Transcript Lecture 1: Course Introduction and Overview

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
Networking
• Issues:
–
–
–
–
–
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
Interconnections (Networks)
• Examples (see Figure 7.19, page 633):
– 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.
end systems,
hosts
a.k.a.
network,
communication
subnet
Interconnection Network
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
SAN vs. NAS
•
•
•
•
Storage area network
Network-attached storage
Storage virtualization
Continuous data protection
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
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.)
• Motivate the interconnection complexity
incrementally
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
A Simple Example
• What is the format of mesage?
– Fixed? Number bytes?
Request/
Response
1 bit
Address/Data
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)
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
A Simple Example Revisted
• What is the format of packet?
– Fixed? Number bytes?
Request/
Response
Address/Data
CRC
2 bits
32 bits
4 bits
00: Request—Please send data from Address
01: Reply—Packet contains data corresponding to request
10: Acknowledge request
11: Acknowledge reply
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
Low-Latency Message Passing
•
•
•
•
•
Reducing data copying
Interrupt coalescing
Decreasing context switch
More efficient DMA transactions
Wither TCP offload engine?
Network Performance Measures
• Overhead: latency of interface vs. Latency: network
Universal Performance Metrics
Sender
Sender
Overhead
Transmission time
(size ÷ bandwidth)
(processor
busy)
Time of
Flight
Transmission time
(size ÷ bandwidth)
Receiver
Overhead
Receiver
Transport Latency
(processor
busy)
Total Latency
Total Latency = Sender Overhead + Time of Flight +
Message Size ÷ BW + Receiver Overhead
Includes header/trailer in BW calculation?
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
• 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
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
Simplified Latency Model
• Total Latency Overhead + Message Size / BW
• Overhead = Sender Overhead + Time of Flight +
Receiver Overhead
• Example: show what happens as vary
– Overhead: 1, 25, 500 µsec
– BW: 10,100, 1000 Mbit/sec (factors of 10)
– Message Size: 16 Bytes to 4 MB (factors of 4)
• If overhead 500 µsec,
how big a message > 10 Mb/s?
Overhead, BW, Size
Delivered BW
1,000
Effective Bandwidth (Mbit/sec)
o1,
bw1000
100
10
o1,
bw10
o500,
bw100
o25,
bw100
o1,
bw100
o25,
bw10
1
o500,
bw1000
o25,
bw1000
o500,
bw10
0
Msg Size
4194304
1048576
65536
16384
4096
1024
Message Size (bytes)
262144
•How big are
real messages?
256
64
16
0
Cummulative %
Measurement:
Sizes of Message for NFS
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Msgs
Why?
Bytes
0
1024
2048
3072
4096
5120
6144
7168
8192
Pa cke t size
• 95% Msgs, 30% bytes for packets ~ 200 bytes
• > 50% data transfered in packets = 8KB
Impact of Overhead on Delivered
BW
Delivered BW
(MB/sec)
10 00.00
1
10 0.00
10
10 0
10 .0 0
10 00
1.00
1000
100
10
1
0.10
M inTime
one-w ay
µs ecs
Pe ak BW (M B/sec )
• BW model: Time = overhead + msg size/peak BW
Interconnect Issues
• Performance Measures
• Network Media
Network Media
Twisted Pair:
Coaxial Cable:
Plastic Covering
Copper, 1mm think, twisted to avoid
attenna effect (telephone)
"Cat 5" is 4 twisted pairs in bundle
Insulator
Copper core
Fiber Optics
Transmitter
– L.E.D
– Laser Diode
light
source
Used by cable companies:
high BW, good noise
Braided outer conductor immunity
Buffer
Light: 3 parts
Cladding
are cable, light
Total internal
source, light
reflection
detector.
Receiver
– Photodiode Note fiber is
unidirectional;
need 2 for full
Silica core
duplex
Cladding
Buffer
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
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
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
Interconnect Issues
• Performance Measures
• Network Media
• Connecting Multiple Computers
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)
Main Issues
•
•
•
•
Addressing
Routing
Congestion control
Flow control
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
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
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
Deterministic Routing Examples
• 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
110
010
111
• tree: common ancestor
• Deadlock free?
011
100
000
001
101
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,
and putting the whole message into a single switch. (Requires a
buffer large enough to hold the largest packet).
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
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)
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
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
Virtual LAN
• Layer2 technology that tries to achieve
what Layer3 routers can do: limit broadcast
traffic
• Distributed spanning tree protocol (802.1D)
• Per-tree spanning tree
• VLAN to emulate ATM
• Transparent reliable multicast
• IGMP snooping
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
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
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
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
Unified P2P Architecture
• Completely distributed system: don’t even
know who to talk to ?
• Advantages: scalability, fault tolerance, and
anonymity
• Examples
– KaZaA
– Routing protocol for wired networks
– Routing protocol for wireless networks
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
Cellular Telephony II
• Orginal 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)
Wireless Networking vs.
Communications
• The name of the game is wireless
communications: modulation, MIMO,
diversity
• Networking part: routing, transport
protocol, handoff, security
Practical Issues for Inteconnection
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?
Practical Issues for Inteconnection
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
Practical Issues
Interconnection
Example
Standard
Fault Tolerance?
Hot Insert?
SAN
Inifiband
Yes
Yes
Yes
LAN
Ethernet
Yes
Yes
Yes
WAN
ATM
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?
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
Crosscutting: Smart Switch vs.
Smart Network Interface Card
Less Intelligent
More Intelligent
Large Ethernet
Switch
Small Ethernet
Myrinet
Inifiband
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
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
reparability
– Often need to scale
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 multiprocessor 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
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
Addressing Cluster Weaknesses
• Network performance: SAN, especially
Inifiband, 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?
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)
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!
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 1Gbit 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
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 Linix 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
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
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
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
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
Co-location 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
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
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
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)
Putting It All Together: Cell Phones
• 1999 280M handsets
sold; 2001 500M
• Radio steps/components:
Receive/transmit
–
–
–
–
–
–
Antenna
Amplifier
Mixer
Filter
Demodulator
Decoder
Putting It All Together: Cell Phones
• about 10 chips in 2000, which should shrink,
but likely separate MPU and DSP
• Emphasis on energy efficiency
From “How Stuff Works” on cell phones: www.howstuffworks.com
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)
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
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
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 wideband 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.
From “How Stuff Works” on cell phones: www.howstuffworks.com
Single-Chip PC
• What constitutes a PC?
• Can they all be packaged into one chip?
100 million transistors
• $100 Notebook computer