10/3/2015 1r. smith - university of st thomas - minnesota cisc 370 - class today homework – next...

04/19/23 1R. Smith - University of St Thomas - Minnesota

CISC 370 - Class Today CISC 370 - Class Today

• Homework – Next ThursdayHomework – Next Thursday• POTS RecapPOTS Recap• Efficiency of Packets vs CircuitsEfficiency of Packets vs Circuits• Frame Relay & ATMFrame Relay & ATM• Firewall LabFirewall Lab

Upcoming HomeworkUpcoming Homework

• I’ll post some Chapter 12/13 homework soonI’ll post some Chapter 12/13 homework soon– Outline: April 23Outline: April 23

• I’ll e-mail comments to your groupI’ll e-mail comments to your group– Revised Outline (optional): Apr 30Revised Outline (optional): Apr 30

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



The Plain Old Telephone SystemThe Plain Old Telephone System

• POTSPOTS– Architecture Architecture – SS-7SS-7

• WANs from the POTS folksWANs from the POTS folks– X.25X.25– Frame RelayFrame Relay– ATMATM

POTS ServicesPOTS Services

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

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



Trade-offs between choicesTrade-offs between choices

• Cost structure: per link, per connection, per Cost structure: per link, per connection, per packet, distance sensitive, etc.packet, distance sensitive, etc.

• Switched vs unswitchedSwitched vs unswitched

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

• Reliability and flow control: network or Reliability and flow control: network or endpoint responsibility?endpoint responsibility?

Computing the Trade-OffsComputing the Trade-Offs

• The Parameters:The Parameters:– Efficiency = ratio of information bits / all bits transmittedEfficiency = ratio of information bits / all bits transmitted– N = Number of hops between systems (ex. 2)N = Number of hops between systems (ex. 2)– L = Message length in bits (ex. 4096)L = Message length in bits (ex. 4096)– B = data rate in bits per second (ex. 8192)B = data rate in bits per second (ex. 8192)– P = size of a packet, assume it’s a fixed size (512)P = size of a packet, assume it’s a fixed size (512)– H = header size in bits (overhead per packet, ex. 32)H = header size in bits (overhead per packet, ex. 32)– S = call setup time for circuits/connections (VCs, ex. 0.4 sec)S = call setup time for circuits/connections (VCs, ex. 0.4 sec)– D = propagation delay when going between hops (ex. 0.002)D = propagation delay when going between hops (ex. 0.002)

• Calculate delays, efficiency of these:Calculate delays, efficiency of these:– Circuit switchingCircuit switching– Datagram-based packet switchingDatagram-based packet switching– Packet switched connections (“virtual circuits”)Packet switched connections (“virtual circuits”)


Efficiency of Fixed vs Variable LengthEfficiency of Fixed vs Variable Length

• The question becomes:The question becomes:– Is it more efficient to carry a ‘packet length’ field and deal with Is it more efficient to carry a ‘packet length’ field and deal with

variable sized data packets,variable sized data packets,– Or is it more efficient to use fixed-size packets w/o an explicit Or is it more efficient to use fixed-size packets w/o an explicit

lengthlength

• ImplicationsImplications– Easier to implement and manage fixed size packetsEasier to implement and manage fixed size packets

• No “external fragmentation” in the buffer of a switchNo “external fragmentation” in the buffer of a switch– ““Packet fill delay” – waiting to fill a part-empty packetPacket fill delay” – waiting to fill a part-empty packet– If typical packets are part-full, then we waste spaceIf typical packets are part-full, then we waste space

• ““Internal fragmentation”Internal fragmentation”



X-25 Network ProtocolX-25 Network Protocol

• Telco industry’s first - unsuccessful - attempt Telco industry’s first - unsuccessful - attempt to build a networking protocolto build a networking protocol

• Designed a "smart network“Designed a "smart network“• Misused the notion of a protocol stackMisused the notion of a protocol stack

– used it to establish independence among protocol designers at used it to establish independence among protocol designers at different levels -different levels -

– led to serious inefficienciesled to serious inefficiencies– Flow control and error correction replicated at layers 2 and 3Flow control and error correction replicated at layers 2 and 3


X.25 ArchitectureX.25 Architecture

• Telcos took as an article of faith that Telcos took as an article of faith that connections are fundamentalconnections are fundamental– Per-connection overhead in individual network switchesPer-connection overhead in individual network switches

• Makes it “circuit switched” Makes it “circuit switched” – Personally, I implemented X.25 over the Arpanet backbone Personally, I implemented X.25 over the Arpanet backbone

without such foolishness and it worked fine.without such foolishness and it worked fine.• Sufficient to embed it in switches nearest the endpointsSufficient to embed it in switches nearest the endpoints• Flow control took some fine-tuning, but that worked, too.Flow control took some fine-tuning, but that worked, too.

• Charges and ServicesCharges and Services– Cost per packet - I remember this; probably a link cost, tooCost per packet - I remember this; probably a link cost, too– Multiple channels per link possibleMultiple channels per link possible– Switched and unswitched channels possible ('permanent' Switched and unswitched channels possible ('permanent'

virtual circuits)virtual circuits)


Frame RelayFrame Relay

• A "dumber network" than X.25A "dumber network" than X.25– closer to “end to end” Internet architecture conceptcloser to “end to end” Internet architecture concept

• WAN with unreliable datagrams and no flow WAN with unreliable datagrams and no flow controlcontrol– Relies on end-to-end protocols like TCP to handle flow control Relies on end-to-end protocols like TCP to handle flow control

and error correctionand error correction– 'Smarter' than datagrams – 'Smarter' than datagrams –

