EEL 5708

Networking

Lotzi Bölöni

Fall 2002

EEL 5708

Acknowledgements

• All the lecture slides were adopted from the slides of David Patterson (1998, 2001) and David E. Culler (2001), Copyright 1998-2002, University of California Berkeley

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Networks

• Facets people talk a lot about:– direct (point-to-point) vs. indirect (multi-hop)– topology (e.g., bus, ring, DAG)– routing algorithms– switching (aka multiplexing)– wiring (e.g., choice of media, copper, coax, fiber)

• What really matters:– latency– bandwidth– cost– reliability

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SAN: Storage vs. System

• Storage Area Network (SAN): A block I/O oriented network between application servers and storage

– Fibre Channel is an example

• Usually high bandwidth requirements, and less concerned about latency

– in 2001: 1 Gbit bandwidth and millisecond latency OK

• Commonly a dedicated network (that is, not connected to another network)

• May need to work gracefully when saturated

• Given larger block size, may have higher bit error rate (BER) requirement than LAN

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SAN: Storage vs. System

• System Area Network (SAN): A network aimed at connecting computers

• Aimed at High Bandwidth AND Low Latency.

– in 2001: > 1 Gbit bandwidth and ~ 10 microsecond

• May offer in order delivery of packets• Technologies:

– Infiniband (intended to replace PCI in computers)

– Myrinet

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LAN - Ethernet

• Local Area Network• Basically single protocol: Ethernet

– 10Mbps original standard IEEE 802.3– 100Mbps Fast Ethernet– 1G Gigabit Ethernet– 10 Gbit Ethernet?

• CSMA: Carrier Sense Multiple Access– Collision detection, random retransmission– Shared media line topology– Ethernet switches star topology

• Large packet• Broadcast mode!• Destination based routing. • Connectionless.

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WAN – ATM, SONET

• 155 Mbps and multiples• 53 byte packet (48 payload, 5 byte header)

– Because of voice QoS requirement

• Label switching• Virtual circuit• Switching is simple, but ATM protocols are

VERY complex.• SONET: packaging multiple ATM packets in a

single larger SONET packet– The current core of the internet is almost 100% Sonet

based.

• Many providers are thinking about replacing ATM/SONET with IP/MPLS.

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More Network Background

• Connection of 2 or more networks: Internetworking

• 3 cultures for 3 classes of networks– WAN: telecommunications, Internet– LAN: PC, workstations, servers cost– SAN: Clusters, RAID boxes: latency (System A.N.) or

bandwidth (Storage A.N.)

• Try for single terminology• Motivate the interconnection complexity

incrementally

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Network Performance Measures

• Overhead: latency of interface vs. Latency: network

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Universal Performance Metrics

Sender

Receiver

SenderOverhead

Transmission time(size ÷ bandwidth)

Transmission time(size ÷ bandwidth)

Time ofFlight

ReceiverOverhead

Transport Latency

Total Latency = Sender Overhead + Time of Flight + Message Size ÷ BW + Receiver Overhead

Total Latency

(processorbusy)

(processorbusy)

Includes header/trailer in BW calculation?

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Total Latency Example• 1000 Mbit/sec., sending overhead of 80 µsec &

receiving overhead of 100 µsec. • a 10000 byte message (including the header), allows

10000 bytes in a single message• 2 situations: distance 100 m vs. 1000 km• Speed of light ~ 300,000 km/sec (50% in the media)

• Latency0.01km = 80 + 0.01km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 260 µsec

• Latency0.5km = 80 + 0.5km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 263 µsec

• Latency1000km = 80 + 1000 km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 6931

• Long time of flight => complex WAN protocol

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Interconnect Issues

• Performance Measures• Network Media

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Network Media

Copper, 1mm thin, twisted to avoidantenna effect (telephone)"Cat 5" is 4 twisted pairs in bundle

Used by cable companies: high BW, good noise immunity

Light: 3 parts are cable, light source, light detector.Note fiber is unidirectional; need 2 for full duplex

Twisted Pair:

Coaxial Cable:

Copper coreInsulator

Braided outer conductor

Plastic Covering

Fiber Optics

Transmitter– L.E.D– Laser Diode

Receiver– Photodiode

lightsource Silica core

Total internalreflection

Cladding

Cladding

Buffer

Buffer

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Fiber• Multimode fiber: ~ 62.5 micron diameter vs. the 1.3 micron

wavelength of infrared light. Since wider it has more dispersion problems, limiting its length at 1000 Mbits/s for 0.1 km, and 1-3 km at 100 Mbits/s. Uses LED as light

• Single mode fiber: "single wavelength" fiber (8-9 microns) uses laser diodes, 1-5 Gbits/s for 100s kms

– Less reliable and more expensive, and restrictions on bending

– Cost, bandwidth, and distance of single-mode fiber affected by power of the light source, the sensitivity of the light detector, and the attenuation rate (loss of optical signal strength as light passes through the fiber) per kilometer of the fiber cable.

