure of a packet is also called "the frame" of the packet

Summary Part 2

Media: Electricity, Light, and RF

Digital vs. Analog

RS232, serial line

Modems

à Used for long distance communications

Delay/Throughput

Nyquist Theorem

• Relates Bandwidth and Throughput
• Encourages the use of computer encoding
• Does not consider noise

Shannon Theorem

• Considers Noise
• Encourages a limit in throughput in real systems

Multiplexing

• TDM
• FDM