Transcript 2014-summer_Lec02

TOBB ETÜ ELE46/ELE563
Communications Networks
Lecture 01
May 6, 2014
Fall 2011
Tuesday 10:30 – 12:20 (310)
Thursday 15:30 – 17:20 (372)
İsrafil Bahçeci
Office: 168
[email protected]
Ders Bilgileri - I
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Bu derste neler öğreneceğiz?
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İnternet olgusu hayatımızın çok önemli bir parçasıdır.
Her ne kadar bilgisayar ağı kavramı İnternetten ibaret
olmasa da İnternet en önemli ve en yaygın bilgisayar
ağlarının başında gelmektedir. Neden?
İnterneti olanaklı kılan teknolojiler ve yapı blokları
nelerdir?
Bu yapı taşlarını kullanarak nasıl bir mimari
oluşturulmuştur ki böylesi etkin ve yaygın bir iletişim
sistemi ortaya çıkmıştır?
Bu sistemin eksikleri var mıdır? Varsa neledir?
Ders Bilgileri - II
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Kaynak kitap
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Computer Networks, Andrew Tanenbaum , 5th Edition,
Prentice Hall 2011
Yarımdcı kitap
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Computer Networks: A Systems Approach, 5th Edition,
Morgan Kaufmann (an Elsevier Company) by L. L.
Peterson and B. S. Davie
Ders Bilgileri - III
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Notlandırma
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Arasınav: %30
Sonsınav: %60
Proje (seçmeli): %10
Objective of this Lecture
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Requirements placed on the network
The idea of network architecture
Key elements in implementation of a network
architecture
Key metrics to evaluate the performance of computer
networks
An Example
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URL (Uniform Resource Locator)
HTTP (Hyper Text Transfer Protocol)
Click in your browser “http://www.mkp.com/pd3e”
One click and as many as 17 messages are exchanged
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Assuming the web page can be downloaded with a single
message
Six messages to translate www.mkp.com into 213.38.165.180
Three messages to set up TCP between the browser and the
server
Four messages for the browser to send the HTTP “get” request
and the server to reply with the requested page (+ ACKs)
Four messages to tear down the connection
Requirements: Connectivity:
Links Nodes
Point-to-point
(a)
(b)
Multiple access
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Scalability
Link
Node
Indirect connectivity → switching
Switched Network
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Clouds
Switched network
Circuit switched (telephone system)
Packet switched (computer networks)
Packets, messages
Store-and-forward
Interconnection of Networks
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internetwork (internet)
Router, gateway
address
routing
Unicasting, broadcasting, multicasting
Requirements:
Cost-effective resource sharing
L1
R1
L2
R2
Sw itch 1
L3
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Sw itch 2
R3
Efficient utilization of the links
Multiplexing
Analogy: CPU
TDM
FDM
More efficient multiplexing: statistical multiplexing
Multiplexing over a link
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How to service packets
FIFO
Round robin
Quality of Service (QoS)
Congestion
Support for Common Services
Host
Host
Application
Channel
Host
Application
Host
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Host
Application processes are communicating
Should each application perform all the complex
functionality to communicate ?
Common services, hide complexity (abstraction)
Application level process communicate over logical channels
What functionality should the logical channel provide?