– Typically glass fiber, since has better characteristics than the less expensive plastic fiber

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Wave Division Multiplexing Fiber

• Send N independent streams on single fiber!

• Just use different wavelengths to send and demultiplex at receiver

• WDM in 2000: 40 Gbit/s using 8 wavelengths

• Plan to go to 80 wavelengths => 400 Gbit/s!• A figure of merit: BW* max distance

(Gbit-km/sec)• 10X/4 years, or 1.8X per year

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Compare Media• Assume 40 2.5" disks, each 25 GB, Move 1 km• Compare Cat 5 (100 Mbit/s), Multimode fiber (1000

Mbit/s), single mode (2500 Mbit/s), and car• Cat 5: 1000 x 1024 x 8 Mb / 100 Mb/s = 23 hrs• MM: 1000 x 1024 x 8 Mb / 1000 Mb/s = 2.3 hrs• SM: 1000 x 1024 x 8 Mb / 2500 Mb/s = 0.9 hrs• Car: 5 min + 1 km / 50 kph + 10 min = 0.25 hrs• Car of disks = high BW media

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Interconnect Issues

• Performance Measures• Network Media• Connecting Multiple Computers

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Connecting Multiple Computers

• Shared Media vs. Switched: pairs communicate at same time: “point-to-point” connections

• Aggregate BW in switched network is many times shared

– point-to-point faster since no arbitration, simpler interface

• Arbitration in Shared network?– Central arbiter for LAN?– Listen to check if being used (“Carrier

Sensing”)– Listen to check if collision

(“Collision Detection”)– Random resend to avoid repeated

collisions; not fair arbitration; – OK if low utilization

(A. K. A. data switching interchanges, multistageinterconnection networks,interface message processors)

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Connection-Based vs. Connectionless

• Telephone: operator sets up connection between the caller and the receiver

– Once the connection is established, conversation can continue for hours

• Share transmission lines over long distances by using switches to multiplex several conversations on the same lines

– “Time division multiplexing” divide B/W transmission line into a fixed number of slots, with each slot assigned to a conversation

• Problem: lines busy based on number of conversations, not amount of information sent

• Advantage: reserved bandwidth

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Connection-Based vs. Connectionless

• Connectionless: every package of information must have an address => packets

– Each package is routed to its destination by looking at its address

– Analogy, the postal system (sending a letter)– also called “Statistical multiplexing”– Note: “Split phase buses” are sending packets

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Routing Messages

• Shared Media– Broadcast to everyone

• Switched Media needs real routing. Options:

– Source-based routing: message specifies path to the destination (changes of direction)

– Virtual Circuit: circuit established from source to destination, message picks the circuit to follow

– Destination-based routing: message specifies destination, switch must pick the path

» deterministic: always follow same path» adaptive: pick different paths to avoid

congestion, failures» Randomized routing: pick between several

good paths to balance network load

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• mesh: dimension-order routing

– (x1, y1) -> (x2, y2)

– first x = x2 - x1,

– then y = y2 - y1,

• hypercube: edge-cube routing

– X = xox1x2 . . .xn -> Y = yoy1y2 . . .yn

– R = X xor Y– Traverse dimensions of differing

address in order

• tree: common ancestor• Deadlock free?

Deterministic Routing Examples

001

000

101

100

010 110

111011

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Store and Forward vs. Cut-Through

• Store-and-forward policy: each switch waits for the full packet to arrive in switch before sending to the next switch (good for WAN)

• Cut-through routing or worm hole routing: switch examines the header, decides where to send the message, and then starts forwarding it immediately

– In worm hole routing, when head of message is blocked, message stays strung out over the network, potentially blocking other messages (needs only buffer the piece of the packet that is sent between switches).

