Examples of networks

Public switched telephone network (PSTN)

• the old system used an operator who would connect wires using a jumper cable
• components:
  • local loops — analog twisted pairs to houses/businesses
  • trunks — digital fiber optic links between switching offices
  • switching offices — move calls from one trunk to another

the local loop:

• two wires from telephone company end office into houses
• to send bits, must convert to analog signals to transmit, and then back again
• enter the ‘modem’ (modulator demodulator)
  • inserted between digital computer and analog telephone system
  • built into computer (e.g. telephone modems) or in a separate box (DSL and cable modems)
  • low rates at <56 kbps, early way to connect to internet

• DSL (Digital Subscriber Lines) — telephone/computers attached to same old phone line, rates vary with line
• Fiber to the Home (FttX) — relies on deployment of fiber optic cables, one wavelength shared among many houses, no need for amplifiers

trunks & multiplexing (PCM: pulse code modulation)

• calls are carried using TDM (time division multiplexing), but that only works with digital data
• codec (coder decoder) converts analog signal to digital by sampling at 8 kHz (since telephone channel has 4 kHz bandwidth)
• each sample is quantised to an 8-bit number

switching

• circuit switching — establish a physical path, *before *any data is sent (the old way of doing stuff)
  • wasted bandwidth if reserved bandwidth is not used for traffic
  • guaranteed service, but wasted resources
• packet switching — packets are sent as soon as they’re available, don’t need a path in advance

  • routers have to use store-and-forward transmission to send each packet

  • no fixed path, different packets can follow different paths and could come out of order

  • no bandwidth is reserved, packets might wait to be forwarded, introducing queuing delay

    • queuing delay for M/M/1 system: Q = 1/(1-ρ) * T
    • ρ is line utilisation

  • however, no bandwidth is wasted because of no traffic on reserved channel like in circuit switching

  • guaranteed service and no resources wasted

Mobile Telephone System Generations:

• 1G: analog voice. modulation based on FM radio. ex: AMPS
• 2G: analog voice and digital data. modulation based on QPSK. ex: GSM
• 3G: digital voice and data. modulation based on CDMA. ex: UMTS
• LTE: digital data including voice. modulation based on OFDM. ex: LTE
• 4G: based on CDMA and 802.16m.

Why “cell/cellular”?

• Based on idea of spatial regions called cells
• Moving across cells causes handoffs
• Frequencies are reused across non-adjacent cells
• Smaller cells can be used to support more mobile phones

2G GSM:

• mobile has handset and SIM (Subscriber Identity Module) card with creds
• mobiles tell HLR (home location register) whereabouts for incoming calls
• cells keep track of visiting mobiles
• air interface has FDM channels of 200 KHz, with eight-slot TDM frame every ~4.6 ms

3G GSM:

• high quality voice, messaging, multimedia, internet access
• not compatible with 2G GSM
• air interface is based on CDMA over 5 MHz channels

4G:

• pure IPv6 packet switching, no circuit switching
• no voice except as VoIP
• 1 Gbps for stationary, 100 Mbps for moving user
• uses carrier aggregation (multiple bands) and OFDMA (orthogonal freq div mux access)
  • OFDMA: each channel is broadcast in parallel on different freq bands

Cable Television

• internet over cable uses cable television plant
• data is sent on shared cable tree, not on DSL
• cable modems at customer implement physical layer of DOCSIS