Introduction to Telephony, Cable and Internet …koushik/shivkuma-teaching/sp2003/bon...Can’t run thick bundles! Instead, send many calls on the same wire Multiplexing (a.ka. Sharing)

Shivkumar KalyanaramanRensselaer Polytechnic Institute

1

Introduction to Telephony, Cable and Internet

Technologies

Based in part upon slides of S. Keshav (Ensim), J. Bellamy’s book, Prof. Raj Jain (OSU), L. Peterson (Princeton), J. Kurose (U Mass)

http://www.pde.rpi.edu/Or

http://www.ecse.rpi.edu/Homepages/shivkuma/


[email protected]


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Connectivity: direct (pt-pt, N-users), indirect (switched, inter-networked)

Telephony, Internet, Cable Networks: Basic ConceptsConcepts: Topologies, Framing, Multiplexing, Flow/Error Control, Reliability, Multiple-access, Circuit/Packet-switching, Addressing/routing, Congestion control Data link/MAC layer: SLIP, PPP, LAN technologies …Interconnection DevicesS. Keshav book (Chapter 2), Opt Nets (Sec 11.1, 13.1, 13.2)

Overview


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Connectivity...Building Blocks

links: coax cable, optical fiber...nodes: general-purpose workstations...

Direct connectivity:point-to-point

multiple access


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Connectivity… (Continued)

Indirect Connectivityswitched networks

=> switches

inter-networks

=> routers


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What is “Connectivity” ?

Direct or indirect access to every other node in the network

Connectivity is what you get instead of a direct physical link

Key Tradeoff: Performance characteristics worse!


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Connectivity …Internet:

Best-effort(no performance guarantees)Packet-by-packet

A pt-pt link: Always-connectedFixed bandwidthFixed delay Zero-jitter


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Telephony


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Telephone Network: What is It?Specialized to carry voice traffic

Aggregates like T1, SONET OC-N can also carry dataAlso carries

Telemetry, video, fax, modem callsInternally, uses digital samplesSwitches and switch controllers are special purpose computers

Pieces:1. End systems2. Transmission3. Switching4. Signaling


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Telephone Network: What is It?Single basic service: two-way voice

low end-to-end delayguarantee that an accepted call will run to completion

Endpoints connected by a circuit, like an electrical circuitSignals flow both ways (full duplex)Associated with reserved bandwidth and buffer

resources


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Telephone Network Design

Fully connected coresimple routingtelephone number is a hint about how to route a call

But not for 800/888/700/900 numbers: these are pointers to a directory that translates them into regular numbers

hierarchically allocated telephone number space


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Telephone Network Design


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Telephone Pieces: End Systems


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Telephone Pieces: End Systems

Transducers: key to carrying voice on wiresDialerRingerSwitch-hook


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Last-Mile Transmission EnvironmentWire gauges:19, 22, 24, 26 gauge(smaller better)Diameters: 0.8, 0.6, 0.5, 0.4 mm (larger better)Various forms of noise: (twisting reduces noise)

Bridged-tap noise: bit-energy diverted to extension phone socketsCrosstalkHam radioAM broadcast

Insertion loss: -140 dBm noise floor 100 million times more sensitive than normal modemsBandwidth range = 600 kHzNotch effects in insertion loss due to bridged-tapsTransmission PSD = -40dBm => 90 dBm budget


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2-wire vs 4-wire: Sidetones and Echoes

Both trans & reception circuits need two wires

4 wires from every central office to home

Alternative: Use same pair of wires for bothtransmission and receptionSignal from transmission flows to receiver: sidetoneReverse Effect: received signal at end-system bounces

back to CO (esp if delay > 20 ms): echoSolutions: balance circuit (attenuate side-tone) + echo-

cancellation circuit (cancel echoes).


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DialingPulse

sends a pulse per digitcollected by central office (CO)Interpreted by CO switching system to place call or activate special features (eg: call forwarding, prepaid-calls etc)

Tonekey press (feep) sends a pair of tones = digitalso called Dual Tone Multifrequency (DTMF)

CO supplies the power for ringing the bell.Standardized interface between CO and end-system => digital handsets, cordless/cellular phones


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Telephone Pieces: Transmission MuxingTrunks between central offices carry hundreds of conversationsCan’t run thick bundles! Instead, send many calls on the same wire

Multiplexing (a.ka. Sharing)Analog multiplexing

Band-limit call to 3.4 KHz and frequency shift onto higher bandwidth trunkobsolete

Digital multiplexingfirst convert voice to samples1 sample = 8 bits of voice8000 samples/sec => call = 64 Kbps


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Transmission Multiplexing (contd)How to choose a sample?