– Cut through routing lets the tail continue when head is blocked, accordioning the whole message into a single switch. (Requires a buffer large enough to hold the largest packet).

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Cut-Through vs. Store and Forward

• Advantage– Latency reduces from function of:

number of intermediate switches X by the size of the packet

to time for 1st part of the packet to negotiate the switches + the packet size ÷ interconnect BW

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Congestion Control• Packet switched networks do not reserve

bandwidth; this leads to contention (connection based limits input)

• Solution: prevent packets from entering until contention is reduced (e.g., freeway on-ramp metering lights)

• Options:– Packet discarding: If packet arrives at switch and no room in

buffer, packet is discarded (e.g., UDP)– Flow control: between pairs of receivers and senders;

use feedback to tell sender when allowed to send next packet» Back-pressure: separate wires to tell to stop» Window: give original sender right to send N packets before

getting permission to send more; overlaps latency of interconnection with overhead to send & receive packet (e.g., TCP), adjustable window

– Choke packets: aka “rate-based”; Each packet received by busy switch in warning state sent back to the source via choke packet. Source reduces traffic to that destination by a fixed % (e.g., ATM)

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Protocols: HW/SW Interface

• Internetworking: allows computers on independent and incompatible networks to communicate reliably and efficiently;

– Enabling technologies: SW standards that allow reliable communications without reliable networks

– Hierarchy of SW layers, giving each layer responsibility for portion of overall communications task, called protocol families or protocol suites

• Transmission Control Protocol/Internet Protocol (TCP/IP)

– This protocol family is the basis of the Internet– IP makes best effort to deliver; TCP guarantees delivery– TCP/IP used even when communicating locally: NFS uses

IP even though communicating across homogeneous LAN

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Protocol Family Concept

Message TH

Message

TH

Message TH

Message

Message TH Message TH TH

Actual

Actual Actual

Actual

Physical

Logical

Logical

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Protocol Family Concept

• Key to protocol families is that communication occurs logically at the same level of the protocol, called peer-to-peer,

• but is implemented via services at the next lower level• Encapsulation: carry higher level information within

lower level “envelope”• Fragmentation: break packet into multiple smaller

packets and reassemble• Danger is each level increases latency if implemented as

hierarchy (e.g., multiple check sums)

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Message

TCP/IP packet, Ethernet packet, protocols

• Application sends message

TCP data

TCP Header

IP Header

IP DataEH

Ethernet Hdr

Ethernet Hdr

• TCP breaks into 64KB segments, adds 20B header

• IP adds 20B header, sends to network

• If Ethernet, broken into 1500B packets with headers, trailers (24B)

• All Headers, trailers have length field, destination, ...

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Example Networks

• Ethernet: shared media 10 Mbit/s proposed in 1978, carrier sensing with expotential backoff on collision detection

• 15 years with no improvement; higher BW?• Multiple Ethernets with devices to allow

Ethernets to operate in parallel!• 10 Mbit Ethernet successors?

– FDDI: shared media (too late)– ATM (too late?)– Switched Ethernet– 100 Mbit Ethernet (Fast Ethernet)– Gigabit Ethernet– 10 Gigabit Ethernet in 2002?

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Connecting Networks

• Bridges: connect LANs together, passing traffic from one side to another depending on the addresses in the packet.

– operate at the Ethernet protocol level– usually simpler and cheaper than routers

• Routers or Gateways: these devices connect LANs to WANs or WANs to WANs and resolve incompatible addressing.

– Generally slower than bridges, they operate at the internetworking protocol (IP) level

– Routers divide the interconnect into separate smaller subnets, which simplifies manageability and improves security

• Cisco is major supplier; basically special purpose computers

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Comparing NetworksSAN LAN WAN

FC-AL Infini-band

10 MbEthernet

100 MbEthernet

1000 MbEthernet

ATM

Length(meters)

30/1000 17/100 500/2500 200 100

Datalines

2 1, 4, 12 1 1 4/1 1

Clock(MHz)

1000 2500 10 100 1000 155/622

Switch? Opt. Yes Optional Opt. Yes Yes

Nodes <=127 ~1000 <=254 <=254 <=254 ~10000

Material Copper/ fiber

Copper/fiber

Copper Copper Copper/fiber

Copper/fiber

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Comparing NetworksSAN LAN WAN

