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
- 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
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
- 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
- "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
- 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