256 quantization levels, logarithmically spaced (why?)sample value = amplitude of nearest quantization levelTwo choices of levels (µ law and A law)

Time division multiplexingTrunk carries bits at a faster bit rate than inputsn input streams, each with a 1-byte bufferOutput interleaves samplesNeed to serve all inputs in the time it takes one sample to arrive

=> output runs n times faster than inputOverhead bits mark end of frame (why?)


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Transmission MultiplexingMultiplexed trunks can be multiplexed furtherNeed a standard! (why?)US/Japan standard is called Digital Signalinghierarchy (DS)

Digital Signal Number

Number of previous level circuits

Number of voice circuits

Bandwidth

DS0 1 64 KbpsDS1 24 24 1.544MbpsDS2 4 96 6.312 MbpsDS3 7 672 44.736 Mbps


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Telephone Pieces: Switching


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Telephone Pieces: SwitchingProblem:

each user can potentially call any other usercan’t have (a billion) direct lines!

Switches establish temporary circuitsSwitching systems come in two parts: switch and switch controller


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Switching System Components


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Switch: What does it do?Transfers data from an input to an output

many ports (up to 200,000 simultaneous calls)need high speeds

Some ways to switch:1. space division switching: eg: crossbarif inputs (or crosspoints) are multiplexed, need a schedule (why?)


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Crossbar Switching Elements


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Switching (Contd)Another way to switch

time division (time slot interchange or TSI)also needs a service schedule (why?)

To build larger switches we combine space and time To build larger switches we combine space and time division switching elementsdivision switching elements


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Telephone pieces: SignalingA switching system has a switch and a switch controllerSwitch controller is in the control plane

does not touch voice samplesManages the network

call routing (collect dialstring and forward call)alarms (ring bell at receiver)billingdirectory lookup (for 800/888 calls)


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SignalingSwitch controllers are special purpose computersLinked by their own internal computer network

Common Channel Interoffice Signaling (CCIS) network

Earlier design used in-band tones, but was hackedAlso was very rigid (why?)

Messages on CCIS conform to Signaling System 7 (SS7)


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Signaling (contd)One of the main jobs of switch controller: keep track of state of every endpointKey is state transition diagram


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Telephony Routing of Signaled Calls

Circuit-setup (I.e. the signaling call) is what is routed. Voice then follows route, and claims reserved resources. 3-level hierarchy, with a fully-connected coreAT&T: 135 core switches with nearly 5 million circuitsLECs may connect to multiple cores


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Telephony Routing algorithmIf endpoints are within same CO, directly connectIf call is between COs in same LEC, use one-hop path between COsOtherwise send call to one of the coresOnly major decision is at toll switch

one-hop or two-hop path to the destination toll switch.

Essence of telephony routing problem:which two-hop path to use if one-hop path is full(almost a static routing problem… )


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Features of telephone routingResource reservation aspects:

Resource reservation is coupled with path reservationConnections need resources (same 64kbps)Signaling to reserve resources and the path

Stable loadNetwork built for voice only.Can predict pairwise load throughout the dayCan choose optimal routes in advance

Technology and economic aspects:Extremely reliable switches

Why? End-systems (phones) dumb because computation was non-existent in early 1900s.Downtime is less than a few minutes per year => topology does not change dynamically


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Features of telephone routingSource can learn topology and compute routeCan assume that a chosen route is available as the signaling proceeds through the networkComponent reliability drove system reliability and hence acceptance of service by customers

Simplified topology: Very highly connected networkHierarchy + full mesh at each level: simple routingHigh cost to achieve this degree of connectivity

Organizational aspects:Single organization controls entire coreAfford the scale economics to build expensive networkCollect global statistics and implement global changes

=> Source-based, signaled, simple alternate-path routing


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Telecommunications Regulation HistoryFCC regulations cover telephony, cable, broadcast TV,

wireless etc“Common Carrier”: provider offers conduit for a fee and does not control the content

Customer controls content/destination of transmission & assumes criminal/civil responsibility for content

Local monopolies formed by AT&T’s acquisition of independent telephone companies in early 20th century

Regulation forced because they were deemed natural monopolies (only one player possible in market due to enormous sunk cost)FCC regulates interstate calls and state commissions regulate intra-state and local callsBells + 1000 independents interconnected & expanded