FC-AL Infini-band

10 MbEthernet

100 MbEthernet

1000 MbEthernet

ATM

Switch? Opt. Yes Optional Opt. Yes Yes

BisectionBW(Mbits/sec)

800sharedor 800 xswitchports

(2000 -24000)xswitchports

10sharedor 10 xswitchports

100sharedor 100 xswitchports

1000 xswitchports

155 xswitchports

Peak linkBW(Mbits/sec)

800 2000,8000,24000

10 100 1000 155/622

Topology Ring orStar

Star Line orStar

Line orStar

Star Star

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Comparing NetworksSAN LAN WAN

FC-AL Infiniband 10 MbEthernet

100 MbEthernet

1000 MbEthernet

ATM

Connec-tionless?

Yes Yes Yes Yes Yes No

Store &forward?

No No No No No Yes

Conges-tioncontrol

Credit-based

Back-pressure

Carriersense

Carriersense

Carriersense

Creditbased

Standard ANSITaskGroupX3T11

InfinibandTradeAssocia-tion

IEEE802.3

IEEE802.3

IEEE802.3ab-1999

ATMForum

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Practical Issues for Interconnection Networks

• Connectivity: max number of machines affects complexity of network and protocols since protocols must target largest size

• Connection Network Interface to computer– Where in bus hierarchy? Memory bus? Fast I/O bus?

Slow I/O bus? (Ethernet to Fast I/O bus, Inifiband to Memory bus since it is the Fast I/O bus)

– SW Interface: does software need to flush caches for consistency of sends or receives?

– Programmed I/O vs. DMA? Is NIC in uncachable address space?

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Practical Issues for Interconnection Networks

• Standardization advantages:– low cost (components used repeatedly)– stability (many suppliers to chose from)

• Standardization disadvantages:– Time for committees to agree– When to standardize?

» Before anything built? => Committee does design?» Too early suppresses innovation

• Reliability (vs. availability) of interconnect

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Practical Issues

Interconnection SAN LAN WAN

Example Infiniband Ethernet ATM

Standard Yes Yes Yes

Fault Tolerance? Yes Yes Yes

Hot Insert? Yes Yes Yes

• Standards: required for WAN, LAN, and likely SAN!

• Fault Tolerance: Can nodes fail and still deliver messages to other nodes?

• Hot Insert: If the interconnection can survive a failure, can it also continue operation while a new node is added to the interconnection?

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Cross-Cutting Issues for Networking

• Efficient Interface to Memory Hierarchy vs. to Network– SPEC ratings => fast to memory hierarchy– Writes go via write buffer, reads via L1

and L2 caches• Example: 40 MHz SPARCStation(SS)-2 vs 50

MHz SS-20, no L2$ vs 50 MHz SS-20 with L2$ I/O bus latency; different generations

• SS-2: combined memory, I/O bus => 200 ns• SS-20, no L2$: 2 busses +300ns => 500ns• SS-20, w L2$: cache miss+500ns => 1000ns

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Crosscutting: Smart Switch vs. Smart Network

Interface CardLess Intelligent More Intelligent

Switch

Small Ethernet

Myrinet

Infiniband

Large Ethernet

NIC

Ethernet

Infiniband Target Channel Adapter

Myrinet

Inifiband Host Channel Adapter

•Inexpensive NIC => Ethernet standard in all computers•Inexpensive switch => Ethernet used in home networks

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Summary: Networking

• Protocols allow heterogeneous networking– Protocols allow operation in the presence of failures– Internetworking protocols used as LAN protocols

=> large overhead for LAN

• Integrated circuit revolutionizing networks as well as processors

– Switch is a specialized computer– Faster networks and slow overheads violate of Amdahl’s

Law

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Clusters

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Cluster

• LAN switches => high network bandwidth and scaling was available from off the shelf components

• 2001 Cluster = collection of independent computers using switched network to provide a common service

• Many mainframe applications run more "loosely coupled" machines than shared memory machines (next chapter/week)

– databases, file servers, Web servers, simulations, and multiprogramming/batch processing

– Often need to be highly available, requiring error tolerance and repairability

– Often need to scale

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Cluster Drawbacks

• Cost of administering a cluster of N machines ~ administering N independent machines vs. cost of administering a shared address space N processors multiprocessor ~ administering 1 big machine

• Clusters usually connected using I/O bus, whereas multiprocessors usually connected on memory bus

