Remote Access Technologies

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Cisco CCNA Study Guide	By Cbrzana
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Remote Access Technologies

Public Switched Telephone Network: built to support traffic between telephones, converts sound waves to analog waves.

1. Frequency: How many times the signal repeats itself from peak to peak in 1 second

2. Amplitude: How strong the signal is (how "high" the peak is)

3. Phase: Refers to where the signal is at a point in time

PSTN evolved to use digital signals in its core, which allowed more voice calls over the same physical lines.

Encoding Scheme: Defines the rules which electrical signals represent 1's and 0's

- > Typically, +5v is 1, -5v is 0

Pulse Code Modulation (PCM): Process of converting from analog-digital, samples 8000 times/sec, each sample is 8 bits, total of 64 kbps, occurs at codec (coder/decoder)

- > To go from digital to analog, reverse process

Analog Modems

Modem: Modulator/demodulator, converts digital to analog so Telco can use bitstream.

PPP Features with Modems: CHAP authentication, dynamically assign IP, supports both synchronous and asynchronous transmission.

Modem Installation and Cabling

External modems connect to PC through COM port or USB port (both serial)

COM Port: Typically female RS-232 connector (25 pins) -or- DB-9 connector

Internal modems (ISA/PCI) connect directly to phone line (RJ-11 connector).

Modem Standards

Standard	Speed	Summary
V.22	1200 bps	Mainly used outside US
V.22bis	2400 bps	1st widely established
V.32	4800/9600 bps	Adjusts speed based on line quality
V.32bis	14.4 kbps	Backward compatible with V.32
V.34	28.8 kbps	Backward compatible with V.32bis and V.32
V.42	28.8 kbps	Error correction features
V.90	56 kbps (upstream) 33 kbps (downstream)	--
V.92	Same as V.90, or 48 kbps each direction	Finds correct speed, allows call-waiting feature.

Integrated Services Digital Network (ISDN)

Provides switched digital WAN services in increments of 64 kbps.

• Require ISDN equivalent of analog modem (not actually a modem)
• Known as a terminal adapter (TA)

ISDN Channels: Two types, Basic Rate Interface (BRI) and Primary Rate Interface (PRI)

Interface	# of Bearer Channels	# of Signaling Channels	Descriptive Term
BRI	2	1 (16 kbps)	2B+D
PRI (T1)	23	1 (64 kbps)	23B+D
PRI (E1)	30	1 (64 kbps)	30B+D

- >B channels transport data, D channel signals new data calls

Signaling: any request to establish a circuit.

Out-of-band signaling: Term used to descrive D channel setting up circuit, and B channels transmitting data.

Typical Uses of ISDN: Routers frequently use as a backup link when primary link is down; telecommuting; dial-on-demand; replace Frame Relay access link/PVC

• Support voice calls concurrently with a data call
• RS-232 from host to TA; RJ-45 (Pins 3+6 transmit, 4+5 receive) from TA to telco
• Lower speeds than DSL or cable connections

Digital Subscriber Line (DSL)

Defines how to transmit data between customer site and the local telco.

• Allows analog voice and digital data signals over the same line
• Local loop must be connected to DSL accees multiplexor (DSLAM)
• Data component always on (no need to signal to set up connection)
• RJ-11 connector (Pin 3 transmit, Pin 4 receive)
• Typically place filter between each phone and wall socket to prevent interference
• Speed doesn't degrade as more users are added

Voice = 0-4000 Hz

Data = > 4000 Hz

ISP's router often at telco location - > called co-location (or co-lo)

DSL Standards: major variations are length of local loop (from house to telco) and asynchronous/synchronous transmission

DSL Type	Standards	Encoding Technique	Speed	Distance Limit
ADSL/G.DMT	ANSI T1.413 Issue 2	Discrete multitone (DMT) or carrierless amplitude phase (CAP)	Downstream speed of 384 to 8 Mbps; upstream speed slower, up to 1.024 Mbps	18,000 ft
G.Lite	ITU-T G.992.1, ITU-T G.992.2	DMT	Downstream speed up to 1.544 Mbps to 6 Mbps; upstream speed up to 640 kbps	18,000 ft
Very-high-datarate DSL (VDSL)	ETSI and ANSI in process	DMT/single-carrier modulation (SCM)	12.96 Mbps to 52.8 Mbps for both upstream and downstream speed	4500 feet
ISDN DSL (IDSL)	ANSI ETR 080	Two binary one quaternary (2B1Q)	144 kbps for both upstream and downstream speed	18,000 ft
Symmetric DSL (SDSL)	None	2B1Q	768 kbps for both upstream and downstream speed	22,000 ft
High-data-rate DSL (HDSL	ITU G.991.1, ANSI TR 28	2B1Q	1.544 or 2.048 Mbps for both upstream and downstream speed	12,000 ft
G.SHDSL	ITU G.991.2	Trellis-coded pulse amplitude modulation (TC PAM)	192 kbps to 2.360 Mbps for both upstream and downstream speed	28,000 ft

Key players in development of DSL:

• American National Standards Institute (ANSI)
• Institute of Electrical and Electronics Engineers (IEEE)
• International Telecommunications Union (ITU)
• European Telecommunications Standards Institute (ETSI)
• DSL Forum

DSL Protocols

• Use Layer 2 Protocol ATM between DSL router and DSL modem
• Uses PPP over ATM (PPPoA)
• PPP provides dynamic IP assignment and CHAP
• ATM provides Virtual Circuit
• Straight-through Ethernet used to directly connect PC and DSL router

Cable Modems

• Provide "always on" internet service
• Cable head-end site: Main site that receives the programming, encodes the signal and transmits distribution cable
• Drop Cable: cable that connects from distribution cable to home users
• Splitting: Allows signal to transmit over several cables within the home (typically an f-connector)
• PC at the house usually uses DHCP to find IP address of default gateway (cable company's router that is at the house)
• Frequency Division Multiplexing
• 30 to 40 Mbps, degrades with more users

Downstream Data: Data going toward the home

Upstream Data: Data going from the home

All downstream data is broadcast to all receivers/homes.

- > To identify individual PC's, uses data-link layer protocol

Multimedia Cable Network Systems (MCNS) MAC.

MCNS also defines encoding:

• Quadrature amplitude modulation (QAM)
• QAM-64  : 6 bits per baud
• QAM-256  : 8 bits per baud

All upstream data uses separate frequency range than downstream, but collision can still occur. Uses time division multiple access (TDMA), where each home has a set period to transmit (multiple times per second).

MCNS, but different encoding schemes:

• Quaternary phase-shift keying (QPSK) - >phase modulation
• QAM-16 - > amplitude modulation

Pros: Speed and availability

Cons: Degrades with more user, less secure due to broadcasts

Comparison Summary

	Analog Modems	ISDN	DSL	Cable Modems
Transport	Telco line	Telco line	Telco line	CATV Cable
Symmetric Speeds?	Yes	Yes	Yes	No
Asymmetric Speeds?	Yes	No	Yes	Yes
Speed Ranges	56kbps/lower	64 kbps per B channel	56 kbps to 2 Mbps	320 kbps to 40 Mbps
Degrades under higher loads?	No	No	No	Yes
Concurrent voice and data?	No	Yes	Yes	Yes
Always on?	No	No	Yes	Yes
Local loop distance issues?	No	No	Yes	No

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