Chapter 6 Remote Access
and Communication Links
6.1
6.2
Remote Access to a LAN
Modem Dial-up
Virtual Private Network
Communication Links
PSTN
ISDN
T1/T3
DSL
Cable TV network
ATM
Chapter 6 Communication Links
6.1 Remote Access to a LAN
 Remote Access means
accessing a LAN by mobile/home users
 Two means:
modem dial-up
VPN
Chapter 6 Communication Links
6.1.1 Modem Dial-up
 Modem dial-up means
connection using ordinary phone lines
also called
Public Switched Telephone Network (PSTN)
Plain Old Telephone Service (POTS)
Both the remote user and the LAN must have
a registered phone number
Remote Access Service (RAS)
of the server
must be activated
RAS client program
needed by the user
Chapter 6 Communication Links
6.1.1 Modem Dial-up
A. Modem (1/2)
 Modem
converts between
digital
• computer
analog
• phone line
Chapter 6 Communication Links
6.1.1 Modem Dial-up
A. Modem (1/2)
 Modem (cont’)
Modulation means
the amplitude (or frequency) of analog signal is changed
according to data
sender modem modulates data
Demodulation means
getting data from a modulated signal
receiver modem demodulates the analog signals
Modems are needed at both ends
for several remote users, a pool of modems are needed
Chapter 6 Communication Links
6.1.1 Modem Dial-up
B. Quality
 Quality is poor
error-rate can be as high as 40%
because PSTN was designed for voice communication
Re-transmission is quite often
Note: poor quality does NOT mean that data received are
inaccurate!
Chapter 6 Communication Links
6.1.1 Modem Dial-up
C. Bandwidth
 Maximum bandwidth of PSTN is 56 Kbps
Modems negotiate the highest possible transmission rate
If one modem is 56 Kbps, but the other is 14.4 Kbps,
the highest transmission rate is only 14.4 Kbps.
Chapter 6 Communication Links
6.1.1 Modem Dial-up
D. Dial-up
 Dial-up is necessary (for standard line, not leased line)
the connection is not permanent
known as “dial-on-demand”
Opposite is “always-on connection”
e.g. DSL broadband
Chapter 6 Communication Links
6.1.1 Modem Dial-up
E. Leased line
 Leased line means
line reserved and dedicated for transmission
“Always-on connection”
higher quality
less error rate
bandwidth can be guaranteed
Chapter 6 Communication Links
6.1.1 Modem Dial-up
F. Security
 Security is low
data transmitted using RAS over PSTN are usually not
encrypted
Chapter 6 Communication Links
6.1.1 Modem Dial-up
G. Cost
 Cost is
Low for local connections
High for long-distance phone line
 In summary, PSTN is
slow
poor in transmission quality
needs dial-up
has security risk
ubiquitous : easy to access
Chapter 6 Communication Links
6.1.2 Virtual Private Network (VPN) (1/3)
 Virtual Private Network (VPN)
remote access to a LAN
over the Internet
with security using tunnels
 Tunnel
virtual dedicated channel, with following characteristics:
A. Encryption
even encrypted data are intercepted, confidentiality can be
guaranteed
B. Authentication
Access control is implemented, such as login
C. Message integrity
Prevent message alteration.
Chapter 6 Communication Links
6.1.2 Virtual Private Network (VPN) (2/3)
Chapter 6 Communication Links
6.1.2 Virtual Private Network (VPN) (3/3)
 IPSec (IP security)
industrial technique in VPN
e.g. In Windows:
PPTP (Point-to-Point Tunneling Protocol)
L2TP (Layer Two Tunneling Protocol)
 VPN server
installed in the LAN
 VPN client
installed by user
 VPN is in widespread use today
e.g. access intranet from home, or
connecting LANs to form extranets
Chapter 6 Communication Links
6.2 Communication Links
 Recall: MAN or WAN is constructed by connecting LANs
by routers and long-distance communication links.
 Long-distance communication media include:
Copper wire
Fiber-optic cable
Radio waves
Satellite links
 For copper wire, WAN technologies include
ordinary phone line, ISDN, T1, T3, DSL, cable TV network
and ATM.
The choice depends on
speed, reliability, cost, availability and security.
Chapter 6 Communication Links
6.2.1 PSTN
 Ordinary phone line (PSTN)
is the slowest method of establishing a communication
link.
but, it has highest availability
most easy to access
Chapter 6 Communication Links
6.2.2 ISDN
 Integrated Services Digital Network (ISDN)
dedicated digital communication link
bandwidth 64 Kbps
provided by telephone companies
but better quality than the analog telephone system
error rate less than 1%
Chapter 6 Communication Links
6.2.3 T1 and T3
 T1
dedicated digital communication link
bandwidth 1.5 Mbps
multiplexing 24 separate 64 Kbps channels
expensive
 T3
bandwidth 44.7 Mbps
multiplexing 672 separate 64 Kbps channels
very expensive
Chapter 6 Communication Links
6.2.4 DSL (1/3)
 Digital Subscriber Line (DSL)
 makes use of ordinary phone lines
 plus advanced technology
 broadband




 much higher bandwidth than PSTN
 inexpensive
Originally, the copper wire of PSTN is capable of carrying signals of very high
frequency up to 1 MHz. But, the interface cards installed in the central office of the
telephone company limit the frequency to 4 KHz only. DSL opens up the maximum
capability by simply replacing the old interface cards in the central office of the
telephone companies.
There are several versions of DSL: ADSL, HDSL, SDSL and VDSL, where the first
letters in the acronyms are Asymmetric, High-data-rate, Symmetric and Very-highdata-rate respectively. The most popular version is ADSL.
Asymmetric DSL (ADSL) has a higher download speed than upload speed, for
example, 6 Mbps download and 1 Mbps upload.
ADSL can reserve a small proportion (8 KHz) of total bandwidth for voice signals.
Therefore, ADSL is able to transmit both data and voice at the same time. An ADSL
modem is necessary.
Chapter 6 Communication Links
6.2.4 DSL (2/3)
 Digital Subscriber Line (DSL)
broadband technology
makes use of ordinary phone lines
much higher bandwidth than PSTN
inexpensive
need DSL modem
 Several versions:
ADSL - Asymmetric (most popular)
higher download speed than upload speed
e.g. 6 Mbps download and 1 Mbps upload
HDSL - High-data-rate
SDSL - Symmetric
VDSL - Very-high-data-rate
Chapter 6 Communication Links
6.2.4 DSL (3/3)
 DSL can transmit voice and data at the same time
a small proportion (8 KHz) of total bandwidth can be
reserved for voice signals
 What is technology in DSL?
Originally, PSTN can carry signals up to 1 MHz
But, the telephone company limits to 4 KHz for voice
So, simply replace the equipment, and the old phone lines
can have bandwidth up to 10 Mbps
Chapter 6 Communication Links
6.2.5 Cable TV Network
 Cable TV network
another broadband technology
coaxial cables
bandwidth as high as 10 Mbps
needs: cable modem
can transmit data and television signals at the same time
Chapter 6 Communication Links
6.2.6 ATM
 Asynchronous Transfer Mode (ATM)
very high-speed
expensive
each packet has a fixed length (53 bytes)
no need to check the end of data for each packet
multiplexed to fully utilize the network resources
media include
coaxial cable, twisted-pair, fiber-optic cables.
bandwidth: 155 Mbps to 622 Mbps