Modems and Beyond
Chapter 5
Copyright 2001 Prentice Hall
Revision 2: July 2001
2
Modems

Problem
– Computer is digital
– Telephone line is analog
– Need translation device called a modem
Analog
Signal
Digital
Signal
Modem
3
Modems

Note:
– Device is digital;Transmission line is analog
–
Cannot just say “modem translates between
analog and digital” (For analog device and
digital transmission line, need something else)
Digital
Device
Analog
Transmission Line
4
Modulation

A Modem is a Modulator and Demodulator
–
Modulation is converting outgoing digital device
signals into analog transmission line signals
–
Demodulation is converting incoming analog
transmission line signals into digital device signals
Analog
Signal
Digital
Signal
Modem
5
Modulation

Example: Frequency Modulation
– 1 is a high-frequency vibration
– 0 is a low-frequency vibration
0
1
1
1
Modem Forms

Internal Modem
– On printed circuit board inside PC
– Does not take up desktop space
– Can be complex to install--must open up
computer
6
Modem Forms

External Modem
– Easy to install (just plug into serial port)
– Takes up desk space
– Needs wire to serial port (adds to wiring
clutter)
– Needs electrical power
7
Modem Forms

PC Card Modems
– Fit in PC Card slots on notebooks
– Easy to install
– Do not take up desktop space
– Expensive
– Few desktop PCs have PC Card slots
8
Modem Connection for Internal Modem
Phone Line to
Wall Jack
Phone Line to
Telephone
9
Modem Connection for External Modem
Phone
Line to
Telephone
Serial Cable
to Serial
Port
Phone Line to
Wall Jack
Wall Power:
usually uses
“brick” transformer
10
Modem Standards

Two Modems Must Follow Same Standards
– Speed standards (modulation method)
– Error correction and compression standards
– Facsimile standards
– Modern modem standards are created by the
ITU-T
11
Modem Standards

Training Period
–
When two modems first start talking, they
negotiate standards to use; settle on highest
common standards
12
Modem Speed Standards (ITU-T)

V.90
– Receive at 56 kbps but send at only 33.6 kbps
– Not all phone lines, ISPs support 56 kbps
– Fall back to 33.6 kbps if cannot support

V.34
– 33.6 kbps send and receive

V.32 bis
– 14.4 kbps send and receive
– bis means second (version of the standard)
13
Modem Standards: Error Correction and
Compression

ITU-T
–
–
–

V.42: Error detection and correction
V.42 bis: Compression (up to 4:1)
Most newer modems have both
MNP Level 5
–
–
–
–
Not in Book
Proprietary standard of Microcom
Once widely used
Provides both error correction and compression
Now largely superceded by V.42 and V.42 bis
14
15
New Modem Standards
New

V.92
–
–
–
–

Can transmit faster than 33.6 kbps but only if the line
has unusually high top frequency cutoff
If transmits faster, reception speed falls below 56 kbps
Modem on hold: can talk a short time without breaking
the connection
Cuts call setup time in half
V.44
–
Can compress webpages better, cutting download time
in half.
Modem Standards

Facsimile Modem Standards
–
Most modems also act as fax modems
–
Facsimile requires different modulation than
data transmission
–
V.14: 14.4 kbps
–
V.29: 9.6 kbps
–
Yes: Facsimile is slow!
16
17
V.90 Modems

Telephone Bandwidth is Limited (Chapter 4)
–
Bandwidth limits speed
–
Limits speed to about 35 kbps
–
When you transmit, limited to 35 kbps
35 kbps
18
V.90 Modems

Upload (Sending) Speed is Limited
–
–
Analog-to-digital converter (ADC) at the
telephone company filters your incoming
signal
Limits you to about 35 kbps
PC
V.34
modem
33.6 kbps
ADC
Telephone
Network
19
V.90 Modems

However, Receive at 56 kbps
– ISP connects with digital line to the Telco
– Sends at 56 kbps
– No analog-to-digital converter to filter and so
limit speed
PC
56 kbps
modem
DAC
Telephone
Network
Digital
Link
No
ADC!
56 kbps
ISP
20
V.90 Modems

Receive at 56 kbps
– 56 kbps digital channels inside phone network
– Only local loop is analog today
– Trunk lines and switches support 56 kbps
transmission
Digital Switches and Trunk Lines
PC
56 kbps
modem
DAC
Telephone
Network
56 kbps
ISP
21
V.90 Modems