FCC rulemaking process:Intent to act, solicitation of public comment etc…


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Deregulation of telephony1960s-70s: gradual de-regulation of AT&T due to technological advances

Terminal equipment could be owned by customers (CPE) => explosion in PBXs, fax machines, handsets Modified final judgement (MFJ): breakup of AT&T into ILECs (incumbent local exchange carrier) and IXC (inter-exchange carrier) part

Long-distance opened to competition, only the local part regulated… Equal access for IXCs to the ILEC network1+ long-distance number introduced then…

800-number portability: switching IXCs => retain 800 number1995: removed price controls on AT&T


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Telecom Act of 1996Required ILECs to open their markets through unbundling of network elements (UNE-P), facilities ownership of CLECs….

Today UNE-P is one of the most profitable for AT&T and other long-distance players in the local market: due to apparently below-cost regulated prices…

ILECs could compete in long-distance after demonstrating opening of markets

Only now some ILECs are aggressively entering long distance marketsCLECs failed due to a variety of reasons…But long-distance prices have dropped precipitously (AT&T’s customer unit revenue in 2002 was $11.3 B compared to 1999 rev of $23B)

ILECs still retain over 90% of local marketWireless substitution has caused ILECs to develop wireless business units


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US Telephone Network Structure (after 1984)


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Exchange Area Network


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Cable TV Networks


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Cable Technology Coaxial cable RF distribution networks. Attributes:

Broadcast, low-band reverse channelsMainly one-way video channelsReasonably secure network (private conduit to home)Free from free-space interferencesGood signal capacity (over 1 GHz) and flexibilityMultiple signaling channelsSignificant attenuation that increases proportional to frequency => (active) RF amplification (every 1000 ft)Freq responses of deployed amps and filters limit practical usage of frequencies > 1 GHz


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Cable Building Blocks


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Cable Spectrum: Upto 750 Mhz


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Cable Technology & Architecture Head-end: signal processing center

Each carrier: Baseband analog or digital modulationCarriers multiplexed w/ freq-selective diplex filters:

allows simultaneous info transfer in both directionsTree-and-branch architecture:

Well-suited for one-way broadcast video transmission (same signals to every customer)Accumulates noise & distortions (amplifiers)Affects plant reliability and received signal qualityLimits on the number of amplifiers cascadedLimits on bandwidth in operation (few 100s of MHz): below cable potential…

Makes delivery of “switched” services (separate stream for each customer) difficult


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Tree-and-Branch Architecture


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Fiber Optics For Cable NetworksKey: Leave the laser ON and intensity-modulate with the analog signalSuch analog modulated lasers are very different from their digital counterparts

Low internal noise and high linearity in the rangeReceiver: simple photo-detector -> back to RF spectrum Result: Hybrid fiber-coax infrastructure, with fiber closer to headend

Coax plant serves smaller range (segmentation), but overall HFC reach dramatically increasedAlso, it allows the economical support of remote, smaller clusters of homesEach part could also provide different services to area (micro-market segmentation)Assign different portions of HFC spectrum to diff uses: many virtual networks: sustained investments possible


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Hybrid Fiber Coax (HFC) Networks


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Multiple Services over HFC


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Future Potential of HFC BroadbandDue to smaller loops, the region from 900MHz –1 GHz can be used for data.

Reduced noise in this region => increased bit rate (200 Mbps) per segment…

Future: fiber moves closer, smaller coax-segments, reduced homes per coax run (60 homes), use of frequencies above 1 Ghz using new electronics

Latest DOCSIS 2.0 spec: 256 QAM (=> 8 bits/Hz) or S-CDMA on cable for more robust transmissions


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Cable RegulationVery different from telephony: not common-carrier

Able to control content AND the conduit!Grew by providing an alternative (and extension) to broadcast TV and had initial growth troublesDid not have to offer service on a non-discriminatory basis (unlike common carriers)Asserted first-amendment rights to maintain control over contentNot required to provide access to their distribution system to other providers (some portion of capacity required to be offered to unaffiliated players: eg: CNN)But they reserve rights to appropriately bundle these channels

Limited regulation: basic tier is rate-regulated by local authorities till 1999 based upon FCC rules


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Cable regulation (contd)Cable networks limited in horizontal expansion, and from vertically integrating w/ CNN etc

Note: ILECs like Bell Atlantic in contrast merged with IXCs like GTEAT&T’s cable acquisitions were interesting (and will be explored later…)

Cable service is multi-faceted and varied from area to area => regulation formulation more complicatedOver-builders (satellite providers) got access to independent content providers: otherwise regulation achieved little for cableLocal authorities get revenue from cable regulationHFC dominates franchise regulation talks, but cable providers are not obligated to provide broadband access..


