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WAN Technology
Ch 12: Long-Distance Technology
Ch 13: WAN Technologies
Ch 14: ATM
CS420 (Cotter) 1
LAN to WAN Connectivity
• Inherent Problem: LAN technologies are
distance limited. (10BaseT segment = 185
meters)
• Connectivity between LANs requires a
different transport technology
Phone Company
Modem Modem
IBM Compatible IBM Compatible
CS420 (Cotter) 2
LAN to WAN Connectivity
• Modem known as DSU/CSU (or DCE)
– Data Communications Equipment
• Computer known as DTE
– Data Termination Equipment
Computer
comm Stds
Phone Company
Modem Modem
IBM Compatible IBM Compatible
Telco comm
Stds
CS420 (Cotter) 3
Network Access
• Modems
• Digital Subscriber Line
• Cable Modems
CS420 (Cotter) 4
Modems
• Function
– Designed to convert digital signal into an
analog signal that can pass over a standard
telecom facility
– Frequency limited to about 300 Hz to 3400 Hz
– Handles all synchronization and signal
handling needed for telco.
CS420 (Cotter) 5
Modem Evolution
• 110 baud • 14.4k - (ITU-T V.32bis)
• 300 - (Bell 103) • 28.8k - (ITU-T V.34)
• 1200 - (Bell 212a) • 33.6k - (ITU-T V.34)
• 2400 - (CCITT V.22bis) • “56k” - (ITU-T V.90)
• 4800 - (CCITT V.32)
• 9600 - (CCITT V.32)
• CCITT --> ITU-T
CS420 (Cotter) 6
Current Modem Standards
• V.32 [32-QAM] => 9,600 bps
• V.32bis [64-QAM] => 14,400bps (2400baud)
• V.34 => 28,000 bps/33,600bps
– 4096-QAM
– Data compression
• V.90 =>56K Modem
– Forward, QAM => – 33,600 bps
– Reverse, PCM => – 56,700 bps
CS420 (Cotter) 7
V.34 Functionality
• Adaptive call setup
– negotiate on bandwidth, signal level, carrier
frequency.
• Support echo cancellation
CS420 (Cotter) 8
V.34 Handshake Process
• V.8 Handshake
– Pass information on supported standards,
available modulation modes, error correction
• Line Probing
– Find out what frequencies (and levels) are
supported by this connection
• Equalizer & Echo Canceller Training
• Final Handshake
CS420 (Cotter) 9
V.34 Line Probing
• Frequency response tested every 150 Hz
at 2 signal levels.
L1
L2
CS420 (Cotter) 10
V.34 Line Rates
• Line Probing will determine maximum
bandwidth and optimum signal level
• Transmission will be tuned to individual
subscriber line.
CS420 (Cotter) 11
Network Access
• Modems
• Digital Subscriber Line
• Cable Modems
CS420 (Cotter) 12
Digital Subscriber Line (DSL)
• Functions
– Provide high speed digital access using
existing telecommunications facilities
• Shared access
– Same facilities (wires) used for both POTS
and DSL
– Allows telco to maximize use of existing
facilities. Allows faster implementation of high
speed Internet access.
CS420 (Cotter) 13
DSL Configuration
Internet
DSL Admin
Telephone
Telephone
IBM Compatible
LP filter LP filter
PSTN
DSL Modem
Public switch
Home
Central Office
CS420 (Cotter) 14
DSL Types
• SDSL
– Symmetric DSL. Used mostly for Business
– Disables POTS during usage
– Provides symmetric transmission rates
• VDSL
– Very High bit-rate DSL. Uses fiber optics.
– High Data Rates (downstream ~55 mbps)
– Much shorter loops (1000 ft.)
CS420 (Cotter) 15
ADSL Technology
• Asymmetric DSL
– Upstream speed: 64 - 640 kbps
– Downstream speed: 1.5 - 6.1 Mbps
– (rates are loop distance limited)
• How it works
– Uses existing copper facilities, but uses
bandwidth not used by POTS
CS420 (Cotter) 16
ADSL Technology
Carrier Amplitude/Phase (CAP)
POTS Upstream Downstream
Frequency 1 Mhz
0-4khz 25-160khz 240khz-1.5Mhz
CS420 (Cotter) 17
ADSL Technology
Discrete Multi-tone (DMT)
247 channels (4khz each) 1 Mhz
CS420 (Cotter) 18
Network Access
• Modems
• Digital Subscriber Line
• Cable Modems
CS420 (Cotter) 19
Cable Modems
• Functions
– Provide high speed digital access using
existing cable television facilities
• Shared access
– Same facilities (coax) used for both CATV
and cable modem
– Allows CATV suppliers to maximize use of
existing facilities. Allows faster
implementation of high speed Internet
access.
CS420 (Cotter) 20
Cable Modem Architecture
Home Home
IBM Compatible
CMTS
Cable Modem
Multiplexer
Home Home
Television
Home
Cable TV
Cable Office
CS420 (Cotter) 21
Cable Modem Technology
• Cable Modem Data Rates
– Upstream (to Internet) - 400KBps
– Downstream (from Internet) - 4-7MBps
Upstream Downstream
5Mhz 42Mhz 850Mhz
CS420 (Cotter) 22
Cable Modem Technology
• Standards (US)
– DOCSIS/MCNS (Data over cable services
and Interface Specifications for Multimedia
Cable Network Systems)
• Data Formats:
SYNC MPEG HEADER MPEG Payload
Gap Sync(16bit) ATM Header ATM Payload Gap
CS420 (Cotter) 23
Access Technology Tradeoffs
• Modems
– Inexpensive hardware / service
– Very slow data rate
• DSL
– Access line private to user
– Data rates are distance dependent
• Cable Modem
– Rates independent of distance to C.O.