Receive at 56 kbps
–
Telephone company transmits in analog to
subscriber at 56 kbps
–
Digital to analog converter (DAC) does not
limit speed to less than 56 kbps. No filtering.
PC
56 kbps
modem
DAC
56 kbps
Telephone
Network
Digital
Link
ISP
22
V.90 Modem

Telephone Company Does Not Have to Do
Anything Differently
–
–
ADCs and DACs are already in place for
ordinary voice service
Lack of change in phone system allowed fast
implementation
ADC
PC
56 kbps
modem
DAC
Telephone
Network
Digital
Link
ISP
23
V.90 Modem

ISP Does Have to Do Things Differently
–
–
–
–
PC
Digital line to telephone network
Special equipment at ISP
Does not use a V.90 modem
If used V.90 modem, could only send at 33.6
kbps
56 kbps
modem
DAC
Telephone
Network
Digital
Link
No
V.90
Modem!
ISP
24
V.90 Modem

User Needs a V.90 Modem
–
Does not need a new phone line
–
Not all phone lines will work, but it is never
worth the cost to install a new line for V.90
modems
–
Same for V.92 modems
New
Alternatives to Ordinary Phone Lines & Modems

Faster Speeds; Called Broadband Services

Also Higher Cost
25
–
Translation devices more expensive than modems
–
Faster (more expensive) transmission line to ISP
–
ISP may charge more because of heavier
transmission load
–
Faster interface than 232 serial port may be
needed
26
Transmission Line Versus ISP

Transmission line may be provided by a
transmission carrier, not the ISP

Users have to pay separate fees to carrier and ISP

Some ISPs also provide transmission line
– But not all do
– Transmission line costs must always be
considered
PC
Transmission Line
ISP
27
Translator Devices

Device versus Line
Device
Transmission Line
Translator
Analog Line
Analog Device
Digital Device
Digital Line
Codec
Modem
DSU
ISDN
Integrated Services Digital Network
Codecs
DSUs
29
ISDN

Get ISDN line from telephone company
– Multiplexes (mixes) three channels on one UTP wire
pair to the desktop
B
Channel
1101
ISDN
Modem
B
Channel
D Channel:
control signals
2B+D
Multiplexed
Onto One
Set of Wires
30
ISDN

Two B Channels are 64 kbps
– Original idea: one for voice, one for data
B
Channel
1101
B
Channel
31
ISDN

Can now “bond” the two B channels for 128 kbps
data if ISP supports it

D Channel is 16 kbps (for supervisory signaling)
Bonding
ISDN
Modem
B
B
Bonding
Telephone
Network
ISP
128
kbps
ISDN Costs

Must install an ISDN line
– Will cost more than telephone line

Actually, could use your existing phone line
– But then would need one B channel for
telephony
32
ISDN Costs

Line is expensive
– $60-$80/mo plus installation fee
– ISP charge is separate; may charge more for
ISDN access

Dial Up: Not always connected
– Do not have to pay for full-time use
– Good if usage is small per day, say to upload
sales data from retail once per night
– Good for backup to always-on services
New

Need “ISDN modem” (expensive)
33
ISDN Modem

ISDN “Modem” is a Misnomer
– Modem is for for digital device, analog line
– ISDN line is digital

ISDN modem really contains
34
–
Codec to link analog telephone to digital ISDN
line
–
Data service unit (DSU) to translate between
PC digital format and ISDN digital format
(voltage levels, timing, etc.)
35
ISDN Modem

Codec
–
Translates analog device signal into digital
signal for propagation
–
Constantly samples the intensity of the voice
analog signal
Sample
0110010
1/8,000 sec
36
ISDN Modem

Codec
– Divides each second into 8,000 sampling periods
– Only measures intensity of voice signal in each
Intensity
Value
0110010
Sample
1/8,000 sec Sampling Period
37
ISDN Modem

Codec
– Measures voice intensity as an 8-bit intensity
value (0-255)
– Overall, sends 8 bits 8,000 times per second (64
kbps)
Intensity
Value
0110010
Sample
1/8,000 sec Sampling Period
38
ISDN Modem

Codec
– This is why telephone channels are 64 kbps
 Designed
for digitized voice
 Often steal 8 kbps for supervisory signaling, so 56
kbps
Intensity
Value
0110010
Sample
1/8,000 sec Sampling Period
39
DSU (Data Service Unit)

DSU translates between different digital formats
– Device and line are both digital, but still must
have translator
 Different
bit rates
 Different number of possible states
 Different voltage levels for the states
 Different ways to represent ones and zeros
Digital Line
DSU
Cable Modem Service
Broadband Internet Access
41
Cable Modem Service

