2. EE898 Lec 4.1 11/12/2004 EE898.02 Architecture of Digital Systems Lecture 4 Interconnection Networks and Clusters Prof. Seok-Bum Ko

3. EE898 Lec 4.2 11/12/2004 Networks Goal: Communication between computers Eventual Goal: treat collection of computers as if one big computer, distributed resource sharing Theme: Different computers must agree on many things –Overriding importance of standards and protocols –Error tolerance critical as well Warning: Terminology-rich environment

4. EE898 Lec 4.3 11/12/2004 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

5. EE898 Lec 4.4 11/12/2004 Interconnections (Networks) Examples (Figure 8.1, page 788): –Wide Area Network (ATM): 100-1000s nodes; ~ 5,000 kilometers –Local Area Networks (Ethernet): 10-1000 nodes; ~ 1-2 kilometers –System/Storage Area Networks (FC-AL): 10-100s nodes; ~ 0.025 to 0.1 kilometers per link a.k.a. network, communication subnet a.k.a. end systems, hosts Interconnection Network

6. EE898 Lec 4.5 11/12/2004 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

7. EE898 Lec 4.6 11/12/2004 SAN: Storage vs. System System Area Network (SAN): A network aimed at connecting computers –Myrinet is an example Aimed at High Bandwidth AND Low Latency. –in 2001: > 1 Gbit bandwidth and ~ 10 microsecond May offer in order delivery of packets Given larger block size, may have higher bit error rate (BER) requirement than LAN

8. EE898 Lec 4.7 11/12/2004 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

9. EE898 Lec 4.8 11/12/2004 ABCs of Networks Starting Point: Send bits between 2 computers Queue (FIFO) on each end Information sent called a “message” Can send both ways (“Full Duplex”) Rules for communication? “protocol” –Inside a computer: »Loads/Stores: Request (Address) & Response (Data) »Need Request & Response signaling

10. EE898 Lec 4.9 11/12/2004 A Simple Example What is the format of message? –Fixed? Number bytes? Request/ Response Address/Data 1 bit 32 bits 0: Please send data from Address 1: Packet contains data corresponding to request Header/Trailer: information to deliver a message Payload: data in message (1 word above)

11. EE898 Lec 4.10 11/12/2004 Questions About Simple Example What if more than 2 computers want to communicate? –Need computer “address field” (destination) in packet What if packet is garbled in transit? –Add “error detection field” in packet (e.g., Cyclic Redundancy Chk) What if packet is lost? –More “elaborate protocols” to detect loss (e.g., NAK, ARQ, time outs) What if multiple processes/machine? –Queue per process to provide protection Simple questions such as these lead to more complex protocols and packet formats => complexity

12. EE898 Lec 4.11 11/12/2004 A Simple Example Revised What is the format of packet? –Fixed? Number bytes? Request/ Response Address/Data 2 bits32 bits 00: Request—Please send data from Address 01: Reply—Packet contains data corresponding to request 10: Acknowledge request 11: Acknowledge reply 4 bits CRC

13. EE898 Lec 4.12 11/12/2004 Software to Send and Receive SW Send steps 1: Application copies data to OS buffer 2: OS calculates checksum, starts timer 3: OS sends data to network interface HW and says start SW Receive steps 3: OS copies data from network interface HW to OS buffer 2: OS calculates checksum, if matches send ACK; if not, deletes message (sender resends when timer expires) 1: If OK, OS copies data to user address space and signals application to continue Sequence of steps for SW: protocol –Example similar to UDP/IP protocol in UNIX

14. EE898 Lec 4.13 11/12/2004 Network Performance Measures Overhead: latency of interface vs. Latency: network

15. EE898 Lec 4.14 11/12/2004 Universal Performance Metrics Sender Receiver Sender Overhead Transmission time (size ÷ bandwidth) Transmission time (size ÷ bandwidth) Time of Flight Receiver Overhead Transport Latency Total Latency = Sender Overhead + Time of Flight + Message Size ÷ BW + Receiver Overhead Total Latency (processor busy) (processor busy) Includes header/trailer in BW calculation?

