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
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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
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Used to build dashboards
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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
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Variables and ASN.1
So Far
• Network Management (SNMP)
– ASN.1
• ADSL
• Cable Internet
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Public Switched Telephone Network
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• 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)
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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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So
urc
e: W
ikip
ed
ia
Before automatic switching
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Automatic Switching
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Local Offices
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Local Offices
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NYC Long Lines Building
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Sourc
e:
Wik
ipedia
Switches (4ESS – 1970s)
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Sourc
e:
http://w
ww
.ph
world.o
rg/s
witch/w
eess.h
tm
Switches (5ESS – 1980s)
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Sourc
e:
http://w
ww
.ph
world.o
rg/s
witch/w
eess.h
tm
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.)
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28.8kbps 28.8kbps/33.6kbps
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Modems
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
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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
𝑠𝑦𝑚𝑏𝑜𝑙
c)QAM-64: 6𝑏𝑖𝑡𝑠
𝑠𝑦𝑚𝑏𝑜𝑙
d)QAM-128: 7𝑏𝑖𝑡𝑠
𝑠𝑦𝑚𝑏𝑜𝑙
• At high speeds, add Trellis Coded Modulations (TCM)
for error correction
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Modulation Techniques
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Modem Limits
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• 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
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Source: Wikipedia
Noisy-Channel Coding Thm
Theorem establishes
how much data can be
sent over a link with
noise on it
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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
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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
• Solution: Shorten distance to local offices
Bezeq Local Offices
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Bezeq Communication Centers
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So
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e:H
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retz
DSL Speeds
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• 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
𝑠𝑦𝑚𝑏𝑜𝑙
• 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
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So Far
• Network Management (SNMP)
– ASN.1
• ADSL
• Cable Internet
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Community Antenna Television
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• 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
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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
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Sharing the Cable
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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
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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
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Cable Modem Bootstrapping
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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
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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
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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)
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Cable versus ADSL
Conclusion
• Network Management (SNMP)
– ASN.1
• ADSL
• Cable Internet
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