chapter 6 remote access and communication links 6.1 remote access to a lan remote access to a...
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Chapter 6 Remote Access Chapter 6 Remote Access and Communication Linksand Communication Links
6.1 Remote Access to a LAN6.1 Remote Access to a LAN
Modem Dial-upModem Dial-up
Virtual Private NetworkVirtual Private Network
6.2 Communication Links6.2 Communication Links
PSTNPSTN
ISDNISDN
T1/T3T1/T3
DSLDSL
Cable TV networkCable TV network
ATMATM
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 servermust be activated
RAS client programneeded 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 meansgetting data from a modulated signalreceiver modem demodulates the analog signals
Modems are needed at both endsfor 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. CostG. 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. AuthenticationAccess control is implemented, such as login
C. Message integrityPrevent 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-high-data-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 speede.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