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Data Networking and the Internet


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Recall: Indirect Connectivity…

Indirect Connectivityswitched networks

=> switches

inter-networks

=> routers


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Inter-Networks: Networks of Networks

=

…

Internet

…

… …

The internet is just a big switch providing indirect connectivity


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Recall: Connecting N users: Directly…

Pt-pt: connects only two users directly…How to connect N users directly ?

What are the costs of each option? Does this method of connectivity scale ?

A B

. . .

Full meshBus


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Point-to-Point Connectivity Issues

A B

Physical layer: coding, modulation etcLink layer needed if the link is shared bet’napps; is unreliable; and is used sporadicallyNo need for protocol concepts like addressing, names, routers, hubs, forwarding, filtering …


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Link Layer: Serial IP (SLIP)Simple: only framing = Flags + byte-stuffingCompressed headers (CSLIP) for efficiency on low speed links for interactive traffic.Problems:

Need other end’s IP address a priori (can’t dynamically assign IP addresses)No “type” field => no multi-protocol encapsulationNo checksum => all errors detected/corrected by higher layer.

RFCs: 1055, 1144


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Link Layer: PPPPoint-to-point protocolFrame format similar to HDLCMulti-protocol encapsulation, CRC, dynamic address allocation possible

key fields: flags, protocol, CRC Asynchronous and synchronous communications possibleLink and Network Control Protocols (LCP, NCP) for flexible control & peer-peer negotiationCan be mapped onto low speed (9.6Kbps) and high speed channels (SONET)


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Connecting N users: Directly ...Bus: Low cost vs broadcast/collisions, MAC complexityFull mesh: High cost vs simplicityNew concept:

Address to identify nodes. Needed if we want the receiver alone to consume the packet!

. . .

Full meshBus

Problem: Direct connectivity does not “scale”….


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How to build Scalable Networks?

Scaling: system allows the increase of a key parameter. Eg: let N increase…

Inefficiency limits scaling …

Direct connectivity is inefficient & hence does not scale

Mesh: inefficient in terms of # of linksBus architecture: 1 expensive link, N cheap links. Inefficient in bandwidth use


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Filtering, forwarding …Filtering: choose a subset of elements from a set

Don’t let information go where its not supposed to…Filtering => More efficient => more scalable

Filtering is the key to efficiency & scaling

Forwarding: actually sending packets to a filtered subset of link/node(s)

Packet sent to one link/node => efficient

Solution: Build nodes which focus on filtering/forwardingand achieve indirect connectivity

“switches” & “routers”


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Connecting N users: IndirectlyStar: One-hop path to any node, reliability, forwarding function“Switch” S can filter and forward!

Switch may forward multiple pkts in parallel for additional efficiency!

S Star


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Connecting N users: Indirectly …Ring: Reliability to link failure, near-minimal links All nodes need “forwarding” and “filtering” Sophistication of forward/filter lesser than switch

Ring


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RingStarS

Tree

Topologies: Indirect Connectivity


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Protocol Issues in Data NetworksPt-Pt connectivity:

FramingError control/ReliabilityFlow control & Windowing protocols

Multiplexing, VirtualizationCircuit vs Packet Switching: a muxing view

MAC arbitration schemes: Random access/CSMA, TDMA, CDMA

Interconnection components: repeater, hub, bridge, switch, router


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Reliability: Types of errors & effectsForward channel bit-errors (garbled packets)Forward channel packet-errors (lost packets)Reverse channel bit-errors (garbled status reports)Reverse channel bit-errors (lost status reports)

Protocol-induced effects: Duplicate packetsDuplicate status reportsOut-of-order packetsOut-of-order status reportsOut-of-range packets/status reports (in window-based transmissions)


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Temporal Redundancy ModelPackets • Sequence Numbers

• CRC or Checksum

Status Reports • ACKs• NAKs, • SACKs• Bitmaps

Timeout

Retransmissions• Packets• FEC information


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Reliability MechanismsMechanisms:

Checksum: detects corruption in pkts & acksACK: “packet correctly received”Duplicate ACK: “packet incorrectly received”Sequence number: identifies packet or ack

1-bit sequence number used both in forward & reversechannel

Timeout only at senderReliability capabilities achieved:

