ADSL part 2, Cable Internet, Cellular 20 June 2016 Lecture 12 20 June 2016 SE 428: Advanced Computer Networks 1
Topics for Today ADSL Cable Internet Cellular Radio Networks 20 June 2016 SE 428: Advanced Computer Networks 2
Old Style: Modems Bit rate: Modulation determines bits per symbol: bits symbols symbol s = bits s a)quadrature Phase Shift Keying (QPSK) 2bits symbol b)quadrature Amplitude Modulation (QAM-16): 4bits symbol c)qam-64: 6bits symbol d)qam-128: 7bits symbol At high speeds, add Trellis Coded Modulations (TCM) for error correction 20 June 2016 SE 428: Advanced Computer Networks 3
Modulation Techniques 20 June 2016 SE 428: Advanced Computer Networks 4
Modem Limits V.34bis modem has 14 bits symbol 2400 symbols s = 33,600bps (use compression to increase) Above 33,600, reach Shannon limit for telephone system and local loops (~35kpbs) Solution: Remove one local loop (ISP 2 receives only digital signals; ISP 1 has modems) Could get 70kbps, but channel is 4000Hz wide 8000 samples/second max (Nyquist Theorem) Bits per sample (in US) is 7 (1 used for control) 56,000bps (V.90) 33.6kbps upstream, 56kbps downstream Noise often prevents even reaching 33.6kbps and most users download more than upload V.92 has 48kbps upstream 20 June 2016 SE 428: Advanced Computer Networks 5
Nyquist Rate Theorem establishes that for a link with bandwidth B, maximum symbol rate is 2 B Source: Wikipedia 20 June 2016 SE 428: Advanced Computer Networks 6
Noisy-Channel Coding Thm Theorem establishes how much data can be sent over a link with noise on it Source: Wikipedia 20 June 2016 SE 428: Advanced Computer Networks 7
Digital Subscriber Lines (DSL) Developed by phone companies to compete with cable TV speeds of 10Mbps Called Broadband by marketing Specifies physical layer (may run ATM for link layer) Many standards, we ll discuss Asymmetric DSL (ADSL) Symmetric Digital Subscriber Line (SDSL) ISDN Digital Subscriber Line (IDSL) High Bit Rate Digital Subscriber Line (HDSL) Single Pair High Speed Digital Subscriber Line (G.SHDSL) Phone designed for voice: Filters remove everything not in 300Hz to 3400Hz range Hence Nyquist Theorem problem (~4000Hz) Solution: Get rid of the filter 20 June 2016 SE 428: Advanced Computer Networks 8
DSL Length Limitations Graph for Cat 3 UTP with new cables, good thickness Meaning: If you re too far from the local office, you can t get good high speed DSL Solution: Shorten distance to local offices 20 June 2016 SE 428: Advanced Computer Networks 9
Bezeq Local Offices 20 June 2016 SE 428: Advanced Computer Networks 10
Source:Haaretz Bezeq Communication Centers 20 June 2016 SE 428: Advanced Computer Networks 11
DSL Speeds Line has about 1.1MHz spectrum Divide into 256 independent channels 4312.5Hz Channel 0: Voice Channels 1-5: Clear to avoid crosstalk Channels 6-256: 1 upstream control, 1 downstream control, rest data Could do 50% upstream and 50% downstream, but most do 80-90% downstream Some do 32 upstream, rest down Can reach 24Mbps downstream, 3.3Mbps upstream (ADSL2+) 20 June 2016 SE 428: Advanced Computer Networks 12
ADSL Details Modulation similar to V.90: 4000 baud ( symbols ) s 15 bits QAM modulation with up to symbol ADSL modem is 250 QAM modems working in parallel Example: 224 downstream channels 15 bits/symbol 4000 baud 224 15 bits 4000 symbols = 13.44Mbps symbol s Splitter separates voice (< 26kHz) from data (> 26kHz) At home use a Network Interface Device (NID) for building or microfilter on each jack At telco, Digital Subscriber Line Access Multiplexer splits off data 20 June 2016 SE 428: Advanced Computer Networks 13
So Far ADSL Cable Internet Cellular Radio Networks 20 June 2016 SE 428: Advanced Computer Networks 14
Community Antenna Television 1940: Rural US had no TV reception people put big antennas on hills to grab TV signals and pass along via coaxial cable By 1970: 1000s of operators Fees paid for infrastructure Spliced new cables, added amplifiers for new subscribers 1974: Home Box Office (HBO) cable-only channel via satellite Many followers Lead to large companies buying up small ones, connecting cities to distribute new channels Similar to how telephone service grew to enable long distance 20 June 2016 SE 428: Advanced Computer Networks 15
