Transcript No Slide Title

CSIS 625 Week 10
Packet Switching
X.25, Frame Relay, ATM
Copyright 2001 - Dan Oelke
CSIS 625
For use by students of CSIS 625 for purposes of this class only.
1
Overview
• Packet Switching technologies
– X.25
• PVCs & SVCs
• PADs and PSEs
– Frame Relay
• PVCs
• FRADs and switches
– ATM
• Cells & Adaptation layers
• Switching Policies
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Overview
• Telephony service and the telephone
network
– History
– Network components
– Lines vs. Trunks
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What is X.25
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A protocol suite defined in ITU standards
Covers Physical, Data Link and Network layers
Called Physical, Frame or Link, and Packet layer
Defines DTE  DCE interface
Has both link layer and packet layer error
detection and retransmission
• Very robust - developed for high-noise and
unreliable communications links.
• The nodes inside the X.25 cloud are PSE - Packet
Switching Exchanges
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X.25 Layers
• Physical Layer
– X.21 is the defined standard
– RS-232 (EIA-232) is often used
– V.35 is often used
• Link Layer
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LAPB - Link Access Protocol - Balanced
LAPB is a subset of HDLC
I-Frames - encapsulates packet layer data
S-Frames - Flow and error control
U-Frames - set up and disconnect link layer
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X.25 Layers
• Packet Layer
– PLP - Packet Layer Protocol
– Responsible for End-to-end delivery of packets
– Virtual Circuits
• Packet layer multiplexes multiple virtual circuits over the link.
• Up to 4095 virtual circuits
• Virtual Circuits
– LCN - Logical Channel Number - the arbitrary number
that identifies the virtual circuit
– One LCN is established for the local or near-end
DTEDCE interface and another for the remote or farend DTEDCE interface
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X.25 Virtual Circuits
• Permanent Virtual Circuit -PVC
– Established by the network operator.
– Use LCNs starting at 0 and going up
• Switched Virtual Circuits - SVC
– Established by DTE signaling to the DCE that it wishes
to establish a connection to the remote DTE.
– Uses X.121 addresses plan (14 digits - kind of like
phone number)
– DTE picks local LCN number (starts with 4095 and
works down)
– DCE picks far-end LCN numbers (starts with numbers
above PVCs)
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X.25 PAD
• PAD - Packet assembler/disassembler
• A device that connects to X.25 network as a DTE,
and has connections for dumb-terminal type
devices.
• Device knows how to take individual characters
and put them into a packet
– And the inverse
– PAD knows how to handle Enter key, arrow keys, etc.
– PAD can handle local echo, line buffering, etc.
• A PAD is a DTE to the X.25 network
• A PAD is a DCE for a dumb-terminal
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Frame Relay
• Also a packet Switched Service, like X.25
• Derived initially from ITU-T I.122 ISDN framemode bearer services
• Decoupled from ISDN by The Group of Four,
Cisco, Stratacom, DEC and Northern Telecom,
which became the initial Frame Relay proposal
group
• Currently Frame Relay Forum handles all of the
FR-related standardization work
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Frame Relay
• Designed to hide network specifics from the user
• The only standardized parts are the UNI, or UserNetwork Interface, and LMI, or Logical
Management Interface
• Designed to provide flexible service -- Bandwidth
on Demand
• Takes into account modern, “intelligent” computer
systems and reliable communication systems
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Frame Relay - FRADs and switches
• FRAD - Frame Relay access device.
– Like X.25 PAD
– FRAD is a DTE to the Frame Relay network
– FRAD is a DCE for the computers connection to the
Frame Relay network
• Frame Relay Switch - node inside the frame relay
cloud.
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Frame Relay Specifications
• Physical Layer
– No specification provided
• Data Link Layer
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Employs a simplified version of HDLC frame
Handles flow control
Has facilities to perform congestion notification
Uses DLCI -- Data Link Layer Identifier -- as an address
• Utilizes primarily PVCs, although some proprietary
versions support SVCs as well
• Uses end-to-end error recovery, implemented either by
upper layer protocols in user’s application or the router
– Bad packets are discarded by the network.
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Frame Relay - Routing
• Based on DLCI, Data Link Layer Control
Identifier
• Although switching is a Network Layer
functionality, it is generally accepted that Frame
Relay does not implement true switching, and thus
“relaying” is treated as Data Link Layer function
• Switch accepts a DLCI on an input port, and,
using internal tables, routes it to the output port,
modifying the DLCI in the process.
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Frame Relay - Policing
• CIR - Committed Information Rate is a guaranteed
level of service between two points.
• Typically frame relay service is purchased with a
CIR, and some burst rate allowed.