An error-free channelA forward & reverse channel with bit-errorsDetects duplicates of packets/acksNAKs eliminatedA forward & reverse channel with packet-errors (loss)


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Stop and Wait Flow Control

Data

Ack

Ack

Data

tframe

tprop

α =tprop

tframe

=Distance/Speed of SignalFrame size /Bit rate

=Distance × Bit rateFrame size × Speed of Signal

=1

2α + 1

U=2tprop+tframe

tframe

U

αLight in vacuum= 300 m/µs

Light in fiber = 200 m/µsElectricity = 250 m/µs


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Data

Ack

tframe

tprop

U=Ntframe

2tprop+tframe

=

N2α+1

1 if N>2α+1

Sliding Window Protocols


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Multiplexing: The Method of Sharing Costly Resources

Multiplexing = sharingAllows system to achieve “economies of scale”Cost: waiting time (delay), buffer space & loss Gain: Money ($$) => Overall system costs less

Full Mesh Bus


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VirtualizationThe multiplexed shared resource with a level of indirection will seem like a unshared virtual resource!

I.e. Multiplexing + indirection = virtualization

We can “refer” to the virtual resource as if it were the physical resource.

Eg: virtual memory, virtual circuits… Connectivity: a virtualization created by the Internet!Indirection requires binding and unbinding…

Eg: use of packets, slots, tokens etc

. . .

Physical Bus

=A B

A B

Virtual Pt-Pt Link


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Statistical MultiplexingReduce resource requirements (eg: bus capacity) by exploiting statistical knowledge of the system.

Eg: average rate <= service rate <= peak rate

If service rate < average rate, then system becomes unstable!!

First design to ensure system stability!!

Then, for a stable multiplexed system: Gain = peak rate/service rate. Cost: buffering, queuing delays, losses.

Useful only if peak rate differs significantly from average rate.

Eg: if traffic is smooth, fixed rate, no need to play games with capacity sizing…


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Stability of a Multiplexed SystemAverage Input Rate > Average Output Rate

=> system is unstable!

How to ensure stability ?1. Reserve enough capacity so that

demand is less than reserved capacity 2. Dynamically detect overload and adapt

either the demand or capacity to resolve overload


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What’s a performance tradeoff ? • A situation where you cannot get something for nothing!

• Also known as a zero-sum game.

R=link bandwidth (bps)L=packet length (bits)a=average packet arrival rate

Traffic intensity = La/R


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What’s a performance tradeoff ?

La/R ~ 0: average queuing delay smallLa/R -> 1: delays become largeLa/R > 1: average delay infinite (service degrades unboundedly => instability)!

Summary: Multiplexing using bus topologies has both direct resource costs and intangible costs like potential instability, buffer/queuing delay.


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How to design large inter-networks? Circuit-Switching

Divide link bandwidth into “pieces”Reserve pieces on successive links and tie them together to form a “circuit”Map traffic into the reserved circuitsResources wasted if unused: expensive.

– Mapping can be done without “headers”. – Everything inferred from timing.


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How to design large inter-networks? Packet-Switching

Chop up data (not links!)into “packets”

Packets: data + meta-data (header)

“Switch” packets at intermediate nodes

Store-and-forward if bandwidth is not immediately available.

Bandwidth division into “pieces”Dedicated allocationResource reservation


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Packet Switching

45 Mbsqueue of packetswaiting for output

link

Cost: self-descriptive header per-packet, buffering and delays due to statistical multiplexing at switches.

Need to either reserve resources or dynamically detect and adapt to overload for stability

10 MbsEthernet

statistical multiplexingCA

1.5 MbsB

D E


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Spatial vs Temporal Multiplexing Spatial multiplexing: Chop up resource into chunks. Eg: bandwidth, cake, circuits…

Temporal multiplexing: resource is shared over time, I.e. queue up jobs and provide access to resource over time. Eg: FIFO queueing, packet switching

Packet switching is designed to exploit both spatial & temporal multiplexing gains, provided performance tradeoffs are acceptable to applications.

Packet switching is potentially more efficient => potentially more scalable than circuit switching !


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Protocol Issues in Data Networks (Contd)

Pt-Pt connectivity:FramingError control/ReliabilityFlow control & Windowing protocols

Multiplexing, VirtualizationCircuit vs Packet Switching: a muxing view

MAC arbitration schemes: Random access/CSMA, TDMA, CDMA

Interconnection components: repeater, hub, bridge, switch, router


80

Multi-Access LANsHybrid topologies:

Uses directly connected topologies (eg: bus), orIndirectly connected with simple filtering components (switches, hubs).

