QMCS 370 - Class Today

1
QMCS 370 - Class Today

• POTS
• Architecture
• SS-7
• WANs from the POTS folks
• X.25
• Frame Relay
• ATM

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POTS

• Plain Old Telephone System
• Driven by analog circuit traditions
• Classic circuit switching
• Architecture based on phone system evolution

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Circuit operation

• Like connections, with differences
• Phase 1 establishment
• One endpoint initiates other accepts
• Establish the route through intermediate nodes
• Allocate resources at intermediate nodes
• Phase 2 data transfer
• Data follows the established path and data rate
• Phase 3 disconnect
• Initiated by an endpoint
• Deallocate resources at the nodes

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Architecture

• Subscribers
• End user points
• Dumb endpoints in traditional systems
• Subscriber lines
• The last mile of telecommunications
• AKA subscriber loop or local loop
• Exchanges
• Connects to subscriber lines
• Connnects to trunks leading to other exchanges
• Trunks
• Connects exchanges together

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Exchanges (Offices)

• End Office connects to subscriber lines
• 19,000 exchanges in the US
• Intermediate Exchange
• connects end offices together
• Long Distance Office
• connects end offices and other long distance
  offices

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Trunks

• Types
• Connecting trunk between nearby exchanges
• Intercity trunk between more distant exchanges
• Channel Implementation
• Frequency Division Multiplexing
• Synchronous Time Division Multiplexing
• Architecture diagram
• Subscriber loop, end office connecting trunk,
  intermediate exchange, long distance office,
  intercity trunk

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Signalling and Control Function

• Controlling a call and giving status
• Consider a classic telephone
• What control and status things have you heard?
• Control signals?
• Status signals?
• In-band versus out of band signalling
• Where do we have in-band signalling on phone
  connections?
• Does TCP/IP use in-band or out-of-band
  signalling? Ethernet?

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Can we figure out how calls work?

• What elements of network technology do we need?
• What have we experienced in phone calls?
• What do we know about POTS architecture?
• How does this manifest itself through circuit
  switching?

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Telephone connection sequence

• Both phones are on-hook, One goes off-hook
• End office sends dial tone
• Caller dials a number
• Switch uses this as the called address
• If called address is not busy, make it ring
• Send ring tone to caller
• If called phone goes off hook, connect the call
• Turn off the ring signal
• Continue the connection till a phone goes on-hook

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Office-to-office connection

• Originating office finds a free connection on an
  interoffice trunk
• Sends a request for a digit register to receive
  the called number
• Destination sends a wink when it has a digit
  register for originator to use
• Originator sends the number to the destination
  office
• The destination connects to the end subscriber
  loop, or continues through another office

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Signaling System 7 (SS7)

• Today, trunks use SS7 for control signaling
• Packet technology POTS office architecture
• Offices are now called switches
• Highly redundant
• Supports modern capabilities
• Phone numbers not tied to hardware (subscriber
  loop)
• Phone numbers roam geographically
• Remote voice mail
• Toll free numbers (800 etc)
• Special charge numbers (900 etc)

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Elements of SS7

• These devices are deployed redundantly
• Service Switching Points (SSPs)
• Connect to subscribers local loops
• Connect to STPs via SS7
• Sends queries to SCPs to find out how to route a
  call
• Service Transfer Points (STPs)
• A packet switch tailored to handle SS7
• Routes data based on phone numbers
• Firewalling traffic from external networks
• Service Control Points (SCPs)
• Centralized databases
• Links particular phone numbers to particular
  subscribers
• Provides routing information for reaching
  subscribers

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WANs - The Telcos' parting attempts at relevance

• o They really are mired in an existing business
  model and customer base
• o Makes it hard for them to deal with the
  changing data comm landscape
• o You can almost see how modern services like ATM
  reflect demands by particular (large) customers
  with particular expectations
• o Telcos still exist because they can meet these
  demands and charge high tariffs for them.

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Classic WAN Lineup

• o "Leased Lines" - dedicated point to point
  connections (archaic!!)
• Most of these were a fixed (huge!) cost per month
• Cost tied to distance of connection
• Analog - an ancient and relatively slow service
  (56K)
• Digital Data Service - a slightly less ancient
  and slow service (56K)
• T-1 - the workhorse for early Internet sites
  1.54M
• T-3 - something of an improvement 44.7M

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Newer Services

• Frame Relay - more recent service
• (talk more about it in a minute)
• 44.7M
• Charge per month for the connecting port
• Added charge per month for each virtual circuit's
  capacity
• No extra charge for longer distances
• Synchro Optical Net (SONET) 51.4M to ...
• Standard designation for optical hardware
  connections
• OC numbers
• OC-1 (or STS-1) at 50Mb/sec
• thru
• OC-192 (STS-192) at 9.6Gb/sec
• STS-768 at 38 Gb/sec.. etc.

