Transcript Specialized network services

Lecture 8
Specialized Network Services
Overview
• This chapter explores:
– high-speed services
– digital network services
• Will discuss:
– how they work
– speed and capacity
– applications in which these services are used
Services to be Discussed
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T-1 and T-3
ISDN
ATM
DSL
Sonet
What in common?
• All technologies to be discussed today are
digital services
• Meet requirements for high speed and
accurate transmission
T-1 -- Voice or Data
• Developed in 1960’s to save on cabling
• Scheme designed to carry 24 voice channels
over one telephone circuit
• In order to reduce number of cables wired
between telephone company switches
• Available to user locations in 1983
Circuit
A path for electrical transmissions
between two points
Initial Users and Applications
• Large organizations, universities, financial
institutions
• Used to connect locations in support of
applications such as: payroll, inventory
• Cost was high
• Better than hand carrying huge data tapes
from location to location
T-1 in 80’s
• High rates
• Service took months to install
• Needed long lead times to plan for
installation
– T-1 digital, needed to be connected to analog
system by using channel banks
Channel Banks
• Multiplexing device used to connect T-1
circuits to analog PBX and CO systems
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samples signals from PBX systems
digitizes these signals
sends down the T-1 channel
channel bank on other end decodes digital
signal back to analog signal
• PCM: pulse code modulation (page 183)
Channel Banks
• Also used to split up the use of T-1 line into
voice and data applications
• Controls the communication to the devices
to which it connects, PBX or computer
• Can be used to break out each of the 24 T-1
channels
Sampling
• Converting continuous signals, like voice or
video, into discrete values, e.g. digital
signals
• Sampling rate:
– The number of times per second that an analog
signal is measured and converted to a binary
number, most commonly used algorithm is
8,000 per second
PCM
• Pulse Code Modulation
– most common method of encoding an analog
voice signal into a digital bit stream
Digital Signal Levels
• DS-O: 64,000BPS
• DS-1: 1,544,000BPS
• speed at which T-1 and T-2 lines run
DS levels
• DS-O
• refers to each speed of
each channel in T-1
• 64Kbps
• DS-1
• refers to entire T-1 line
• entire bandwidth of
the circuit
• 1,544Mbps
European, Japanese and
US T-standards
4000..4
Level
US
Japan
Europe
DS-0
1
1
1
T-1(DS 1) 24
24
30
T-2(DS 2) 96
96
120
T-3(DS 3) 672
480
480
T-4(DS 4) 4032
5760
19.20
Time Division Multiplexing
• All T carrier signals are based on division
multiplexing
• each device which communicates over a Tline is assigned a time slot
• 8 devices communicating, 8 time slots assigned
• assigned for duration of the communication, even
when no information being transmitted
• TDM: not efficient in handling a WAN
Idle Time Slots
• Pauses in data transmission result in idle
time slots
• In a network with millions of time slots,
empty time slots represent waste or
inefficient use of transmission capacity
• ATM and IP do not assign specific time
slots to each device
T-1 Configurations
• T-1 circuit has 24 lines available for voice,
data, or video
– voice usually connected to a PBX
– data usually bypasses the PBX
– video can bypass or be routed through the PBX,
depending on weather it can transmit at high
speeds
Digital Cross Connects
• Used to connect multiple sites with
dedicated lines
• Digital cross connects provided by telco’s
• Used to divide a T-1 connection over
several users
Digital Cross Connectivity
6 lines
T-1
6 lines
Mux
12 lines
Digital
Cross
Connect
T-3
• Equivalent to 28 T-1 lines or 672 channels
28 X 24 = 672
• Total speed 44,736, includes bits for
overhead and 28 X 1.544Mbs
• Used for businesses with large calling
volume
• Fractional T-3 service, partial use of a T-3
circuit
ISDN
• Integrated Services Digital Network
– BRI ISDN: basic rate
– PRI ISDN: primary rate
• Worldwide public standard for sending
voice, video, and data or packets over
public switched network
Characteristics ISDN
• Digital connectivity: consistent, high quality
service
• Out of channel signaling: call set up fast, total
bandwidth of each channel dedicated to
transfer of user data
• Switched service: fees based on usage time
• Standard interface: all ISDN users can
interface with each other
ISDN in the “Real World”
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Works on copper wiring
Availability an issue
Takes long time to install
Poor support from providers
1% of US access lines equipped in US
Must be within 18,000 feet from central
office
Basic Rate Interface
• Two bearer channels (voice, data, video)
• 64Kps
• One signaling channel
• 16Kps
Uses Basic Rate Interface
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Internet access
desktop video conferencing
Centrex ISDN multi-line telephones
use of D signal for credit card verification
downloading software
work-at-home
connecting remote local area networks
ISDN for work at home
• To share line for voice and other:
– need a terminal adapter with an NT-1 (network
terminating device)
– used to connect non-ISDN equipment to the
ISDN line
ISDN and Analog Lines
• ISDN is a digitally based technology
• Can not transmit to an analog line
• Can only communicate with ISDNequipped services
• Must have matching modems on each end
of the connection
Pricing BRI
• Ranges considerably
• Installation fees $150-300
• Monthly fees $10-40 more than charge for
analog line
• Per minute, sometimes flat rate for 140-300
monthly hours
Primary Rate Interface ISDN
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PRI: has 24 channels
23 channels are bearer channels
1 channel used for signaling
Similar to a T-1 both have 24 channels
– T-1 in-band signaling
– PRI out-of-band signaling on 24th channel
• Trunk connection
Trunk Connection
A communication line between
two
switching systems.