Limited scalability due to limited filtering

Medium Access Protocols:ALOHA, CSMA/CD (Ethernet), Token Ring …Key: Use a single protocol in network

Concepts: address, forwarding (and forwarding table), bridge, switch, hub, token, medium access control (MAC) protocols


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MAC Protocols: a taxonomyThree broad classes:

Channel Partitioningdivide channel into smaller “pieces” (time slots, frequency)allocate piece to node for exclusive use

“Taking turns”: Token-basedtightly coordinate shared access to avoid collisions

Random Accessallow collisions“recover” from collisions

Goal: efficient, fair, simple, decentralized


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Channel PartitioningMAC protocols. Eg: TDMA

TDMA: time division multiple accessAccess to channel in "rounds" Each station gets fixed length slot (length = pkt trans time) in each round Unused slots go idle Example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle


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“Taking Turns” MAC protocols - 1Channel partitioning MAC protocols:

share channel efficiently at high loadinefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!

Random access MAC protocolsefficient at low load: single node can fully utilize channelhigh load: collision overhead

“Taking turns” protocolslook for best of both worlds!


84

“Taking Turns” MAC protocols - 2Polling:

Master node “invites” slave nodes to transmit in turnRequest to Send, Clear to Send messagesConcerns:

polling overhead latencysingle point of failure (master)

Token passing:Control token passed from one node to next sequentially.Token messageConcerns:

token overhead latencysingle point of failure

(token)


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“Taking Turns” Protocols –3Reservation-based a.k.a Distributed Polling:

Time divided into slotsBegins with N short reservation slots

reservation slot time equal to channel end-end propagation delay station with message to send posts reservationreservation seen by all stations

After reservation slots, message transmissions ordered by known priority


86

Random Access ProtocolsAloha at University of Hawaii: Transmit whenever you likeWorst case utilization = 1/(2e) =18%CSMA: Carrier Sense Multiple Access Listen before you transmitCSMA/CD: CSMA with Collision DetectionListen while transmitting. Stop if you hear someone else.Ethernet uses CSMA/CD.Standardized by IEEE 802.3 committee.


87

10Base5 Ethernet Cabling RulesThick coaxLength of the cable is limited to 2.5 km, no more than 4 repeaters between stationsNo more than 500 m per segment ⇒ “10Base5”

2.5m500 mTransceiver

Repeater

Terminator


88

10Base5 Cabling Rules (Continued)No more than 2.5 m between stationsTransceiver cable limited to 50 m

2.5m500 mTransceiver

Repeater

Terminator


89

Inter-connection DevicesRepeater: Layer 1 (PHY) device that restores data and collision signals: a digital amplifier

Hub: Multi-port repeater + fault detectionNote: broadcast at layer 1

Bridge: Layer 2 (Data link) device connecting two or more collision domains.

Key: a bridge attempts to filter packets and forward them from one collision domain to the other.It snoops on passing packets and learns the interface where different hosts are situated, and builds a L2 forwarding tableMAC multicasts propagated throughout “extended LAN.”Note: Limited filtering intelligence and forwarding capabilities at layer 2


90

Interconnection Devices (Continued)Router: Network layer device. IP, IPX, AppleTalk. Interconnects broadcast domains.

Does not propagate MAC multicasts.

Switch:Key: has a switch fabric that allows parallel forwarding pathsLayer 2 switch: Multi-port bridge w/ fabricLayer 3 switch: Router w/ fabric and per-port ASICs

These are functions. Packaging varies.


91

Interconnection DevicesExtended LAN=Broadcast domainH H B H H

LAN=CollisionDomain

Router

Application Application

RouterBridge/SwitchRepeater/Hub

Gateway

NetworkDatalinkPhysical

TransportNetworkTransport

DatalinkPhysical


92

Ethernet (IEEE 802) Address Format

OUI(Organizationally Unique ID)

10111101

G/L bit(Global/Local)

G/I bit(Group/Individual)

48-bit flat address => no hierarchy to help forwardingHierarchy only for administrative/allocation purposesAssumes that all destinations are (logically) directly connected.

Address structure does not explicitly acknowledge indirect connectivity

=> Sophisticated filtering cannot be done!


