lecture 13: snmp, adsl, cable internet · • network management (snmp) – asn.1 • adsl •...

12 Jan 2020 SE 428: Advanced Computer Networks 1

SNMP, ADSL, Cable Internet

12 January 2020

Lecture 12

Topics for Today

• Network Management (SNMP)

– ASN.1

• ADSL

• Cable Internet

Source:

• Stallings 14

• Tanenbaum 2.5


Network Management• When you have hundreds of servers and routers, it’s hard to

manage them all manually → remote protocol– Lets network admin track status

• Simple Network Management Protocol (SNMP)– Request/Reply protocol (GET/SET)

• Requests from network nodes information types (Management Information Base – MIB):– System: General parameters

– Interfaces: Physical addresses, packets sent on each interface

– Address translation: ARP and translation tables

– IP: Routing table, number of successfully routed packets, reassembly, drops

– TCP: Connections, timeouts, resets, per connection information

– UDP: Datagrams sent and received


Used to build dashboards


Abstract Syntax Notation (ASN.1)

• Basic Encoding Rules (BER)

• Data items in the form: <tag, length,

value>

– Tag: 8 bit field (can be multibyte)

– Length: how many bytes follow

• Less than 127 B, length has the length

• >127B, length has how many B in the

length

– Value can nest other data items

ASN.1 in SNMP

• Variables listed in dot

notation

– 1.3.6.1.2.1.4.3: IP field

called ipInReceives

(number of IP datagrams

received)

– 1.3.6.1.2.1: all MIB fields

– 4 is the IP group

– 3 is the ipInReceives field


Variables and ASN.1

So Far


– ASN.1

• ADSL

• Cable Internet


Public Switched Telephone Network


• Easiest way to connect two people/companies → string a

cable between them

– On private property, ok

– Over/under public property? No

• Scaling problems → Public Switched Telephone Network

About PSTN (POTS)

• Built to move voice,

not data

– Cable between 2

computers →

1,000,000,000𝑏𝑝𝑠

– Dialup over PSTN →

56,000𝑏𝑝𝑠

• Last mile: 2 copper

lines from phone to

local end office

(Category 3 UTP)


Hierarchical Switching

• End office → Regional

(Toll) Office via toll

connecting trunks

• Toll Office →

Intermediate

Switching Office via

intertoll trunks

• May use coaxial

cables, microwaves,

or fiber


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Before automatic switching


Automatic Switching


Local Offices


NYC Long Lines Building


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Switches (4ESS – 1970s)


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Switches (5ESS – 1980s)


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Old Style: Modems

Local loop is analog → bottleneck for high speeds

• Attenuation: loss of energy (dB per km)

• Distortion: Fourier components propagate at different

speeds

• Noise: Energy from other sources (thermal, crosstalk,

etc.)


28.8kbps 28.8kbps/33.6kbps


Modems

https://www.youtube.com/watch?v=gsNaR6FRuO0

https://www.youtube.com/watch?v=gsNaR6FRuO0

Old Style: Modems

Modem converts analog ↔ binary:

• Bandwidth: Range of frequencies that pass with

minimum attenuation

– Physical property of medium

• Baud: Samples per second → how many symbols

(pieces of information) per second: 𝑠𝑦𝑚𝑏𝑜𝑙𝑠

𝑠


Old Style: Modems

• Bit rate: Modulation determines bits per symbol:

𝑏𝑖𝑡𝑠

𝑠𝑦𝑚𝑏𝑜𝑙×

𝑠𝑦𝑚𝑏𝑜𝑙𝑠

𝑠=

𝑏𝑖𝑡𝑠

𝑠

a)Quadrature Phase Shift Keying (QPSK) 2𝑏𝑖𝑡𝑠

𝑠𝑦𝑚𝑏𝑜𝑙

b)Quadrature Amplitude Modulation (QAM-16): 4𝑏𝑖𝑡𝑠


c)QAM-64: 6𝑏𝑖𝑡𝑠


d)QAM-128: 7𝑏𝑖𝑡𝑠


• At high speeds, add Trellis Coded Modulations (TCM)

for error correction


Modulation Techniques


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

Nyquist Rate

Theorem establishes

that for a link with

bandwidth 𝐵, maximum

symbol rate is 2 × B


Source: Wikipedia

Noisy-Channel Coding Thm

Theorem establishes

how much data can be

sent over a link with

noise on it


Source: Wikipedia

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


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

Bezeq Local Offices


Bezeq Communication Centers


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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+)

ADSL DetailsModulation similar to V.90:

• 4000 baud (𝑠𝑦𝑚𝑏𝑜𝑙𝑠

𝑠)

• QAM modulation with up to 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


– ASN.1

• ADSL

• Cable Internet


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

Internet over CableIntercity 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


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)

• European channels start at 65𝑀𝐻𝑧 and are 6 −

8𝑀𝐻𝑧 wide (PAL and SECAM higher resolutions)

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

𝑠𝑦𝑚𝑏𝑜𝑙) due to too much noise (12𝑀𝑏𝑝𝑠)

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


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

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

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


Cable Modem Communication

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)


Cable versus ADSL

Conclusion


– ASN.1

• ADSL

• Cable Internet


lecture 13: snmp, adsl, cable internet · • network management (snmp) – asn.1 • adsl •...

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