• Cluster of N machines has N independent memories and N copies of OS, but a shared address multi-processor allows 1 program to use almost all memory

– DRAM prices has made memory costs so low that this multiprocessor advantage is much less important in 2001

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Cluster Advantages

• Error isolation: separate address space limits contamination of error

• Repair: Easier to replace a machine without bringing down the system than in an shared memory multiprocessor

• Scale: easier to expand the system without bringing down the application that runs on top of the cluster

• Cost: Large scale machine has low volume => fewer machines to spread development costs vs. leverage high volume off-the-shelf switches and computers

• Amazon, AOL, Google, Hotmail, Inktomi, WebTV, and Yahoo rely on clusters of PCs to provide services used by millions of people every day

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Addressing Cluster Weaknesses

• Network performance: SAN, especially Infiniband, may tie cluster closer to memory

• Maintenance: separate of long term storage and computation

• Computation maintenance:– Clones of identical PCs– 3 steps: reboot, reinstall OS, recycle– At $1000/PC, cheaper to discard than to figure out what

is wrong and repair it?

• Storage maintenance:– If separate storage servers or file servers, cluster is no

worse?

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Clusters and TPC Benchmarks

• “Shared Nothing” database (not memory, not disks) is a match to cluster

• 2/2001: Top 10 TPC performance 6/10 are clusters (4 / top 5)

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Putting it all together: Google

• Google: search engine that scales at growth Internet growth rates

• Search engines: 24x7 availability• Google 12/2000: 70M queries per day, or

AVERAGE of 800 queries/sec all day• Response time goal: < 1/2 sec for search• Google crawls WWW and puts up new index

every 4 weeks• Stores local copy of text of pages of WWW

(snippet as well as cached copy of page)• 3 collocation sites (2 CA + 1 Virginia)• 6000 PCs, 12000 disks: almost 1 petabyte!

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Hardware Infrastructure

• VME rack 19 in. wide, 6 feet tall, 30 inches deep

• Per side: 40 1 Rack Unit (RU) PCs +1 HP Ethernet switch (4 RU): Each blade can contain 8 100-Mbit/s EN or a single 1-Gbit Ethernet interface

• Front+back => 80 PCs + 2 EN switches/rack

• Each rack connects to 2 128 1-Gbit/s EN switches

• Dec 2000: 40 racks at most recent site

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Google PCs• 2 IDE drives, 256 MB of SDRAM, modest Intel

microprocessor, a PC mother-board, 1 power supply and a few fans.

• Each PC runs the Linux operating system• Buy over time, so upgrade components:

populated between March and November 2000

– microprocessors: 533 MHz Celeron to an 800 MHz Pentium III,

– disks: capacity between 40 and 80 GB, speed 5400 to 7200 RPM

– bus speed is either 100 or 133 MH– Cost: ~ $1300 to $1700 per PC

• PC operates at about 55 Watts• Rack => 4500 Watts , 60 amps

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Reliability

• For 6000 PCs, 12000s, 200 EN switches• ~ 20 PCs will need to be rebooted/day• ~ 2 PCs/day hardware failure, or 2%-3% / year

– 5% due to problems with motherboard, power supply, and connectors– 30% DRAM: bits change + errors in transmission (100 MHz)– 30% Disks fail– 30% Disks go very slow (10%-3% expected BW)

• 200 EN switches, 2-3 fail in 2 years• 6 Foundry switches: none failed, but 2-3 of 96 blades of

switches have failed (16 blades/switch)• Collocation site reliability:

– 1 power failure,1 network outage per year per site– Bathtub for occupancy

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Google Performance: Serving

• How big is a page returned by Google? ~16KB

• Average bandwidth to serve searches

70,000,000/day x 16,750 B x 8 bits/B

24 x 60 x 60

=9,378,880 Mbits/86,400 secs

= 108 Mbit/s

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Google Performance: Crawling

• How big is a text of a WWW page? ~4000B• 1 Billion pages searched • Assume 7 days to crawl• Average bandwidth to crawl

1,000,000,000/pages x 4000 B x 8 bits/B

24 x 60 x 60 x 7

=32,000,000 Mbits/604,800 secs

= 59 Mbit/s

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Google Performance: Replicating Index

