Ethernet Wi-Fi Detail

Wi-Fi/Ethernet LAN Strategy

• Wi-Fi and Ethernet are LANs, Local Area Networks (e.g. a room)
• Many computers, 1 shared medium
• Ethernet - shared wire
• Wi-Fi - shared radio channel
• 1. Every computer has its own address (addr)
• 2. Every computer listens to the shared channel all the time
• Packet "to:" field is the addr of the intended recipient
• How to send: wait for silence, broadcast the packet
• Everyone gets the packet - what "shared" means
• How to receive: look for a packet to: your addr

Wi-Fi Picture

Wi-Fi Broadcast

• Suppose the router has a packet that's supposed to go to computer2
• The router broadcasts that packet .. it goes to everyone on the LAN
• The non-intended recipients are supposed to ignore the packet

Ethernet/Wi-Fi Scenarios

Theme: Shared!

1. Busy LAN

• Suppose many many people are using the LAN
• (alternately one person is making lots of traffic)
• What does each computer observe?
• The shared radio channel is busy with many packets (i.e. not silent)
  Each computer gets fewer opportunities to use the channel
• Basically each computer gets a smaller "slice" as there are more computers
• It's shared!
• The system slows but does not break

2. Corruption

• Computer receives a packet, but the checksum does not match
  Maybe the receiver is at the outer-limit of radio range
• What does this mean?
• The packet was corrupted in transit
• The receiver will request re-send of that packet
• User observes: data comes through correct, but slowly (if many re-sends)

3. Bad Guy

• Bad guy intercepts packets intended for others
• Bad guy computer does not discard packets to: someone else
• It's a shared medium, they really can see all the packets!
• This is why we have encryption/https - a later lecture

Megabits Math

• "Bandwidth" speed of an internet connection - bits per second
• A typical LAN speed: 100 megabits per second (wired or wireless)
• Slow home internet connection is maybe 1 megabit
• Fast home internet is 20 megabits or more
• You local LAN speed it the fastest
• Your "upstream" internet connection speed is typically slower

Networking Math

• Recall 1500 byte packet, 1500 * 8 = 12000 bits
• One strategy: convert to bits first
• Q1: How many megabytes per second can a 100 mbps network send ignoring overhead?
• Q2: How many seconds are required to send 1500 byte packet on 100 megabit network?
• Q3: I have a 38 MB image. How long to send at 100 mbps?
• Here computing the "ideal"/no-overhead speed
• In reality, networking has a lot of overhead + possible sharing
• Actual obtained speed might be 50-80% of ideal

Solutions:

Q1 1 megabyte is 8 million bits
 How many times per second can we send 8 million bits?
 100 million bits-per-sec / 8 million bits
 100 / 8 = 12.5
 12.5 MB per sec
 In reality less due to overhead


Q2 1500 bytes is 12000 bits
 12000 / 100 million is 0.00012 seconds
 This packet ties up the wire very briefly!


Q3: We have 12.5 MB per second from Q1,
 so 38 / 12.5 = about 3 seconds