Lecture 2 - Notes

January 7, 2016

Overview of Networks

What is a network?

• End Systems, e.g., computers, phones, servers.
• Communication Links, e.g., cell broadcasts, broadband
• Intermediate Systems, e.g., routers, switches

Types of Networks

• Broadcast
  • Cable TV
• Switched
  • Circuit Switching
    • Public Switched Telephone Network (PSTN)
  • Packet Switching
    • The Internet

Packet Switching

definition: Packet switching is a method of transmitting data where the data is transformed into chunks --- called packets --- which are then shared across a network. Some properties of packet switching are

• Each packet contains an address that directs the packet to its destination
• A channel is occupied during the transmission of the packet only, and upon completion of the transmission the channel is made available for the transfer of other traffic
• Increases network efficiency by allowing data to be transmitted between multiple start and end points along the same connection in the network

Types of Delay

Propagation Delay

definition: The propagation delay is the amount of time that it takes for the head of the packet to reach its destination.

$$d_{prop} = \frac{d}{s}$$

where $d$ is the distance between the nodes and $s$ is the speed.

Queueing Delay

definition: The queueing delay is the time that a packet waits in a queue before being able to be processed. It is the least predictable delay and is based on the number of packets being sent at a particular time.

Processing Delay

definition: The processing delay the time it takes for a router to process the packet header, i.e, The time it takes to decide where the packet needs to go, to look for errors and to decrypt and encrypt the packet.

Transmission Delay (Store-And-Forward Delay)

definition: The transmission delay is the time that it takes for a packet's bits to be sent into the channel. It happens because it takes time to put the bits of a packet on the channel before those bits are actually sent and is very dependant on the packets size. The transmission delay can be expressed as,

$$d_{trans} = \frac{L}{R}$$

where $L$ is the number of bits and $R$ is the rate of transmission (often in bits per second).

End-To-End Delay

The total amount of time for a packet to reach its destination is

$$d_\text{end to end} = d_\text{transmission} + d_\text{propagation} + d_\text{processing} + d_\text{queue}$$

Example

You have a packet that is 7.5 Mbits which goes through 3 routers and each router processes at 1.5 Mbps.

$$\underset{\text{routers}}3 \cdot \frac { 7.5 ~\text{Megabits} } {\left( \frac{1.5 ~\text{Megabits}}{\text{Second}} \right)} = 15 ~\text{seconds}$$

So the total transmission time is 15 seconds