Internet over Cable Intercity cables turned to high bandwidth fiber optic Coaxial just for cables to houses (last mile) Hybrid Fiber Coax (HFC) Fiber nodes convert optical to coax Fiber much higher bandwidth than coax, so many coax lines share a fiber line Began to offer internet over cable 20 June 2016 SE 428: Advanced Computer Networks 16
Internet over Cable Problem: Cable TV is download only, but internet is up and down Coax cable is shared! Solution: Split up long cables and connect to fiber nodes Put 500-2000 houses on a single coax cable More subscribers more splitting and fiber nodes 20 June 2016 SE 428: Advanced Computer Networks 17
Sharing the Cable Cable internet lives with Cable TV on the coax In US, CATV uses 54 550MHz region (FM radio in 88MHz 108MHz) US Channels are 6MHz wide, including buffer (guard bands) European channels start at 65MHz and are 6 8MHz wide (PAL and SECAM higher resolutions) 20 June 2016 SE 428: Advanced Computer Networks 18
Sharing the Cable Cable modems can use up to 750MHz Use lower band (5 42MHz) for upstream and > 500MHz for downstream Since TV is downstream, < 54MHz is upstream and > 54MHz is downstream (easy filtering) i.e. upstream bandwidth is lower than downstream Downstream Modulation: 6 8MHz channel with QAM-64 (27Mbps net out of 36Mbps) or QAM-256 (39Mbps net) Upstream Modulation: QPSK ( 2 bits ) due to too much noise (12Mbps) symbol 20 June 2016 SE 428: Advanced Computer Networks 19
Cable Modems Some providers use proprietary modems CableLabs along with major cable providers produced Data Over Cable Service Interface Specification (DOCSIS) HOT in Israel is built using DOCSIS 3 European version is EuroDOCSIS Modem is always online When it boots, uses a bootstrapping protocol to get connected to headend 20 June 2016 SE 428: Advanced Computer Networks 20
Cable Modem Bootstrapping 1. Modem scans downstream channels for bootstrapping message sent out by headend every so often System parameters 2. Modem announces itself on an upstream channel 3. Headend acks and assigns upstream and downstream channels Assigns minislot for requesting upstream bandwidth (size varies, 8B is common) Modems may share minislots contention 20 June 2016 SE 428: Advanced Computer Networks 21
Cable Modem Bootstrapping 4. Modem finds distance to headend by sending ranging packet and waiting for response Distance key to get timing right Headend announces minislot rounds, but not all hear it at the same time due to distance 5. Sends DHCP packet to ISP 6. Establishes secret (shared) encryption key with headend, authenticates 20 June 2016 SE 428: Advanced Computer Networks 22
Cable Modem Communication Sending to Headend 1. Computer passes data to cable modem 2. Modem requests #minislots needed (using minislot request channel) 3. Headend accepts: ACKs with minislots assigned More packets can be requested using header field 4. Headend rejects or contention: No ACK Modem waits random time and retries Exponential backoff Receiving from Headend One sender, so no contention Downstream is usually larger than upstream Use 204B packet size (184B + error correcting codes + overhead) Compatibility with MPEG-2 and TV 20 June 2016 SE 428: Advanced Computer Networks 23
Cable versus ADSL Cable Internet Coax cables have more bandwidth than telephone, but: Coax carries TV too Can t give hard bandwidth guarantees Depends on usage and how many are online Coax cable is shared by subscribers Similar to cell phones Encryption essential (check the ciphers) Requires dedicated cables Often less reliable than telephone (more things to go wrong) ADSL Adding more users doesn t affect others directly Each has its own local loop ADSL stays within its target bandwidth most of the time Limited by distance to local office Uses phone lines (which most people have) More secure (no shared medium) 20 June 2016 SE 428: Advanced Computer Networks 24
So Far ADSL Cable Internet Cellular Radio Networks 20 June 2016 SE 428: Advanced Computer Networks 25
Cellular Wireless Networks Key technology for mobiles and wireless nets Developed to increase mobile phone capacity Based on multiple low power transmitters Area divided into cells In a tiling pattern to provide full coverage Each with own antenna Each with own range of frequencies Served by base station Adjacent cells use different frequencies to avoid crosstalk 20 June 2016 SE 428: Advanced Computer Networks 26