Uses PRI ISDN
• Video conferencing (128 to 384 Kbps)
• Receiving the calling party numbers at large
call centers
• backing up LAN-to-LAN connections
• backing up dedicated lines
• Internet service providers for dial in from BRI
ISDN customers
• Corporate sites for remote access
PBXs with PRI Trunks
• Calls and signals sent separately
• PBX may send the signal to a database to
match the number and then pull the data
– call centers
– call screening
PRI Bandwidth on Demand
• PRI: use of channels dynamic always open
for voice or data
• setup signals notifies network how to route
the call
– voice: public network
– data: data network
• T-1 channels permanently set aside for
voice or data purposes
Digital Subscriber Line
• Initially used to transmit video and TV
signals from CO to end-users over copper
• Main application has changed to Internet
access
• ADSL: asymmetric digital subscriber line
– line has different speeds from and to customer
– upstream slower than downstream connection
Digital Subscriber Line
• Many different flavors
– speeds may vary
– distance from CO varies
• Has potential to relieve network congestion
caused by modem traffic on PSTN
• DSL traffic routed onto a data network
when it hits the CO switch
DSL Flavors
• ADSL: asymmetric, upstream slower than
downstream
• DSL Lite: Universal DSL, ITU-based
standard for residential consumers, offered
by local exchange providers
• HDSL: high-bit-rate, two competing flavors
• VDSL: very-high-bit-rate for TV and
Internet usage
DSL Applications
• Alternative to cable modem for Internet
• CLEC: to serve data only communications
• IXC: to provide high speed local voice,
Internet access, and support telecommuting
• Internet service providers
Obstacles DSL
• Availability been slow, now popular
• Cost high to install for providers
– loading coils and bridge caps must be removed
by providers
• Not easy to install for end users
Technical Explanation DSL
• Uses unused frequencies available in the
telephone wire for data transmission
• Voice carried between 300-3300 Hrz
• DSL uses rest of frequencies available for
data communications
• DMT: discrete multi-tone modulation used
to send data in frequencies over 4,000Hz
Technical Explanation DSL
• DMT used compression algorithms to
encode and compress signals
• Different frequencies are used for the
upstream and downstream portions of the
channel
• Routing functionality enables multiple users
to share one DSL line
DSL Lite
– Developed to lower the cost of providing DSL
service
• also referred to as: Universal DSL
• known as a splitterless DSL
• save telco’s from having to dispatch a technician to
install splitter
– Works consistently on longer telephone
company loops, within 3 miles of telco CO
– no internal wiring changes required on part of
user
Splitter
• Used with full rate ADSL
• sits between the customer demarcation point
and the ADSL line and another at telco
• separates the low frequencies voice signals
from the high frequency data signals
• small in size
Concatenation
• Puts streams of data into one “fat” or one
high bandwidth contiguous stream
• Joining several fibers together end-to-end
• To link together in a series or chain
DSLAM’s
• Digital subscriber lines access multiplexers
• Located at network providers site
• Take traffic from multiple DSL lines and
combine into higher speeds before sending
it out to Internet or data network
• Can stream video together rather than
separating it with data
DSL for Internet Access
• Competes against cable modems, T-1, ISDN
and high speed modems
• Suitable for:
– Internet access
– Telecommuters to corporate databases
– High bandwidth applications
• Telecom Act notable impetus to this service
Frame Relay
• Network access method offered by local and
long distance providers
• Shared by multiple users
• Acts like a private, dedicated network
• Leasing dedicated lines not necessary
• Alternative to those building their network
Frame Relay
• First implemented in 1992
• Allows customers to transmit data between
multiple locations
• Used for:
– LAN connectivity
– Internet access
– connections for remote users
Frame Relay
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network managed by provider
less hardware required at each location
capacity on frame relay flexible
has internal backup routes
Frame Relay
• Shared network
• Fast, due to low error checking
• Customer must check data upon arrival
– X.25: packet network, oldest most widely used,
slower due to extensive error checking into
every packet transmitted
• Excels at LAN-to-LAN communications
ATM
• Asynchronous transfer mode
• High speed, carrier voice, data, video, and
multimedia images (13.22 Gbps)
• Carries multiple types of traffic over a
single connection
• Higher cost, higher speed, more flexible
service than frame relay
ATM
• Packages data that it switches into discrete
cells
• Requires less handling than variable-sized
packages
• Each cell:
– 53 bytes
– 5 bytes of these 53 are header cells
ATM Header Cells
• Identifies type of information contained in
the cell (voice, data, video)
• Prioritization of cells based on content
• voice and video need constant transmission rate
• prioritization done to avoid delays
• Other header content used for
– routing
– putting the cells in the correct sequence
– error checking
ATM Switching Hardware
• ATM is fast because cells are switched in
the hardware
• ATM switch does not look up each cell’s
address in software
• Route set up upon investigation of first cell
• All cells with same header routed the same
SONET
• Equipment and software enable network
providers to carry traffic from many types
of customer equipment in a uniform way on
backbone fiber optic cabling
• High speed portion of SONET network
called backbone or core
• Sonet can handle traffic coming from: T-1
lines, ATM, T-3 lines, and other sources
SONET
• Is a standard way to multiplex high speed
traffic from various vendors onto fiber
• Advantages:
– lower bandwidth pipes can be fed into SONET
multiplexers and carried at high speeds
• High reliability due to ring configuration