– Access lines shared with neighbor
CS420 (Cotter) 24
Telecommunications Transport
• Telecommunications Carrier Systems
• Fiber Transport
• Asynchronous Transfer Mode
CS420 (Cotter) 25
Carrier Systems
• Provided a means to multiplex several
individual signals over a single facility.
• Analog Carrier
– Used FDM to carry multiple signals.
• Digital Carrier
– Carries a high speed digital signal, and
divides that bandwidth among multiple
channels.
– T-Carrier Systems based on Digital Standard
rates
CS420 (Cotter) 26
T-Carrier Systems
• DS0 - 64kbps
– 8000 samples/sec of 8 bits
– A single voice channel
• DS1 - 1.544 mbps
– 24 DS0 channels
– 8 kbps signaling
• DS3 - 44.736 mbps
– 28 DS1 channels
CS420 (Cotter) 27
Fiber Optic Transport
• Characteristics
– Multi-mode fiber - larger diameter fiber.
Allows light waves to reflect from sides.
– Single mode fiber - a few wavelengths in
diameter. Acts like a waveguide. More
expensive, but can propagate farther.
CS420 (Cotter) 28
Single mode vs. multi-mode
Multi-mode fiber
Step index
Multi-mode fiber
graded index
Single mode fiber
CS420 (Cotter) 29
Fiber Optic characteristics
Multimode Multimode singlemode
Step index Graded index
Core size 50 – 62.5 50-62.5 8-9
Microns Microns Microns
Typical data 5 Mhz –Pl 1Ghz – 1 Ghz - ?
rates 50Mhz – Gl 10 Ghz 100k Ghz
Signal loss 4-6 db/km 1-3 db/km .25-.4
db/km
CS420 (Cotter) 30
Optical Carrier Standards
• Based on STS standards
• OC-1
– 51.840 mbps (810 channels @64kbps)
• OC-3
– 155.520 mbps (3 OC-1 channels)
– OC-3C (a single 155mbps channel)
• OC-12
• OC-48
CS420 (Cotter) 31
SONET
• Synchronous Optical Network
– Optical transmission scheme used to
multiplex multiple signal channels over a
single fiber.
• SONET Frame
– Frame size dependent on signal rate.
– Each frame represents the amount of data
sent in 125 microseconds (1/8000 sec).
– Allows easy synchronization with digital voice
circuits (8k sample rate).
CS420 (Cotter) 32
SONET STS-1 Frame
overhead Payload
...
...
90 columns
CS420 (Cotter) 33
ATM
• Connection-oriented Networking
• Asynchronous Transfer Mode
– International Standards approach to defining
a fast, small packet connection-oriented
communications protocol
– Based on a 53 octet packet size
• 5 octet header
• 48 octet payload
CS420 (Cotter) 34
ATM
• Built around the idea of using virtual channels
and virtual paths to route packets between ATM
switches.
• Virtual Path includes many Virtual Channels. An
endpoint is known by its identifiers - VPI / VCI
• A connection might use several different virtual
paths concatenated together. Each link would
have its own VPI/VCI.
CS420 (Cotter) 35
ATM Routing Example
S
A B C D
H G F E D
CS420 (Cotter) 36
VP / VC Routing Example
AA AB BC CD DE EE
A B C D E
VPI Link VPI Link VPI Link VPI Link VPI Link
3 null 4 8 BC 8 17 CD 17 Null 12 Null
3 17 12
VPI VCI Link VPI VCI Link VPI VCI Link
10 4 6 AB 6 12 18 DE 18 8 5 EE
VPI Link VPI Link VPI Link VPI Link VPI Link
4 null 8 4 BA 17 8 CB 12 null 8 null
4 12 8
VPI VCI Link VPI VCI Link VPI VCI Link
6 3 10 AA 18 17 6 DC 5 12 18 ED
CS420 (Cotter) 37
ATM Header
0 1 2 3 4 5 6 7
Flow cntrl VPI (1 st 4 bits)
VPI (2nd 4 bits) VCI (1st 4 bits)
VCI (middle 8 bits)
VCI (last 4 bits) Pay. type Prio
CRC
CS420 (Cotter) 38
ATM Services
• Permanent Virtual Circuits
– A permanently provisioned connection
between two endpoints.
– Survives power failures, link failures, etc.
• Switched Virtual Circuits
– Connections dynamically created and
removed when communication has been
completed.
CS420 (Cotter) 39
ATM Services
• Quality of Service Specifications
– CBR - Constant Bit Rate Service.
Guaranteed capacity. Most expensive
– VBR – Variable Bit Rate Service. Allows a
range of capacity. Not guaranteed at max
rates.
– ABR – Available Bit Rate Service. Spare
capacity. For non-critical traffic. Least
expensive.
CS420 (Cotter) 40
ATM Critique
• Expense
• Connection Setup Latency
• Cell Tax
• Specification of Service Requirements
• Lack of Efficient Broadcast
• Complexity of QoS
• Assumption of Homogeneity
CS420 (Cotter) 41
Summary
• Network Access
– Telecom Modems
– ADSL
– Cable Modems
• Telecommunications Transport
– Telecommunications Carrier Systems – T1
– Fiber Transport - SONET
– ATM - Asynchronous Transfer Mode
CS420 (Cotter) 42