• Frames that exceed the CIR have the Discard
Eligibility (DE) bit in the header set.
• Frames with the DE bit set may be discarded by
switching nodes that encounter congestion.
• The Frame Relay switch must set the DE bit
– Can’t trust a FRAD owned by the subscriber.
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Frame Relay - Policing
• Burst rate is the maximum rate that information
can be sent at.
– Sender may use the burst rate for up to some time limit
(2 seconds for example)
• It may not be wise for a service provider to allow
high burst rates
– When network is initially set up, the subscriber gets all their burst
data through because congestion is very rare.
– Subscriber gets used to this kind of service.
– More subscribers sign on.
– Congestion starts to occur and packets get dropped
– Subscriber is irate because they aren’t getting the service they are
used to.
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ATM
• Goal of ATM is to allow all data: voice video and data to
co-exist on the same network.
– The be-all and end-all of networking protocols
• Everything in ATM is based on Cells.
– A cell is 53 bytes long
• 5 bytes for header
• 48 bytes of data
– 32 bytes wanted by Europeans and
– 64 bytes wanted by Americans.
– 32 bytes => 4 milliseconds which means no need for echo
cancellation
– Americans have to do echo cancellation anyway because of
distance, and wanted something more efficient for data
applications
– Typical committee, they compromised so no one is happy
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ATM - Why short fixed length cells?
• The use of relatively short cells makes sure that if high
priority traffic comes in, it doesn’t have to wait very long
behind a big packet.
• Fixed length cells allow for easier hardware
implementations.
• Easier implementations means that very high speed circuits
can be created to allow higher speed applications to work.
• Fixed length cells mean that buffer memory is always
efficient.
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ATM - why short, fixed length cells aren’t important
• Now with much higher speed interfaces the amount of time
to wait, is very small even for “big” packets.
• Memory and processor power is much cheaper now
• Variable size packets are more efficient for data traffic
– Data traffic now makes up as much traffic as voice, and it it
growing rapidly
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ATM – UNI and NNI
• UNI – User-to-Network Interface
• NNI – Network-to-Network Interface
• In ATM networks, a distinction is made
between an end point connecting to an ATM
switch (UNI) and two ATM switches
connecting together (NNI)
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ATM – VPI/VCI
• In an ATM network, a virtual circuit identifier is
identified by a pair of numbers, the VPI and VCI.
– VPI – Virtual Path Identifier
– VCI – Virtual Circuit Identifier
• Some ATM switches switch only on VPI
– A cell comes in, and the switch sends it to another port
based on it’s VPI
– A new VPI is written into the cell
– The VCI is left untouched in this process.
• Some ATM switches switch on VPI & VCI
– A cell comes in and the switch sends it to another port
based on both the VPI and VCI
– A new VPI and VCI is written into the cell
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ATM – Header fields (UNI)
• 5 bytes in the header. The fields are:
– GFC (4 bits) – General Flow Control Identifier
• Used for flow control between the network and the DTE
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VPI (8 bits) – Virtual Path Identifier
VCI (16 bits) – Virtual Circuit Identifier
PTI (3 bits) - Payload Type Indicator
CLP (1 bit) – Cell loss priority
• Marked with a 0 means that it is to be discarded before cells
marked with a 1.
– HEC (8bits) – Header Error Control
• An 8-bit CRC to catch errors in the header
• Does not catch errors in the data body
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ATM – Header fields (NNI)
• 5 bytes in the header. The fields are:
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VPI (12 bits) – Virtual Path Identifier
VCI (16 bits) – Virtual Circuit Identifier
PTI (3 bits) - Payload Type Indicator
CLP (1 bit) – Cell loss priority
HEC (8bits) – Header Error Control
• NNI interfaces have more VPIs, based on the idea
that inside of the ATM cloud, there might be more
VPI switches than full VPI/VCI switches.
• This is all nice and good – but not normally used
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Payload Type
• 3 bits
• First one defines management or not
• Enumerated out:
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000 –no congestion, no signaling
010 – no congestion, signaling
001 – congestion encountered, no signaling
001 – congestion encountered, signaling
100 – Management, link associated management
101 – Management, end to end management
110 – Management, resource management
111 – reserved
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ATM – Service Classes
• CBR – Constant Bit Rate
– For real-time audio or video
– Similar to that service of a dedicated T1 line
• VBR – Variable Bit Rate
– VBR-RT – Variable Bit Rate – Real Time
• For those services that use compression to create a
variable bit rate stream, but still need real-time
characteristics.
– VBR-NRT – Variable Bit Rate – Non-real time
• Ditto – but don’t require real-time
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ATM – Service Classes
• ABR – Available Bit Rate
– Delivers a minimum cell rate
– If network capacity is available, higher cell
rates are achievable.
