index.md (4184B)
1 +++ 2 title = "Examples of networks" 3 +++ 4 5 # Examples of networks 6 **Public switched telephone network (PSTN)** 7 8 - the old system used an operator who would connect wires using a jumper cable 9 - components: 10 - local loops — analog twisted pairs to houses/businesses 11 - trunks — digital fiber optic links between switching offices 12 - switching offices — move calls from one trunk to another 13 14  15 16 the local loop: 17 18 - two wires from telephone company end office into houses 19 - to send bits, must convert to analog signals to transmit, and then back again 20 - enter the ‘modem’ (modulator demodulator) 21 - inserted between digital computer and analog telephone system 22 - built into computer (e.g. telephone modems) or in a separate box (DSL and cable modems) 23 - low rates at <56 kbps, early way to connect to internet 24 25  26 27 - DSL (Digital Subscriber Lines) — telephone/computers attached to same old phone line, rates vary with line 28 - Fiber to the Home (FttX) — relies on deployment of fiber optic cables, one wavelength shared among many houses, no need for amplifiers 29 30 trunks & multiplexing (PCM: pulse code modulation) 31 32 - calls are carried using TDM (time division multiplexing), but that only works with digital data 33 - codec (coder decoder) converts analog signal to digital by sampling at 8 kHz (since telephone channel has 4 kHz bandwidth) 34 - each sample is quantised to an 8-bit number 35 36 switching 37 38 - circuit switching — establish a physical path, *before *any data is sent (the old way of doing stuff) 39 - wasted bandwidth if reserved bandwidth is not used for traffic 40 - guaranteed service, but wasted resources 41 - packet switching — packets are sent as soon as they’re available, don’t need a path in advance 42 - routers have to use store-and-forward transmission to send each packet 43 - no fixed path, different packets can follow different paths and could come out of order 44 - no bandwidth is reserved, packets might wait to be forwarded, introducing queuing delay 45 - queuing delay for M/M/1 system: Q = 1/(1-ρ) * T 46 - ρ is line utilisation 47 48 - however, no bandwidth is wasted because of no traffic on reserved channel like in circuit switching 49 - guaranteed service and no resources wasted 50 51  52 53 **Mobile Telephone System** 54 Generations: 55 56 - 1G: analog voice. modulation based on FM radio. ex: AMPS 57 - 2G: analog voice and digital data. modulation based on QPSK. ex: GSM 58 - 3G: digital voice and data. modulation based on CDMA. ex: UMTS 59 - LTE: digital data including voice. modulation based on OFDM. ex: LTE 60 - 4G: based on CDMA and 802.16m. 61 62 Why “cell/cellular”? 63 64 - Based on idea of spatial regions called cells 65 - Moving across cells causes handoffs 66 - Frequencies are reused across non-adjacent cells 67 - Smaller cells can be used to support more mobile phones 68 69 2G GSM: 70 71 - mobile has handset and SIM (Subscriber Identity Module) card with creds 72 - mobiles tell HLR (home location register) whereabouts for incoming calls 73 - cells keep track of visiting mobiles 74 - air interface has FDM channels of 200 KHz, with eight-slot TDM frame every ~4.6 ms 75 76  77 78 3G GSM: 79 80 - high quality voice, messaging, multimedia, internet access 81 - not compatible with 2G GSM 82 - air interface is based on CDMA over 5 MHz channels 83 84  85  86 87 4G: 88 89 - pure IPv6 packet switching, no circuit switching 90 - no voice except as VoIP 91 - 1 Gbps for stationary, 100 Mbps for moving user 92 - uses carrier aggregation (multiple bands) and OFDMA (orthogonal freq div mux access) 93 - OFDMA: each channel is broadcast in parallel on different freq bands 94 95 **Cable Television** 96 97 - internet over cable uses cable television plant 98 - data is sent on shared cable tree, not on DSL 99 - cable modems at customer implement physical layer of DOCSIS 100 101  102 103 