• How big is Google index? ~5 TB • Assume 7 days to replicate to 2 sites,

implies BW to send + BW to receive• Average bandwidth to replicate new index

2 x 2 x 5,000,000 MB x 8 bits/B

24 x 60 x 60 x 7

=160,000,000 Mbits/604,800 secs

= 260 Mbit/s

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Colocation Sites• Allow scalable space, power, cooling and

network bandwidth plus provide physical security

• charge about $500 to $750 per Mbit/sec/month – if your continuous use measures 1- 2 Gbits/second

to $1500 to $2000 per Mbit/sec/month – if your continuous use measures 1-10 Mbits/second

• Rack space: costs $800 -$1200/month, and drops by 20% if > 75 to 100 racks (1 20 amp circuit)

– Each additional 20 amp circuit per rack costs another $200 to $400 per month

• PG&E: 12 megawatts of power, 100,000 sq. ft./building, 10 sq. ft./rack => 1000 watts/rack

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Google Performance: Total

• Serving pages: 108 Mbit/sec/month• Crawling: 59 Mbit/sec/week, 15 Mbit/s/month• Replicating: 260 Mbit/sec/week, 65

Mb/s/month• Total: roughly 200 Mbit/sec/month• Google’s Collocation sites have OC48

(2488 Mbit/sec) link to Internet• Bandwidth cost per month?

~$150,000 to $200,000• 1/2 BW grows at 20%/month

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Google Costs

• Collocation costs: 40 racks @ $1000 per month + $500 per month for extra circuits

= ~$60,000 per site, * 3 sites~$180,000 for space• Machine costs:• Rack = $2k + 80 * $1500/pc + 2 * $1500/EN

= ~$125k• 40 racks + 2 Foundry switches @$100,000

= ~$5M• 3 sites = $15M• Cost today is $10,000 to $15,000 per TB

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Comparing Storage Costs: 1/2001

• Google site, including 3200 processors and 0.8 TB of DRAM, 500 TB (40 racks) $10k - $15k/ TB

• Compaq Cluster with 192 processors, 0.2 TB of DRAM, 45 TB of SCSI Disks (17+ racks) $115k/TB (TPC-C)

• HP 9000 Superdome: 48 processors, 0.25 TB DRAM, 19 TB of SCSI disk =(23+ racks) $360k/TB (TPC-C)

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Putting It All Together: Cell Phones

• 1999 280M handsets sold; 2001 500M

• Radio steps/components: Receive/transmit

– Antenna– Amplifier– Mixer– Filter– Demodulator– Decoder

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Putting It All Together: Cell Phones

• about 10 chips in 2000, which should shrink, but likely separate MPU and DSP

• Emphasis on energy efficiencyFrom “How Stuff Works” on cell phones: www.howstuffworks.com

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Cell phone steps (protocol)

1. Find a cell• Scans full BW to find stronger signal every 7 secs

2. Local switching office registers call• records phone number, cell phone serial number,

assigns channel• sends special tone to phone, which cell acks if correct• Cell times out after 5 sec if doesn't get supervisory

tone

3. Communicate at 9600 b/s digitally (modem)• Old style: message repeated 5 times• AMPS had 2 power levels depending on distance (0.6W

and 3W)

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Frequency Division Multiple Access (FDMA)

• FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth

• !st generation analog

From “How Stuff Works” on cell phones: www.howstuffworks.com

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Time Division Multiple Access (TDMA)

• a narrow band that is 30 kHz wide and 6.7 ms long is split time-wise into 3 time slots.

• Each conversation gets the radio for 1/3 of time.

• Possible because voice data converted to digital information is compressed so

• Therefore, TDMA has 3 times capacity of analog

• GSM implements TDMA in a somewhat different and incompatible way from US (IS-136); also encrypts the call

From “How Stuff Works” on cell phones: www.howstuffworks.com

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Code Division Multiple Access (CDMA)

• CDMA, after digitizing data, spreads it out over the entire bandwidth it has available.

• Multiple calls are overlaid over each other on the channel, with each assigned a unique sequence code.

• CDMA is a form of spread spectrum; All the users transmit in the same wide-band chunk of spectrum.

• Each user's signal is spread over the entire bandwidth by a unique spreading code. same unique code is used to recover the signal.

• GPS for time stamp. Between 8 and 10 separate calls

space as 1 analog call

From “How Stuff Works” on cell phones: www.howstuffworks.com

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Cell Phone Towers

From “How Stuff Works” on cell phones: www.howstuffworks.com