• UBR – Unspecified Bit Rate
– Best effort delivery that doesn’t guarantee
anything
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ATM – QOS attributes
• Different service classes allow specifying
some or all of these attributes.
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SCR – Sustained Cell Rate
PCR – Peak Cell Rate
MCR – Minimum Cell rate
CVDT – Cell variation delay tolerance
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ATM Adaptation Layers - AAL
• AAL’s are standards that specify how the 48
byte data payload is used.
• AAL1 – Supports constant bit rate
applications such as T1, T3, etc.
– Has a 1 byte header and 47 bytes of data
• AAL2 – support for variable bit rate
applications (like compressed voice)
– Has a 1 byte pointer and 47 bytes of data
– The 47 bytes contain variable length packets
that each have a 3 byte header.
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ATM Adaptation Layers - AAL
• AAL3/4 – combined #3 and #4
– Support for data services
– Each cell has a 2 byte header and 2 byte trailer
– The data packet (up to 64k bytes) is given a 4
byte header and 4 byte trailer and padded to a
multiple of 44.
– The data packet is then sliced into 44 byte
chunks to be put into each cell
– Supports sequencing and error control
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ATM Adaptation Layers – AAL
• AAL5 – data applications that don’t require
sequencing and error control of AAL3/4
• Data packet (up to 64k bytes) has an 8 byte
trailer added and padded up to multiple of
48 bytes.
• Data packet is then sliced into 48 byte
chunks to be put into each cell.
• The PTI signaling bit in the header indicates
when the end of a packet occurs.
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Telecom history
• 1876 Alexander Graham Bell applies for patent on
his telephone.
– Race between Bell and Elisha Gray.
– Bell won in court
• 1891 Almon Strowger, an undertaker, receives
patent for an automatic switcher
• 1921 Graham-Willis act establishes AT&T as a
“natural monopoly”
• 1934 Federal Communications Commission
(FCC) created
– With PUCs - regulates AT&T
• 1940s - Hush-A-Phone device
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Breaking up is hard to do
• 1969 MCI provides inter-city service
• 1974 US Government files antitrust suit against
AT&T
• 1975 Carterphone decision
• 1984 - Modified Final Judgement (MFJ)
• Ma Bell broken up into AT&T and 7 RBOCS.
• RBOCS - Regional Bell Operating Companie
– Nynex, Bell Atlantic, BellSouth, Ameritech, Southwest
Bell, US West, Pacific Telesis
• Local and Intra-Lata long distance is kept as
monopoly while other services are competitive.
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1996 - Telecommunications Act
• Deregulate the local loop.
• Facilities Based
– Carrier provides its own cables to the customer premise
• Non-Facilities Based
– Carrier rents or leases equipment and lines from local telcos at a
discount rate.
• Allows BOCs to enter long distance market
– Provided that they show an open competitive environment exists in
local loop.
• Creates a lot of mergers
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Bell Atlantic merges with Nynex
Bell Atlantic & GTE  Verizon
US West & Qwest  Qwest
SBC, Pac Bell, & Ameritech  SBC
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Hell’s Bells: A Radio history of the
Telephone
– http://www.town.hall.org/Archives/radio/IMS/
HellsBells/
• 8 Parts - 30 minutes each
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Tariffs
• A tariff is a description of a service that
offers an appropriate rate of charge for that
service, and the rules under which the
service is to be provided.
• 50 different regulators rule on tariffs
• FCC governs rates & services for Long
Distance providers
• Tariffs are written by phone companies and
reviewed and enforced by regulators.
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Sound
• Banging of molecules together at a rapid rate
• This banging is called compression and
rarefication
• Rate of compression (pitch, or frequency) per unit
time (seconds) is measured in Hertz (Hz).
• Human voice has frequency range between 100
and 5000 Hertz.
• Sound also has loudness attribute, or amplitude.
• Human ear is responsive to variations in frequency
between 25 and 25000 Hz.
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Sound Processing
• Through empirical testing, Phone Companies
realized that the majority of useful information is
carried in a 3 KHz range
• The actual voice envelope (spectrum) carried by
the phone is 200Hz to 3500Hz.
• Thus, the bandwidth of the transmitted voice is 3.3
KHz
• The actual bandwidth of the voice line is 4KHz,
but margins are suppressed by bandpass filters to
allow multiple channels on the same media
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Telephone Network
• Topology of connections is used
• Twisted Pair connects DEMARK point with the
wire center (frame).
• Wire Center is connected to the Switch (Central
Office)
• Switches are interconnected together to form a
network.
