Networking

Question 1

Ethernet LAN Cable

Answer 1

• Local Area Network
• wire is about as thick as a drinking straw
• often yellow or blue
• one house or one floor of a building
  
  • 100 meters max

Question 2

Sending One Packet Process

Answer 2

• the natural unit of transmission within networking
• 1 packet = 1500 bytes
• Say we have a packet of 1500 bytes of information we want to send. Each byte is 8 bits, so that’s 12000 bits to send, where each bit is a 0 or 1
• The sending computer could read through the 12000 bits in order, and for each 1 bit, put 3 volts between the wires, and for each 0 bit, put 0 volts between the wires
• The receiving computer can follow along, noting the 3v/0v pattern on the wires over time and so receive the 12000 bits

Question 3

Packet Errors - Checksum Re-send

Answer 3

• the receiver of the packet can detect if some of the bits in the packet got corrupted in transmission
• go through all the bytes, and add them all up
• the checksum is the last 2 digits of the sum of all the bytes; send that checksum as an extra byte along with the rest of the packet data
• the receiver can do the same computation – adding up all the bytes – to check that they get the same checksum

Question 4

Ethernet on Multiple Computers

Answer 4

• only one computer should transmit at a time
• to send data, the sender divides their message into small “packets” of, say, around 1500 bytes
• every packet begins with the address of the recipient
• each computers on the LAN has a unique address called the MAC address (Media Access Control)
• there is one wire, and all the computers are connected to it – they share the wire – this is what makes it “broadband” as they all share the one medium
• all computers listen to the wire all the time, picking up packets addressed to them and ignoring packets not for them
• Wi-Fi is similar and the “air” is the shared medium

Question 5

Ethernet Collision

Answer 5

• sometimes two senders send at the same time, and so their packets “collide” on the wire and get garbled
• the network hardware can usually detect this “collision” and so know to stop transmitting, as those sends are ruined
• the senders follow a “wait/re-transmit” protocol to re-send packets – wait a random amount of time – one of Metcalfe’s breakthrough ideas – and then try again when the wire is quiet

Question 6

Ethernet Design Advantages

Answer 6

• Shared – there’s just the one wire and everybody uses it (cheap)
  

Incredibly successful design strategy – getting great performance out of minimal hardware

• Distributed and Collaborative – no central control, depends on each computer following the collaborative protocol in good faith
• Insecure – not to hard to listen and pick up packets not intended for you (shared)
• Performance degrades but does not break as more computers use the shared medium

Question 7

Suppose a computer wants to send a packet on ethernet, as shown in lecture. The computer first waits for what?

Answer 7

Waits for the ethernet wire to be quiet

Question 8

Suppose we are using the same checksum scheme as in lecture: add up all the bytes, and take just the last two digits of the sum. What is the checksum of the following bytes: 101, 202, 103, 100, 210, 120?

Answer 8

36

Question 9

What is the purpose of a packet checksum?

Answer 9

allow the recipient to detect if bytes received were corrupted in transit