ADSL part 2, Cable Internet, Cellular

20 June 2016 SE 428: Advanced Computer Networks ₁

ADSL part 2, Cable Internet,

Cellular

Topics for Today

• ADSL

• Cable Internet

Old Style: Modems

• Bit rate

: Modulation determines bits per symbol:

𝑏𝑖𝑡𝑠 𝑠𝑦𝑚𝑏𝑜𝑙

×

=

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

Modem Limits

20 June 2016 SE 428: Advanced Computer Networks ₅

• 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

Nyquist Rate

Theorem establishes

that for a link with

bandwidth 𝐵, maximum

symbol rate is 2 × B

Noisy-Channel Coding Thm

Theorem establishes

how much data can be

sent over a link with

noise on it

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)

DSL Length Limitations

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• 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

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

ADSL Details

Modulation similar to V.90: • 4000 baud (𝑠𝑦𝑚𝑏𝑜𝑙𝑠_𝑠 )

• QAM modulation with up to _{𝑠𝑦𝑚𝑏𝑜𝑙}15 𝑏𝑖𝑡𝑠

• ADSL modem is 250 QAM modems working in parallel Example: • 224 downstream channels • 15 bits/symbol • 4000 baud 224 × 15 𝑏𝑖𝑡𝑠 𝑠𝑦𝑚𝑏𝑜𝑙× 4000 𝑠𝑦𝑚𝑏𝑜𝑙𝑠 𝑠 = 13.44𝑀𝑏𝑝𝑠

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

So Far

• ADSL

• Cable Internet

Community Antenna Television

20 June 2016 SE 428: Advanced Computer Networks ₁₅

• 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

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

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

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)

Sharing the Cable

20 June 2016 SE 428: Advanced Computer Networks ₁₉

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)

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

Cable Modem Bootstrapping

20 June 2016 SE 428: Advanced Computer Networks ₂₁

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)

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

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 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)

So Far

• ADSL

• Cable Internet

• Cellular Radio Networks

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

Cellular Geometries

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

𝑁 frequencies

Frequency Reuse Patterns

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

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

Microcells

Frequency Reuse Example

Components of cellular network architecture

 connects cells to wired tel. net.

 manages call setup (more later!)

 handles mobility (more later!)

MSC  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

cell

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

SE 428: Advanced Computer Networks

frequency bands

time slots

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

Call Stages

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

2G (voice) network architecture

SE 428: Advanced Computer Networks

BSC

BTS

Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center (MSC)

Mobile subscribers Base station system (BSS)

3G (voice+data) network architecture

Serving GPRS Support Node (SGSN)

Public Internet

GGSN G

Key insight: new cellular data

network operates in parallel (except at edge) with existing cellular voice network

 voice network unchanged in

core

3G (voice+data) network architecture

SE 428: Advanced Computer Networks ₄₅

radio network controller MSC SGSN Public telephone network Gateway MSC G Public Internet GGSN G

radio access network Universal Terrestrial Radio

Access Network (UTRAN)

core network

Conclusion

• ADSL

• Cable Internet