16. EE898 Lec 4.15 11/12/2004 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 Latency 0.01km = 80 + 0.01km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 260 µsec Latency 0.5km = 80 + 0.5km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 263 µsec Latency 1000km = 80 + 1000 km / (50% x 300,000) + 10000 x 8 / 1000 + 100 = 6931 Long time of flight => complex WAN protocol

17. EE898 Lec 4.16 11/12/2004 Universal Metrics Apply recursively to all levels of system inside a chip, between chips on a board, between computers in a cluster, … Look at WAN v. LAN v. SAN

18. EE898 Lec 4.17 11/12/2004 Simplified Latency Model Total Latency = Overhead + Message Size / BW Overhead = Sender Overhead + Time of Flight + Receiver Overhead Effective BW = Message Size / Total Latency

19. EE898 Lec 4.18 11/12/2004 Overhead, BW, Size Msg Size Delivered BW How big are real messages?

20. EE898 Lec 4.19 11/12/2004 Measurement: Sizes of Message for NFS 95% Msgs, 30% bytes for packets ~ 200 bytes > 50% data transferred in packets = 8KB

21. EE898 Lec 4.20 11/12/2004 Interconnect Issues Performance Measures Network Media

22. EE898 Lec 4.21 11/12/2004 Network Media Copper, 1mm think, twisted to avoid attenna 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 core Insulator Braided outer conductor Plastic Covering Fiber Optics Transmitter – L.E.D – Laser Diode Receiver – Photodiode light source Silica core Total internal reflection Cladding Buffer

23. EE898 Lec 4.22 11/12/2004 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

24. EE898 Lec 4.23 11/12/2004 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!

25. EE898 Lec 4.24 11/12/2004 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 = 0.3 hrs Car of disks = high BW media

26. EE898 Lec 4.25 11/12/2004 Interconnect Issues Performance Measures Network Media Connecting Multiple Computers

27. EE898 Lec 4.26 11/12/2004 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, multistage interconnection networks, interface message processors)

28. EE898 Lec 4.27 11/12/2004 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

29. EE898 Lec 4.28 11/12/2004 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

30. EE898 Lec 4.29 11/12/2004 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

31. EE898 Lec 4.30 11/12/2004 mesh: dimension-order routing –(x 1, y 1 ) -> (x 2, y 2 ) –first  x = x 2 - x 1, –then  y = y 2 - y 1, hypercube: edge-cube routing –X = x o x 1 x 2... x n -> Y = y o y 1 y 2... y n –R = X xor Y –Traverse dimensions of differing address in order tree: common ancestor Deterministic Routing Examples 001 000 101 100 010 110 111 011

32. EE898 Lec 4.31 11/12/2004 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, compressing the strung-out message into a single switch. (Requires a buffer large enough to hold the largest packet).

33. EE898 Lec 4.32 11/12/2004 Cut-Through vs. Store and Forward Advantage –Latency reduces from a function of: # of intermediate switches × by the size of the packet to the time for 1st part of the packet to negotiate the switches + transmission time (=the packet size ÷ interconnect BW)

34. EE898 Lec 4.33 11/12/2004 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)

35. EE898 Lec 4.34 11/12/2004 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

36. EE898 Lec 4.35 11/12/2004 Protocol Family Concept Message TH TH TH TH THTH Actual Physical Logical

37. EE898 Lec 4.36 11/12/2004 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)

38. EE898 Lec 4.37 11/12/2004 Message TCP/IP packet, Ethernet packet, protocols Application sends message TCP data TCP Header IP Header IP Data EH 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,...

39. EE898 Lec 4.38 11/12/2004 Example Networks Ethernet: shared media 10 Mbit/s proposed in 1978, carrier sensing with exponential backoff on collision detection 15 years with no improvement; higher BW? Multiple Ethernets with devices to allow Ehternets 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

40. EE898 Lec 4.39 11/12/2004 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

41. EE898 Lec 4.40 11/12/2004 Comparing Networks

42. EE898 Lec 4.41 11/12/2004 Comparing Networks

43. EE898 Lec 4.42 11/12/2004 Comparing Networks

44. EE898 Lec 4.43 11/12/2004 Packet Formats See Fig 8.20 on page 826

45. EE898 Lec 4.44 11/12/2004 Wireless Networks Media can be air as well as glass or copper Radio wave is electromagnetic wave propagated by an antenna Radio waves are modulated: sound signal superimposed on stronger radio wave which carries sound signal, called carrier signal Radio waves have a wavelength or frequency: measure either length of wave or number of waves per second (MHz): long waves => low frequencies, short waves => high frequencies Tuning to different frequencies => radio receiver pick up a signal. –FM radio stations transmit on band of 88 MHz to 108 MHz using frequency modulations (FM) to record the sound signal

46. EE898 Lec 4.45 11/12/2004 Issues in Wireless Wireless often => mobile => network must rearrange itself dynamically Subject to jamming and eavesdropping –No physical tape –Cannot detect interception Power –devices tend to be battery powered –antennas radiate power to communicate and little of it reaches the receiver As a result, raw bit error rates are typically a thousand to a million times higher than copper wire