• Switches make routing decisions based on the
requested destination and available capacity
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Pre-84 Network Topology
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Over 19000 End (Class 5) Offices
Over 940 Toll Centers
170 Class 3 Offices (Primary Centers)
Over 50 Sectional Centers (Class 2)
10 Regional (Class 1) Centers
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Post-84 Network Topology
•
Images from “Voice & Data Communications” by Regis J. “Bud” Bates & Donald W. Gregory.
Third Edition - 2000
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North American Numbering Plan
• Area Code
– 160 (152) in 60’s, 800 right now
• Central Office (Exchange) Code
– 640 originally
– expanded to 800 in 1960’s
Timing
Original
Pre-1995
Post 1995
N = 2-9
X = 0-9
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Area Code
N 0/1 X
N 0/1 X
NXX
Central Office Code
NNX
NXX
NXX
Subscriber Number
XXXX
XXXX
XXXX
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Local Access and Transport Area - LATA
• “Turf” division between Local and Long
Distance Companies
• Based on Geographical area and Population
Density
• Different Rules applied as to who could
route the calls where.
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Classifications of telephone companies
• LEC - Local Exchange Carrier
• ILEC - Incumbent Local Exchange Carrier
• CLEC - Competitive Local Exchange
Carrier
• BLEC - Building or Business Local
Exchange Carrier
• DLEC - Data Local Exchange Carrier
• IXC - Inter Exchange Carrier
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Lines VS Trunks
• Lines
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Low Capacity
Non-intelligent (lack signalling)
Normal Voice Load
Typically dedicated to 1 consumer
Allocated on demand
Can be blocked
• Trunks
– Intelligent (carry signalling information)
– High Capacity
– Statically Allocated Capacity
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Types of Lines and Trunks
• POTS - Plain Old Telephony Service
– normal voice telephony service
• Direct Inward Dial
• Direct Outward Dial
• FX -- Foreign Exchange -- provides dial tone from the
remote (foreign) exchange.
– Typical use -- airline reservation system
• OPX -- Off-premise Extension -- permits a remote phone
to function as a local phone.
– Typical use -- “remote” extensions in business offices.
• Tie Lines -- private point-to-point circuit used to connect
two voice facilities
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Line signaling types
• Loop start
• Ground start
• Reverse Battery
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LEC - Local Echange Carriers
• LEC terminates services at the Network Interface
Unit (or Demarkation Point)
• Customer takes over from there on
• Main Service Provided: Dialtone
• Other services can be purchased.
• Either dedicated or party line services is offered
• Residential Dialing Service (Direct Distance
Dialing)
• Business Services (DID, DOD)
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Pay Phone Service
• Public Phone Service
– Both phone and and line are owned by LEC
– No monthly rental charges
– Accessed by everybody
• Semipublic Service
– Rent a line and set, place phone in controlled location
– Minimum rate guaranteed to the LEC -- controlled by
tariffs
• Private Service
– buy or rent a set, rent a line (flat fee) and/or share of
profits
– placed in restricted location
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WATS
• Wide Area Telecommunications Services
• Primarily a billing service that allows reduced rate
for long-distance and local telecommunications
• Initially, WATS was implemented in the form of
bands that separated the country into 5 regions
• Currently WATS rates can be negotiated between
any geographical localities
• 800/888 Service
– Reverse-billing WATS
– Recipient pays for the call, and not the call initiator
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Access to IEC (Equal Access)
• Either full or partial presence is possible in
any market
– Full presence:
• IEC rents or buys space
• installs a POP (Point of Presence, digital switch)
– Partial presence:
• IEC buys space or rents space from LEC
• installs or rents a frame (wire center)
• runs high-capacity trunks to POP in a different
location
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PBX Systems
• PBX -- Private Branch Exchange
• Typically a small-capacity (up to 5K lines) digital
switch that provides add-on services not available
through LEC
• Add-on services include, but are not limited to
voice mail, transferring, conference calling, etc.
• Interfaces to the LEC via leased trunks
• Typically utilizes digital phone sets
• Up to the customer to maintain the wiring and the
equipment
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Centrex Service
• A PBX-like service offered through LECs
• LEC “partitions” a Class 5 switch, dedicating
some of its processing and voice capacity to
Centrex customers
• Partitioning is typically virtual, or software-only
• Provides the same services as PBXs
• Customer does not have to maintain the wiring or
the equipment -- LEC does it for the customer
• Customer has an option of adding own services
(voice processing, etc) to Centrex, just like to any
PBX
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Key Systems
• Same features as PBXs
• Aimed at smaller customers
• Uses dial-up lines instead of trunks to
interface with LEC
• Modern PBXs are typically packaged as
either Key Systems or PBXs, the only
difference being the LEC interface
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