93

Ethernet (IEEE 802) Address Format

G/L bit: administrativeGlobal: unique worldwide; assigned by IEEELocal: Software assigned

G/I: bit: multicastI: unicast addressG: multicast address. Eg: “To all bridges on this LAN”

10111101

G/L bit(Global/Local)

G/I bit(Group/Individual)

OUI(Organizationally Unique ID)


94

Ethernet & 802.3 Frame Format

Ethernet

❑ IEEE 802.3

Dest.Address

SourceAddress

IP IPX AppleTalk

Type

6 6 2

Size in bytesInfo CRC

4

Dest.Address

SourceAddress Length

6 6 2

IP IPX AppleTalk

LLC CRC

4

Pad

Length

Info

• Maximum Transmission Unit (MTU) = 1518 bytes• Minimum = 64 bytes (due to CSMA/CD issues)


95

Network/Transport Layer IssuesInter-networking: heterogeneity, scaleRoutingCongestion controlQuality of Service (QoS)


96


What is it ?“Connect many disparate physical networks and make them function as a coordinated unit … ” - Douglas ComerMany => scaleDisparate => heterogeneity

Result: Universal connectivity!The inter-network looks like one large switch, User interface is sub-network independent


97


Internetworking involves two fundamental problems: heterogeneity and scale

Concepts: Translation, overlays, address & name resolution, fragmentation: to handle heterogeneityHierarchical addressing, routing, naming, address allocation, congestion control: to handle scaling

Two broad approaches: circuit-switched and packet-switched


98

Scalable Forwarding, Structured Addresses

Address has structure which aids the forwarding process.Address assignment is done such that nodes which can be reached without resorting to L3 forwarding have the same prefix (network ID)A simple comparison of network ID of destination and current network (broadcast domain) identifies whether the destination is “directly” connected

I.e. Reachable through L2 forwarding onlyWithin L3 forwarding, further structure can aid hierarchical organization of routing domains (because routing algorithms have other scalability issues)

Network ID Host ID

Demarcator


99

Flat vs Structured AddressesFlat addresses: no structure in them to facilitate scalable routing

Eg: IEEE 802 LAN addressesHierarchical addresses:

Network part (prefix) and host partHelps identify direct or indirectly connected nodes


100

Internet Routing DriversTechnology and economic aspects:

Internet built out of cheap, unreliable components as an overlay on top of leased telephone infrastructure for WAN transport.

Cheaper components => fail more often => topology changes often => needs dynamic routing

Components (including end-systems) had computation capabilities.

Distributed algorithms can be implemented Cheap overlaid inter-networks => several entities could afford to leverage their existing (heterogeneous) LANs and leased lines to build inter-networks.

Led to multiple administrative “clouds” which needed to inter-connect for global communication.


101

Internet Routing Model2 key features:

Dynamic routingIntra- and Inter-AS routing, AS = locus of admin control

Internet organized as “autonomous systems” (AS).AS is internally connected

Interior Gateway Protocols (IGPs) within AS. Eg: RIP, OSPF, HELLO

Exterior Gateway Protocols (EGPs) for AS to AS routing. Eg: EGP, BGP-4


102

Intra-AS and Inter-AS routing

inter-AS, intra-ASrouting in

gateway A.c

network layerlink layer

physical layer

a

b

b

aaC

A

Bd

Gateways:•perform inter-ASrouting amongst themselves•perform intra-ASrouters with other routers in their AS

A.cA.a

C.bB.a

cb

c


103

Intra-AS and Inter-AS routing: Example

Host h2

a

b

b

aaC

A

Bd c

A.aA.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-ASrouting

between A and B

Intra-AS routingwithin AS B


104

Requirements for Intra-AS RoutingShould scale for the size of an AS.

Low end: 10s of routers (small enterprise)High end: 1000s of routers (large ISP)

Different requirements on routing convergence after topology changes

Low end: can tolerate some connectivity disruptionsHigh end: fast convergence essential to business (making money on transport)

Operational/Admin/Management (OAM) ComplexityLow end: simple, self-configuringHigh end: Self-configuring, but operator hooks for control

Traffic engineering capabilities: high end only


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Requirements for Inter-AS RoutingShould scale for the size of the global Internet.

Focus on reachability, not optimalityUse address aggregation techniques to minimize core routing table sizes and associated control trafficAt the same time, it should allow flexibility in topological structure (eg: don’t restrict to trees etc)

Allow policy-based routing between autonomous systemsPolicy refers to arbitrary preference among a menu of available options (based upon options’ attributes)In the case of routing, options include advertised AS-level routes to address prefixesFully distributed routing (as opposed to a signaled approach) is the only possibility.Extensible to meet the demands for newer policies.