• retains order of transmission on a channelretains order of transmission on a channel– Stallings argues that this works because modern digital Stallings argues that this works because modern digital

transmission methods are more reliable than the analog transmission methods are more reliable than the analog modem-based techniquesmodem-based techniques

– Greatly increased network efficiency and reduced transmission Greatly increased network efficiency and reduced transmission delays by eliminating "smart network" protocol overheaddelays by eliminating "smart network" protocol overhead


Protocol detailsProtocol details

• Multiple channels – Multiple channels – – channel 0 for linking other channels to endpointschannel 0 for linking other channels to endpoints

• Each channel can have its own endpoint – Each channel can have its own endpoint – – either predefined or on a "per call" basiseither predefined or on a "per call" basis– Like ‘virtual circuits’ on X.25Like ‘virtual circuits’ on X.25

• Individual packets carry a channel number or Individual packets carry a channel number or "Data Link Connection Identifier" (DLCI)."Data Link Connection Identifier" (DLCI).


Setting up a connectionSetting up a connection

• Initating host sends a SETUP packet - crosses the Initating host sends a SETUP packet - crosses the network to the destination, delivered to destination network to the destination, delivered to destination host.host.

• Destination host accepts by sending a CONNECT Destination host accepts by sending a CONNECT packet - goes back to the initiating host.packet - goes back to the initiating host.

• The SETUP/CONNECT protocol establishes a channel, The SETUP/CONNECT protocol establishes a channel, assigns a DLCI.assigns a DLCI.

• When connection finished, send a RELEASE to other When connection finished, send a RELEASE to other endend

• Other end responds with RELEASE COMPLETEOther end responds with RELEASE COMPLETE• No big deal - just different names for the same sort of No big deal - just different names for the same sort of

thing.thing.


Congestion controlCongestion control• Not much.Not much.• "Danger Will Robinson" bit – "Danger Will Robinson" bit –

– says that there's congestion in one direction or the other. says that there's congestion in one direction or the other. – "Forward/Backward Explicit Congestion Notification" FECN or BECN)"Forward/Backward Explicit Congestion Notification" FECN or BECN)

• "Sacrificial Lamb" bit – "Sacrificial Lamb" bit – – says this packet is a good one to discard if things are too congested. says this packet is a good one to discard if things are too congested. – "Discard Eligibility" DE"Discard Eligibility" DE

• Implement multiple transmission rates, based on what Implement multiple transmission rates, based on what is paid foris paid for– Committed Info Rate (CIR) - what's paid forCommitted Info Rate (CIR) - what's paid for– Maximum Rate (MR) - what is acceptedMaximum Rate (MR) - what is accepted– Access Rate – Access Rate –

• what the link accepts – what the link accepts – • excess past MR gets discardedexcess past MR gets discarded


•ATM or "Cell Relay“ATM or "Cell Relay“

• A "cell" is a "frame" only it's supposed to be A "cell" is a "frame" only it's supposed to be transmitted faster.transmitted faster.– Dumber and more efficient than X.25Dumber and more efficient than X.25– Cell sequence is preservedCell sequence is preserved

• Basic Features Basic Features – Virtual channelsVirtual channels– Packet format/featuresPacket format/features– Service categoriesService categories


Virtual paths and virtual channelsVirtual paths and virtual channels

• Users see virtual channels as logical Users see virtual channels as logical connectionsconnections

• Virtual paths are a network level property: Virtual paths are a network level property: – represents a set of virtual channels with a common destination represents a set of virtual channels with a common destination

– – – network handles them as an aggregated entity instead of network handles them as an aggregated entity instead of

handling the channels individuallyhandling the channels individually


Packet formatPacket format

• Packet destination = virtual path + virtual Packet destination = virtual path + virtual channel within pathchannel within path

• Payload type = user data vs system data, Payload type = user data vs system data, – also includes info about congestionalso includes info about congestion– poor flow control againpoor flow control again

• Sacrificial lamb bit - "Cell Loss Priority" (CLP)Sacrificial lamb bit - "Cell Loss Priority" (CLP)• 8-bit checksum for the header8-bit checksum for the header

– since bit errors could cause pain to the networksince bit errors could cause pain to the network


ATM Service categories ATM Service categories

• or, "I'm a big customer and you'd better provide me the or, "I'm a big customer and you'd better provide me the category of service I want or I'm calling in the category of service I want or I'm calling in the competition."competition."– + Constant bit rate (CBR) - traditional connection service+ Constant bit rate (CBR) - traditional connection service

– + Variable Bit Rat (VBR) - gives network more flexibility and lower cost + Variable Bit Rat (VBR) - gives network more flexibility and lower cost to the customerto the customer

– + Unspecified Bit Rate (UBR) - 'best effort' service - give it whatever + Unspecified Bit Rate (UBR) - 'best effort' service - give it whatever bandwidth is left overbandwidth is left over

– + Avaliable bit rate (ABR) - specifies a minimum cell rate required + Avaliable bit rate (ABR) - specifies a minimum cell rate required (MCR) and a peak rate (PCR). Connects LANs across ATM(MCR) and a peak rate (PCR). Connects LANs across ATM

– + Guaranteed Frame Rate (GFR) + Guaranteed Frame Rate (GFR)

• - for connecting to Internet backbone. Has the ATM net understand - for connecting to Internet backbone. Has the ATM net understand frame boundaries, so packets are discareded in "frame" sets frame boundaries, so packets are discareded in "frame" sets instead of individually, possibly from separate frames.instead of individually, possibly from separate frames.

Firewall LabFirewall Lab

• Walk through the labWalk through the lab• Walk through the manualWalk through the manual

• … … let’s visit the lablet’s visit the lab



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