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o "Switched Services

• gee, a choice of destinations!
• Dial-up analog - the classic modem connection
  56K
• X.25 packet switching - now archaic 56K
• ISDN -
• a first attempt at integrated ditigtal service
• up to 1.54M
• cost per month plus connect time charge long
  distance charges
• ADSL - something more contemporary, but aging
• up to 9M
• Frame Relay - see, both switched and unswitched
• ATM - the Great White Hope of the telcos
• if this doesn't bring in business, they're
  history
• Pricing structure varies, but is not usually
  distance sensitive

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Trade-offs between choices

• o Cost structure per link, per connection, per
  packet, distance sensitive, etc.
• o Switched vs unswitched
• o Channels per physical link all in one, or
  multiplexed
• o Reliability and flow control network or
  endpoint responsibility?

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X-25 Network Protocol

• Telco industrys first - unsuccessful - attempt
  to build a networking protocol
• Designed a "smart network
• Misused the notion of a protocol stack
• used it to establish independence among protocol
  designers at different levels -
• led to serious inefficiencies
• Flow control and error correction replicated at
  layers 2 and 3

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X.25 Architecture

• Telcos took as an article of faith that
  connections are fundamental
• Embedded per-connection overhead in individual
  network switches
• Personally, I implemented X.25 over the Arpanet
  backbone without such foolishness and it worked
  fine.
• Flow control took some fine-tuning, but that
  worked, too.
• Services
• Cost per packet - I remember this probably a
  link cost, too
• Multiple channels per link possible
• Switched and unswitched channels possible
  ('permanent' virtual circuits)

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Frame Relay

• A "dumber network" than X.25
• closer to end to end Internet architecture
  concept
• WAN with unreliable datagrams and no flow control
• Relies on end-to-end protocols like TCP to
  handle flow control and error correction
• 'Smarter' than datagrams
• retains order of transmission on a channel
• Stallings argues that this works because modern
  digital transmission methods are more reliable
  than the analog modem-based techniques
• Greatly increased network efficiency and
  reduced transmission delays by eliminating "smart
  network" protocol overhead

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Protocol details

• Multiple channels
• channel 0 for linking other channels to endpoints
• Each channel can have its own endpoint
• either predefined or on a "per call" basis
• Like virtual circuits on X.25
• Individual packets carry a channel number or
  "Data Link Connection Identifier" (DLCI).

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Setting up a connection

• Initating host sends a SETUP packet - crosses
  the network to the destination, delivered to
  destination host.
• Destination host accepts by sending a CONNECT
  packet - goes back to the initiating host.
• The SETUP/CONNECT protocol establishes a
  channel, assigns a DLCI.
• When connection finished, send a RELEASE to
  other end
• Other end responds with RELEASE COMPLETE
• No big deal - just different names for the same
  sort of thing.

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Congestion control

• Not much.
• "Danger Will Robinson" bit
• says that there's congestion in one direction or
  the other.
• "Forward/Backward Explicit Congestion
  Notification" FECN or BECN)
• "Sacrificial Lamb" bit
• says this packet is a good one to discard if
  things are too congested.
• "Discard Eligibility" DE
• Implement multiple transmission rates, based on
  what is paid for
• Committed Info Rate (CIR) - what's paid for
• Maximum Rate (MR) - what is accepted
• Access Rate
• what the link accepts
• excess past MR gets discarded

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ATM or "Cell Relay

• A "cell" is a "frame" only it's supposed to be
  transmitted faster.
• Dumber and more efficient than X.25
• Cell sequence is preserved
• Features (to be expanded)
• Virtual channels
• Packet format/features
• Service categories

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Virtual paths and virtual channels

• Users see virtual channels as logical
  connections
• Virtual paths are a network level property
• represents a set of virutal channels with a
  common destination
• network handles them as an aggregated entity
  instead of handling the channels individually

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Packet format

• Packet destination virtual path virtual
  channel within path
• Payload type user data vs system data,
• also includes info about congestion
• poor flow control again
• Sacrificial lamb bit - "Cell Loss Priority" (CLP)
• 8-bit checksum for the header
• since bit errors could cause pain to the network

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ATM Service categories

• or, "I'm a big customer and you'd better provide
  me the category of service I want or I'm calling
  in the competition."
• Constant bit rate (CBR) - traditional
  connection service
• Variable Bit Rat (VBR) - gives network more
  flexibility and lower cost to the customer
• Unspecified Bit Rate (UBR) - 'best effort'
  service - give it whatever bandwidth is left over
• Avaliable bit rate (ABR) - specifies a minimum
  cell rate required (MCR) and a peak rate (PCR).
  Connects LANs across ATM
• Guaranteed Frame Rate (GFR)
• - for connecting to Internet backbone. Has the
  ATM net understand frame boundaries, so packets
  are discareded in "frame" sets instead of
  individually, possibly from separate frames.

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