Service of Cable Television Companies
–
–
–
Deliver up to 10 Mbps downstream to the home
Even this capacity is shared by multiple
subscribers, so real download speed is more
limited
Usually, only about 300 to 500 kbps real
throughput
Not in Book
Cable
Modem
Cable TV
Network
Also ISP
Functions
Cable Modem Sharing in Perspective
42

Sharing is Not as Extreme as it May First Appear
– Usually, shared within Blocks of 500 Houses
– Only Some Households in Block will Subscribe
– Only Some Subscribers will be Online at Any
Moment
– Only Some of Online Subscribers will Send and
Receive at Any Moment
– Only these will Share the Capacity

So Sharing Does Not Reduce Speed Unless the
Adoption Rate is Large
43
Cable Modem Service

Service of Cable Television Companies
– Limited to about 64 kbps to 256 kbps upstream
– Does not tie up telephone line
– Always available
Cable
Modem
Cable TV
Network
Also ISP
Functions
Cable Modem Service

Cost is about $50 per Month
–
Includes ISP service!
–
Installation usually costs $100 to $150 and
includes a cable modem and a network
interface card
–
Cost-competitive with adding a second phone
line to handle your modem communication
44
DSLs
Digital Subscriber Lines
46
Digital Subscriber Lines (DSLs)

Offered by Telephone Companies
– Lines to customer premises are subscriber
lines, which connect subscribers to the
telephone system
– These are digital
– Hence “digital subscriber line”
DSL
Modem
DSL
Telephone
Network
ISP
47
Digital Subscriber Lines (DSLs)

Offered by Telephone Companies
– Several types of digital lines for subscribers
 Some
–
are for homes, others for businesses
For residential customers, usually multiplexes
regular phone, high-speed data
 Can
use existing phone line coming into house
 Can use Internet without tying up phone
DSL
Modem
Existing
Phone Line
Telephone
Network
ISP
48
Digital Subscriber Lines

Most common for home is Asymmetric DSL (ADSL)
–
–
–
–
–
Upstream and downstream speeds are different
Upstream at 64 kbps or more
Downstream at 256 kbps to a few Mbps
Asymmetric speed is good for WWW
About $50 per month incl. ISP; more for faster service
ADSL
DSL
Modem 64 or more kbps
256 kbps-a few Mbps
Telephone
Network
ISP
49
Digital Subscriber Lines
G.Lite (G.992.2) Standard
– New ADSL standard from ITU-T
– Up to 1.5 Mbps downstream speed
– No carrier installation is necessary
– Not being widely adopted by ADSL vendors
New yet

ADSL
DSL
Modem 64 or more kbps
Up to 1.5 Mbps
Telephone
Network
ISP
50
Digital Subscriber Lines

Digital Subscriber Line Access Multiplexer
– Telco must install a DSLAM at end switching
office to serve DSL users
DSL
DSL
DSL
Telephone
DSLAM
Network
ISP
51
Digital Subscriber Lines

New
Splitting Voice and Data
–
Voice and data are split at home by the DSL
modem
–
Voice and data are also split at the telephone
company’s first switching office
Computer
Phone
Data
Network
DSL
Modem
DSL
DSLAM
Voice
Network
DSLs Versus Cable Modem Service

Both Expected to Sell Well

Forecast for Worldwide Installations in 2004
– Cable modem lines: 9 million
– DSLs: 10 million
– Source: Insight Research

Cable is Available Mostly in Residential Areas

Cable is only widespread in some countries,
especially the United States
52
Broadband Access Lines in Perspective


53
Limited to Transmission Between Customer
Premises and the ISP
–
ISP Speed can Limit Service Speed
–
Internet Backbone Speed can Limit Service Speed
–
Server Host Access Line Speed can Limit Service
Speed
–
Server Host Processing Speed can Limit Service Speed
Broadband Access Lines are Not Panaceas
Wireless Internet Access

Reach the Internet by Radio
– Terrestrial (uses earth stations)
– Satellite-based
ISP
54
GEO Satellites

In geosynchronous orbit
– Appear to be stationary in the sky

Far from the ground
–
36,000 km (22,300 miles)
–
Need much power to send/receive
–
Need dish antennas to concentrate signals
–
Must point dish at the satellite
–
Impractical for portable computers
55
VSATs