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λiµλ

µi

If information about λ i , λ and µ is known in a central location where control of λ i or µ can be effected with zero time delays,

the congestion problem is solved!Unfortunately, we have incomplete info, require a distributed solution with time-varying time-delays

The Congestion Problem

λ1

λn

CapacityDemand

•Problem: demand outstrips available capacity


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Congestion: A Close-up View

knee – point after which throughput increases very slowlydelay increases fast

cliff – point after whichthroughput starts to decrease very fast to zero(congestion collapse)delay approaches infinity

Note (in an M/M/1 queue)delay = 1/(1 – utilization)

Load

Load

Thro

ughp

utD

elay

knee cliff

congestioncollapse

packetloss


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Congestion Control vs. Congestion Avoidance

Congestion control goalstay left of cliff

Congestion avoidance goalstay left of knee

Right of cliff: Congestion collapse

Load

Thro

ughp

ut

knee cliff

congestioncollapse


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Goals of Congestion ControlTo guarantee stable operation of packet networks

Sub-goal: avoid congestion collapse

To keep networks working in an efficient status Eg: high throughput, low loss, low delay, and high utilization

To provide fair allocations of network bandwidth among competing flows in steady state

For some value of “fair” ☺109


110

CC Techniques: Self-clockingPrPb

Ar

Ab

ReceiverSender

As

Implications of ack-clocking:

More batching of acks => bursty traffic

Less batching leads to a large fraction of Internet traffic being just acks (overhead)


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CC Techniques: Additive Increase/Multiplicative Decrease (AIMD) Policy

x0

x1

x2

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocation

x2

Assumption: decrease policy must (at minimum) reverse the load increase over-and-above efficiency line

Implication: decrease factor should be conservatively set to account for any congestion detection lags etc


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Quality of Service: What is it?

Multimedia applications: network audio and video

network provides application with level of performance needed for application to function.

QoS


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Fundamental QoS Problems

B

Scheduling DisciplineFIFO

B

In a FIFO service discipline, the performance assigned to one flow is convoluted with the arrivals of packets from all other flows!

Cant get QoS with a “free-for-all”Need to use new scheduling disciplines which provide “isolation” of performance from arrival rates of background traffic


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Fundamental QoS ProblemsConservation Law (Kleinrock): Σρ(i)Wq(i) = K

Irrespective of scheduling discipline chosen:

Average backlog (delay) is constantAverage bandwidth is constant

Zero-sum game => need to “set-aside” resources for premium services


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QoS Big Picture: Control/Data Planes

Internetwork or WANWorkstationRouter

Router

RouterWorkstation

Control Plane: Signaling + Admission Control orSLA (Contracting) + Provisioning/Traffic Engineering

Data Plane: Traffic conditioning (shaping, policing, markingetc) + Traffic Classification + Scheduling, Buffer management


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Internet RegulationFCC has largely had a hands-off policyEarly development of internet in part was influenced by high cost of telecom links

Packet switching developed as better multiplexing technologyCommon-carriage regulation has affected Inet:

Eg: modems were like fax machine for the common carrierUse of basic service (eg: telephony) to provide enhanced service (eg: internet access) => not subject to FCC or state jurisdiction

Led to community bulletin-boards, ISPs, value-added networks (frame-relay?)…Home-to-ISP treated as local call (even if crossed state-boundaries)

ILECs prohibited from offering inter-LATA servicesDSL viewed as basic service => must unbundle DSL to allow 3rd

parties to offer internet access over ILEC DSL


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Summary: List of Internet Problems

Basics: Direct/indirect connectivity, topologiesLink layer issues:

Framing, Error control, Flow control Multiple access & Ethernet:

Cabling, Pkt format, Switching, bridging vs routingInternetworking problems: Naming, addressing, Resolution, fragmentation, congestion control, traffic management, Reliability, Network Management


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Additional Reading

Internet Design Philosophy:Saltzer, Reed, Clark: "End-to-End arguments in System Design" Clark: "The Design Philosophy of the DARPA Internet Protocols": RFC 2775: Internet Transparency: In HTML

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/readings/e2e.pdf

http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp2001/readings/ccr-9501-clark.pdf

http://www.landfield.com/rfcs/rfc2775.html

Documents

Introduction to Telephony, Cable and Internet …koushik/shivkuma-teaching/sp2003/bon...Can’t run thick bundles! Instead, send many calls on the same wire Multiplexing (a.ka. Sharing)