Lecture 2 (Jan 11, 2018)

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern; badwidth is shared on demand
TDM (Time-Division Multiplexing): Each host gets same slot in revolving TDM frame
Packet switching vs. circuit switching
- Packet switching allows more users to use network
  - 1 MB/s link
  - Each user
    - 100kb/s when active
    - Active 10% of the time
- Circuit switching
  - 10 users
- Packet switching
  - With 35 users, probability > 10 active at the same time is less than 0.0004
    - 0.0004
- Packet switching is good for bursty data
  - Resource sharing
  - Simpler, no call setup
- Excessive congestion: packet delay and loss
  - Protocols needed for reliable data transfer, congestion control
- How to provide circuit-like behavior?
  - Bandwidth guarantees needed for audio/video apps
  - Still an unsolved problem

Throughput: How fast?
- Rate (bits/time) at which bits transferred between sender and receiver.
Delay: How responsive is the network?
Loss: How good is packet delivery?

How do loss and delay occur?
- Packets queue in router buffers
  - Packet arrival rate to link exceeds output link capacity
  - Packets queue, wait for turn
Four sources of packet delay
1. Nodal processing
  - Check bit errors
  - Determine output link
2. Queuing
  - Time waiting at output link for transmission
  - Depends on congestion level of router
Nodal delay (d_nodal = d_proc + d_queue + d_trans + d_prop)
- d_proc = processing delay
  - Typically a few microseconds or less
- d_queue = queuing delay
  - traffic intensity = L * a / R = Packet size (bits) * Average packet arrival rate / Link bandwidth (bps)
    - L * a / R = 0: average queuing delay is small
    - L * a / R = 1: average queuing delay is large
    - L * a / R > 1: more "work" is arriving than can be serviced; average delay is infinite
  - Depends on congestion
- d_trans = transmission delay
  - = L / R = Packet size (bits) / Link bandwidth (bps)
- d_prop = propagation delay
  - = d / s = Length of physical link / Propagation speed in medium (~2*10⁸ m/s)
  - A few microseconds to hundreds of microseconds
- What does "real" internet delay and loss look like?
  - Traceroute program: provides delay measurement from source to router