56
Very Small Aperture Terminals (VSATs)
–
Small dishes (1 meter in diameter or less)
–
Do not concentrate signal as well as large
dishes, so inefficient
–
Do not focus as precisely as large dishes, so
satellites cannot be spaced closely together
–
However, inexpensive
–
Used when there must be many ground stations
LEO Satellites

Omnidirectional
Antenna
Low Earth Orbit (LEO) Satellites
–
Below First Van Allen Radiation band,
which peaks at 3,000 km (1,800 miles)
–
A typical orbit is 1,000 km or 600 miles
New: Not in Book
57
LEO Satellites

Omnidirectional
Antenna
Low Earth Orbit (LEO) Satellites
–
Need far less power to reach than 36,000 km
(22,300 mile) geosynchronous satellites
–
So can get by with omnidirectional antenna
–
Can use phone of reasonable size, cost
–
Access anywhere is the world, although
local laws may limit legality
58
59
MEO Satellites

Medium Earth Orbit (MEO) Satellites
–
Between First and Second Van Allen Radiation
Band, which peaks at 15,000 km (about 9,000
miles)
–
Typical distance is 6,000 km or 4,000 miles
–
Farther away than LEOs, so need more power
–
But travel more slowly across sky, so fewer
hand-offs, fewer satellites needed
New
Ports

Need port fast enough for line

232 Serial port to 115.2 kbps: only V.34, V.90 or
ISDN

USB (universal serial bus)
– 12 Mbps
– Available on all new PCs
– Fast enough for DSLs, cable modems
– Faster version coming (USB-2, ~480 Mbps)
60
Ports

Firewire (IEEE 1394)
– 400 Mbps and faster
– Not available on most new PCs
– Fast enough for DSLs, cable modems

Ethernet NIC (10 Mbps)
– Network interface card used in PC networks
– Printed circuit board
– Must be installed inside PC systems unit
– Fast enough for DSLs, cable modems
61
62
Small Office Service

Serial Router Connects Internal Stations to Shared
Broadband Access Service
Serial Router
Shared
Broadband
Line
Small
Office
ISP
63
Small Office Service
New

Serial Routers (Also Called Access Routers) are
Very Simple
–
One or more ports to connect the serial port to
the LAN
–
One port to connect to the high-speed access
line, which is serial
–
Much less expensive than general-purpose
routers because no complex router forwarding
decision caused by multiple possible output
ports
Serial Router
Small Office Service

64
Serial Router Connects Internal Stations to
Shared Broadband Access Service
–
Companies with several stations pay much
more for line than when connecting a single PC
–
Each machine gets its own IP address via
DHCP
–
Serial router may be a user PC with added
software
–
Often, router is called a gateway (the old name
for router)
Connecting Two PCs at Home to the Internet

65
Physically, One PC (the Gateway PC) Connects to
the Hub or Switch and to the Broadband Line
–
Gateway (router) PC needs two Network Interface
Cards, one for hub, one for broadband modem
Hub or Switch
NIC
ADSL/Cable
Modem
Gateway
PC
Broadband
Line to ISP
Connecting Two PCs at Home to the Internet

66
Gateway PC Needs Gateway Software
–
–
–
On NIC to broadband line, acts like a single station
Gets temporary IP address from ISP DHCP
Sends and receives to ISP using this IP address
Gateway
PC
Send, Receive with
Temporary
IP Address
Connecting Two PCs at Home to the Internet

Gateway PC Needs Gateway Software
–
–
Each PC is set up with a permanent IP address
So gateway PC has two addresses, one bound to each
NIC
Gateway
Program
Permanent
IP Address
Permanent
IP Address
Temporary
IP Address
67
Connecting Two PCs at Home to the Internet

68
Gateway Software Does Network Address
Translation (NAT)
–
–
When a PC sends using permanent IP address
Gateway software replaces source IP permanent address
with temporary IP address ISP expects
Gateway
PC
Permanent
IP Address
Permanent
IP Address
Temporary
IP Address
Connecting Two PCs at Home to the Internet

Gateway Software Does Network Address
Translation (NAT)
–
–
When a PC receives
Gateway software replaces ISP temporary IP
destination address with receiver’s permanent IP
address, passes on to PC
Gateway
PC
Permanent
IP Address
Permanent
IP Address
Temporary
IP Address
69
Connecting Two PCs at Home to the Internet
New

Newer Approach
– Access router connects to broadband modem
– Has built in hub or switch to connect stations
Broadband Modem
UTP
Access Router
With
Hub or Switch
70