47. EE898 Lec 4.46 11/12/2004 Reliability of Wires Transmission bit error rate (BER) of wireless link determined by received signal power, noise due to interference caused by the receiver hardware, interference from other sources, and characteristics of the channel –Path loss: power to overcome interference –Shadow fading: blocked by objects (walls, buildings) –Multipath fading: interference between multiple version of signals arriving different times –Interference: reuse of frequency or from adjacent channels

48. EE898 Lec 4.47 11/12/2004 2 Wireless Architectures Base-station architectures –Connected by land lines for longer distance communication, and the mobile units communicate only with a single local base station –More reliable since 1-hop from land lines –Example: cell phones Peer-to-peer architectures –Allow mobile units to communicate with each other, and messages hop from one unit to the next until delivered to the desired unit –More reconfigurable

49. EE898 Lec 4.48 11/12/2004 Cellular Telephony Exploit exponential path loss to reuse same frequency at spatially separated locations, thereby greatly increasing customers served Divide region into nonoverlaping hexagonal cells (2-10 mi. diameter) which use different frequencies if nearby, reusing a frequency when cells far apart so that mutual interference OK Intersection of three hexagonal cells is a base station with transmitters and antennas Handset selects a cell based on signal strength and then picks an unused radio channel To properly bill for cellular calls, each cellular phone handset has an electronic serial number

50. EE898 Lec 4.49 11/12/2004 Cellular Telephony II Original analog design frequencies set for each direction: pair called a channel –869.04 to 893.97 MHz, called the forward path –824.04 MHz to 848.97 MHz, called the reverse path –Cells might have had between 4 and 80 channels Several digital successors: –Code division multiple access (CDMA) uses a wider radio frequency band –time division multiple access (TDMA) –global system for mobile communication (GSM) –International Mobile Telephony 2000 (IMT-2000) which is based primarily on two competing versions of CDMA and one TDMA, called Third Generation (3G)

51. EE898 Lec 4.50 11/12/2004 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, Infiniband 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 uncacheable address space?

52. EE898 Lec 4.51 11/12/2004 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

53. EE898 Lec 4.52 11/12/2004 Practical Issues InterconnectionSANLANWAN ExampleInfinibandEthernetATM StandardYesYesYes Fault Tolerance?YesYesYes Hot Insert?YesYesYes 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?

54. EE898 Lec 4.53 11/12/2004 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

55. EE898 Lec 4.54 11/12/2004 Crosscutting: Smart Switch vs. Smart Network Interface Card Less IntelligentMore Intelligent Switch Small Ethernet Myrinet Infiniband Large Ethernet NIC Ethernet Infiniband Target Channel Adapter Myrinet Infiniband Host Channel Adapter Inexpensive NIC => Ethernet standard in all computers Inexpensive switch => Ethernet used in home networks

56. EE898 Lec 4.55 11/12/2004 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 lecture) – 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

57. EE898 Lec 4.56 11/12/2004 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

58. EE898 Lec 4.57 11/12/2004 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

59. EE898 Lec 4.58 11/12/2004 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?

60. EE898 Lec 4.59 11/12/2004 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 Processors and 12000 disks

61. EE898 Lec 4.60 11/12/2004 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

62. EE898 Lec 4.61 11/12/2004 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

63. EE898 Lec 4.62 11/12/2004 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

64. EE898 Lec 4.63 11/12/2004 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

65. EE898 Lec 4.64 11/12/2004 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

66. EE898 Lec 4.65 11/12/2004 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

67. EE898 Lec 4.66 11/12/2004 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

68. EE898 Lec 4.67 11/12/2004 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

69. EE898 Lec 4.68 11/12/2004 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

70. EE898 Lec 4.69 11/12/2004 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)

71. EE898 Lec 4.70 11/12/2004 Putting It All Together: Cell Phones 1999 280M handsets sold; 2001 500M Radio steps/components: Receive/transmit –Antenna –Amplifier –Mixer –Filter –Demodulator –Decoder

72. EE898 Lec 4.71 11/12/2004 Putting It All Together: Cell Phones about 10 chips in 2000, which should shrink, but likely separate MPU and DSP Emphasis on energy efficiency

73. EE898 Lec 4.72 11/12/2004 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)

74. EE898 Lec 4.73 11/12/2004 Frequency Division Multiple Access (FDMA) FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth 1st generation analog

75. EE898 Lec 4.74 11/12/2004 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

76. EE898 Lec 4.75 11/12/2004 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

77. EE898 Lec 4.76 11/12/2004 Cell Phone Towers