Chapter 6 Remote Access and Communication Links 6.1 Remote Access to a LAN Remote Access to a LANRemote Access to a LAN Modem Dial-up Modem Dial-up Virtual

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


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


6.1.1 Modem Dial-up

A. Modem (1/2)

Modem converts between

digital • computer

analog• phone line


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


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!


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.


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


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


6.1.1 Modem Dial-up

F. Security

Security is low data transmitted using RAS over PSTN are usually not

encrypted


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


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.





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


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.


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


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%


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


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.


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


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


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


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

Documents

Chapter 6 Remote Access and Communication Links 6.1 Remote Access to a LAN Remote Access to a LANRemote Access to a LAN Modem Dial-up Modem Dial-up Virtual