Download Remote Access

Remote Access
Data Communications/Networking
©Richard L. Goldman
February 13, 2002
Page 1 - © Richard L. Goldman
Remote Access
• Remote access technologies allow users to access a network
and its services from a computer outside the network
• Dial-up access requires:
– Network computer (server) running RAS or NAS with a modem
and a dial-up phone line
• Remote Access Server – Microsoft
• Network Access Server – Novell
– Remote computer (laptop) with modem and a dial-up phone line
– An ISP is not required – This will be a direct “call” from one
computer to another computer.
Page 2 - © Richard L. Goldman
Types of Modulation
• AM – Amplitude Modulation
• FM – Frequency Modulation
• PM – Phase Modulation
– PSK – Phase Shift Keying – Shifts with bit change
– DPSK – Differential Phase Shift Keying – Shifts when
a “1” bit is to be transmitted.
• PCM – Pulse Code Modulation
– Method used by CODEC – Digital “1” is twice the
voltage of a digital “0”.
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Types of Multiplexing
• FDM – Frequency Division Multiplexing
• WDM – Wave Division Multiplexing
• TDM – Time Division Multiplexing
• STDM – Statistical Time Division Multiplexing
Page 4 - © Richard L. Goldman
Modems
• Modulator-demodulator
• Converts digital to analog for transmission over POTS (Plain old
Telephone Service)
• Configure:
– COM 1 or 3 – IRQ 4 – I/O 3F8 or 3E8
– COM 2 or 4 – IRQ 3 – I/O 2F8 or 2E8
• Internal Modems
– Have built in UART (Universal Asynchronous Receiver-Transmitter) – Usually
new (16550 or newer)
– Cheaper
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External Modems
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Uses computer's internal UART – may be old (8250 - 9600bps max) and slow
May be easier to configure
Stats lights
More expensive (case, power supply, cables, etc.)
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ISDN – Integrated Services Digital Network
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Can carry voice and data simultaneously
Uses a standard PSTN voice line
Standard established by the ITU
ISDN service began in mid 1980’s
Requires a Network Termination device(s) to
process the data from the telephone company
• Requires a Terminal Adapter (router) to convert
the digital data to voice and distribute the
communication.
• Must be within 3.4 miles (18,000 feet or 5.5Km)
of the telephone company.
Page 6 - © Richard L. Goldman
BRI (Basic Rate ISDN)
• Basic Rate (2B+D) ISND lines uses two B channels
and one D channel multiplexed over 1 copper pair
• Each B (Bearer) channels can transmit up to 64Kbps
of data or it can be used to transmit voice.
• The D (Data) channel can transmit up 16Kbps and is
used for call setup & management
• Two B channels can be bonded to produce 128Kbps
throughput.
NT1
Data
TelCo
2 B channels
1 D channel
Router
Voice
Page 7 - © Richard L. Goldman
PRI (Primary Rate ISDN)
• Primary Rate (23B+D) ISND lines uses 23 B
channels and one D channel multiplexed over 1
copper pair
• Each B (Bearer) channels can transmit up to
64Kbps of data or it can be used to transmit voice.
• The D (Data) channel can transmit up 16Kbps and is
used for call setup & management
• The B channels can be bonded to produce
1.544Mbps throughput.
NT1
NT2
TelCo
23 B channels
1 D channel
Data
Router
Voice
Page 8 - © Richard L. Goldman
DSL – Digital Subscriber Line
• Uses regular POTS “phone” lines to transmit digital
signals
• Allows voice and digital over single pair
– Uses 0-4KHz for voice
– Uses 4KHz-2.2MHz for data
• ADSL – Asymmetric DSL
– 640Kbps Uplink speed
– 1.544Mbps Downlink speed (9Mbps Max.)
• HDSL – High Data-Rate DSL
• VDSL – Very High Data-Rate DSL
• SDSL – Single Line Digital Subscriber Line
Page 9 - © Richard L. Goldman
Remote Access Protocols
• SLIP – Serial Line Internet Protocol
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Used to transmit TCP/IP over serial connections
Low overhead – No error checking – No packet addressing
Being replaced with more feature rich protocols
Does not support encrypted passwords
• PPP – Point-to-Point Protocol
– Used to implement TCP/IP over point-to-point connections
– uses the Link Control Protocol (LCP) to communicate
between PPP client and host
– high overhead
– automatically configures TCP/IP
• PPTP – Point-to-Point Tunneling Protocol
– PPTP is setup through a PPP connection forming a VPN
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PSTS
“Demarc”
(Demarcation Point)
Local
Loop
(UTP)
Central
Office
Local
Switch
POP
Regional
Switch
Long
Distance
Switch
POP
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X.25 Protocol
• Analog WAN technology employing packet
switching
• Sending router breaks messages into smaller packets
• Packets may travel different routes
• Developed CCIT (now ITU)
• Handles bottom 3 layers only
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Frame Relay
• Digital WAN technology employing packet
switching
• Sending router breaks messages into smaller packets
• Packets may travel different routes
• Receiving router reassembles packets
• By sharing the network you get the minimum that
you contract for (Committed Information Rate CIR) but may receive speeds up to T1 speed
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T-Series Connections
• T-1 - 1.544Mbps – 2 pair STP(or UTP) – 24 DSO channels
- (64Kbps each)
• T-1C - 3.152Mbps
• T-2 - 6.312Mbps
• T-3 - 44.736Mbps - Fiber or Microwave – 672 DSO
channels
• T-4 - 274.176Mbps
Page 14 - © Richard L. Goldman
Remote Connection Terms
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Asymmetrical
Asynchronous
CODEC
CSU/DSU
Demarc
DSL
ISDN
Modem
Mux
POP
POTS
PSTN
RAS
UART
VPN
Not Symmetrical
Not Synchronous
Coder/Decoder
Channel Service Unit/Data Service Unit
Demarcation Point
Digital Subscriber Line
Integrated Services Digital Network
Modulator/Demodulator
Multiplexer
Point of Presence
Plane Old Telephone Services
Public Switched Telephone Network
Remote Access Service
Universal Asynchronous Receiver-Transmitter
Virtual Private Network
Page 15 - © Richard L. Goldman