1. What is the Internet?

• Both PC and server are hosts.
• Network Applications: WhatsApp, Zoom, browsers are examples of applications that are run by hosts to communicate over the network.

2. Network Edge

• Home network, mobile network, institutional networks are all examples of access network, aka network edge.
  • Hosts access the internet through access network.
• Routers provide ip addresses to the hosts, allows them to be connected to the internet.

3. Network Core

• Network core is a mesh of interconnected routers.

3-1. How is Data transmitted through Core?

Circuit Switching:

• Used for telephones mostly.
• Circuits between routers that lead to the destination must be reserved before the data gets transmitted.
• If there are too many devices trying to reserve, some users may not be served.

Packet Switching:

• Used for internet.
• Receiver will receive packets - smaller chunks of the whole data - and combine them to restore the original data.
  
• R is the transmission rate, aka link capacity, link bandwidth.
• A packet will be sent bit by bit.
  • every bit is sent as sin or cosin waves, which is translated to binary numbers.

Packet Switching: Store and Forward

• The entire packet must arrive at a router, if there is one, before being trasmitted on the next link.
• Because of this, the transmission delay is twice as long as what would have been just L / R.
  • so it is 2 * L / R.

Routing and Addressing

• Each packet must carry source and destination information for the routing algorithm to work:
  • routing algorithm chooses the shortest path to get from one end to another.

3-2. Internet Structure: Network of Networks

• Internet is not owned by a single organization!
  
• Each ISP is connected with other ISPs.
  • EX) If you are sending email to the US,
    • Email goes through nus network,
    • Then to Singtel,
    • Then to Hongkong (for example)
    • Then several countries then to the US.

4. Delay, Loss and Throughput in Networks

4-1. How do Delay and Loss Occur?

• Packets get queued in the router.
  • The queue is known as buffer.
• If the queue is full but another packet tries to get it, packet loss may occur.
  • In more technical terms, when packet arrival time exceeds packet departure time, packet loss may occur.
    

4-2. 4 Sources of Packet Delay

Nodal Processing:

• Router processes incoming packet to check integrity.
• If a packet is corrupted, the router will discard it.
• Typically less than a milisecond.

Queuing Delay:

• Time a packet waits in the queue.
• Depends on congestion level of a router.

Transmission Delay:

• L (packet length in bits) / R (Band with - how fast can a bit be converted into em wave in bps)
• How long does it take to push the packet onto the transmission link

Propagation Delay:

• d (length of the physical link) / s (propagation speed in the medium)

The four delays comprise the end-to-end packet delay.

4-3. Throughput

• How many bits can be transmitted in how much time?
• Total bits transmitted / time it took.
  
• F can be cancelled out.

5. Network Protocol

• Protocols regulate communication activities in a network.
  • Define the format and order of messages exchanged between hosts for a specific purpose.