Frame Relay ATM University of St Thomas

					CISC 370 - Class Today
•   Homework – Next Thursday
•   POTS Recap
•   Efficiency of Packets vs Circuits
•   Frame Relay & ATM
•   Firewall Lab




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Upcoming Homework
• I’ll post some Chapter 12/13 homework soon
  – Outline: April 23
     • I’ll e-mail comments to your group
  – Revised Outline (optional): Apr 30

  – Papers: May 12
  – Presentations: May 12 and 14




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The Plain Old Telephone System
• POTS
  – Architecture
  – SS-7
• WANs from the POTS folks
  – X.25
  – Frame Relay
  – ATM




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POTS Services
• Digital POTS
  – Synchro Optical Net (SONET) 51.4M ++
  – ISDN -
  – ADSL - something more contemporary, but aging


• Switched Services”
  – X.25 packet switching - now archaic 56K
  – Frame Relay - see, both switched and unswitched
  – ATM - the Great White Hope of the telcos



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Trade-offs between choices
• Cost structure: per link, per connection, per
  packet, distance sensitive, etc.

• Switched vs unswitched

• Channels per physical link: all in one, or
  multiplexed

• Reliability and flow control: network or
  endpoint responsibility?


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Computing the Trade-Offs
• The Parameters:
  –   Efficiency = ratio of information bits / all bits transmitted
  –   N = Number of hops between systems (ex. 2)
  –   L = Message length in bits (ex. 4096)
  –   B = data rate in bits per second (ex. 8192)
  –   P = size of a packet, assume it’s a fixed size (512)
  –   H = header size in bits (overhead per packet, ex. 32)
  –   S = call setup time for circuits/connections (VCs, ex. 0.4 sec)
  –   D = propagation delay when going between hops (ex. 0.002)
• Calculate delays, efficiency of these:
  – Circuit switching
  – Datagram-based packet switching
  – Packet switched connections (“virtual circuits”)
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Efficiency of Fixed vs Variable Length
• The question becomes:
  – Is it more efficient to carry a ‘packet length’ field and deal with
    variable sized data packets,
  – Or is it more efficient to use fixed-size packets w/o an explicit
    length
• Implications
  – Easier to implement and manage fixed size packets
      • No “external fragmentation” in the buffer of a switch
  – “Packet fill delay” – waiting to fill a part-empty packet
  – If typical packets are part-full, then we waste space
      • “Internal fragmentation”




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X-25 Network Protocol
• Telco industry’s 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
  – Per-connection overhead in individual network switches
     • Makes it “circuit switched”
  – Personally, I implemented X.25 over the Arpanet backbone
    without such foolishness and it worked fine.
     • Sufficient to embed it in switches nearest the endpoints
     • Flow control took some fine-tuning, but that worked, too.
• Charges and 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
• Basic Features
  – 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 virtual 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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Firewall Lab
• Walk through the lab
• Walk through the manual

• … let’s visit the lab




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