Cellular Geometries 20 June 2016 SE 428: Advanced Computer Networks 27
Frequency Reuse Must manage reuse of frequencies Power of base transceiver controlled Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Typically 10 50 frequencies per cell Example: Advanced Mobile Phone Service (AMPS) N cells all using same number of frequencies K total number of frequencies used in systems Each cell has K frequencies N K = 395, N = 7 giving 57 frequencies per cell on average 20 June 2016 SE 428: Advanced Computer Networks 28
Frequency Reuse Patterns 20 June 2016 SE 428: Advanced Computer Networks 29
Frequency Reuse Patterns 20 June 2016 SE 428: Advanced Computer Networks 30
Increasing Capacity Add new channels Not all channels used to start with Frequency borrowing Taken from adjacent cells by congested cells Or assign frequencies dynamically Cell splitting Non-uniform topography and traffic distribution Use smaller cells in high use areas 20 June 2016 SE 428: Advanced Computer Networks 31
Cell Splitting 20 June 2016 SE 428: Advanced Computer Networks 32
Increasing Capacity Cell sectoring Cell divided into wedge shaped sectors (3 6 per cell) Each with own channel set Directional antennas Microcells Move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings Use reduced power to cover a much smaller area Good for city streets, roads, inside large buildings 20 June 2016 SE 428: Advanced Computer Networks 33
Cell Sectors 20 June 2016 SE 428: Advanced Computer Networks 34
Microcells 20 June 2016 SE 428: Advanced Computer Networks 35
Femtocells 20 June 2016 SE 428: Advanced Computer Networks 36
Frequency Reuse Example 20 June 2016 SE 428: Advanced Computer Networks 37
Components of cellular network architecture cell Covers geographical region Base station (BS) analogous to 802.11 AP Mobile users attach to network through BS Air-interface: physical and link layer protocol between mobile and BS MSC connects cells to wired tel. net. manages call setup (more later!) handles mobility (more later!) Mobile Switching Center Mobile Switching Center Public telephone network wired network 20 June 2016 SE 428: Advanced Computer Networks 38
Cellular networks: the first hop Two techniques for sharing mobile-to-bs radio spectrum combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots CDMA: code division multiple access frequency bands time slots 20 June 2016 SE 428: Advanced Computer Networks 39
Cellular System Channels System is fully automated See two types of channels between mobile and base station (BS) Control channels Set up and maintain calls Establish relationship between mobile unit and nearest BS Traffic channels Carry voice and data 20 June 2016 SE 428: Advanced Computer Networks 40
Call Stages 20 June 2016 SE 428: Advanced Computer Networks 41
Other Functions Call blocking If all traffic channels busy Call termination When user hangs up Call drop When BS cannot maintain required signal strength Calls to/from fixed and remote mobile subscriber MTSO connects mobile user and fixed line via PSTN MTSO connects to remote MTSO via PSTN or dedicated lines 20 June 2016 SE 428: Advanced Computer Networks 42
2G (voice) network architecture Base station system (BSS) BTS BSC MSC G Gateway MSC Public telephone network Legend Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center (MSC) Mobile subscribers 20 June 2016 SE 428: Advanced Computer Networks 43
3G (voice+data) network architecture radio network controller Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network voice network unchanged in core data network operates in parallel MSC SGSN G Public telephone network Gateway MSC G GGSN Public Internet Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN) 20 June 2016 SE 428: Advanced Computer Networks 44
3G (voice+data) network architecture radio network controller MSC G Public telephone network Gateway MSC SGSN G GGSN Public Internet radio interface (WCDMA, HSPA) radio access network Universal Terrestrial Radio Access Network (UTRAN) core network General Packet Radio Service (GPRS) Core Network public Internet 20 June 2016 SE 428: Advanced Computer Networks 45
Conclusion ADSL Cable Internet Cellular Radio Networks 20 June 2016 SE 428: Advanced Computer Networks 46