Common Communication Patterns
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Client/Server
Request/reply channel
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(small request message, large reply message) – a
digital library
The opposite
Message stream channel – video on demand
Channel abstractions
Reliability
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Networks can fail
Bit errors
Burst errors
Buffer overload
Software/OS errors
Routing errors
Human errors
Others (power failure)
Network Architecture:
Example of A Layered Network System
Application programs
Process-to-process channels
Host-to-host connectivity
Hardware
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Abstraction
Interface
Hide complexity
Decompose the problem
Monolithic software
Modularity
Protocols
Application programs
Request/reply Message stream
channel
channel
Host-to-host connectivity
Hardware
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Protocols
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Service interface
Peer interface
Service and Peer Interface
Host 1
High-level
object
Protocol
Host 2
Service
interface
Peer-to-peer
interface
High-level
object
Protocol
Example of A Protocol Graph
Host 1
File
application
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Digital
library
application
Video
application
Host 2
File
application
Digital
library
application
RRP: Request Reply Protocol
MSP: Message Stream Protocol
HHP: Host-to-Host Protocol
Video
application
Protocols
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Protocol specification
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Interoperability
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Pseudo code
State transition diagram
Packet formats
Independent implementation
Standardization bodies
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IETF (Internet Engineering Task Force)
ISO (International Standards Organization)
IEEE (Institute of Electrical & Electronics Engineers)
Encapsulation
Host
Host
Application
Application
program
program
Application
Application
program
program
Data
Data
RRP
RRP
RRP
Data
RRP
HHP
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Header
Trailer
Body
Demultiplexing
HHP
HHP
RRP
Data
Data
OSI Reference Model
End host
End host
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Netw ork
Data link
Physical
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Netw ork
Netw ork
Data link
Data link
Physical
Physical
OSI (Open System Interconnection)
Reference model
One or more nodes
w ithin the netw ork
Netw ork
Data link
Physical
Client-server model
REQUEST and REPLY
Applications
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Business: Web, VoIP, e-commerce
Home: Connectivity, instant messaging,
social network, wiki
Mobility
Peer-2-peer model
Network Scale and Hardware
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Broadcast vs. Point-2-Point (unicast)
Scale of network
Bluetooth, medical devices, RFID
Enterprise, home, AP, wireless
router, 802.11 (WiFi), 802.3
(Ethernet), Virtual LANs
City coverage, cable TV +
internet, Wireless MAN (802.16
WiMax)
Large geographical area, country,
continent
Network Scale and Hardware
PAN
LAN
MAN
WAN
Network Scale and Hardware
WAN-VPN
WAN-ISP
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Transmission lines
Routers inter-communication: Routing, forwarding algorithms
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Wireless WANs
• Satellite
• Cellular network
Network of Networks
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ISP (Internet service provder) networks
to connect many different types of
networks
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Subnet: collection of routers and
communication lines
Hosts: connected to subnet
Gateway: interconnects different
netwroks
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Hardware and software translator
Network Software
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Protocol: agreement between
communicating parties
Protocol stack: Efficient simplified
implementation by layers
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Services to higher layers, similar to virtual
machine
5-layer protocol stack
Physical communication
Virtual communication
• Clear, modular
interface design,
• Well-defined
functions
Network Architecture
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Set of protocol
layers
Protocol stack: list
of protocols used by
a certain system
Network Architecture 2
Layer Design
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Software design for higher levels,
hardware/firmware design for lower layers
Design issues
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Error detection, error correction
Path selection, routing
Network evolution: protocol layering
Packet aggregation/de-aggregation, addressing,
ordering
Scalability
Resource allocation, scheduling, statistical multiplexing
Flow control, congestion control
Quality of service
Confidentiality, authentication
Connection Type
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Connection-oriented service: circuit
switch
Connectionless service: packet switch
Service vs. Protocol
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Service: a set of operations that a layer
provides to the layer above it
Protocol: set of rules for the formatting,
packetization, message bit generation
Object operations vs. implementation
Network Architecture Examples
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Open System Interconnection (OSI)
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Generic model, although protocol layers
are not directly used
TCP/IP
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Not a generic model, but widely
implemented protocol layers
Reference Model 1: OSI
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Open Systems
Interconnection (OSI)
reference model
ISO (revised in 1995)
Principles
• New layer for a
different abstraction
• Well-defined
functionality
• Internationally
standardized protocols
• Minimize information
flow across interface
• Distinct functions per
layer
Reference Model 2: TCP/IP
Network Model in 463/563
OSI vs. TCP/IP
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Common
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OSI: services, interfaces, protocols
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Send ip, receive ip packet services
In real-time implementation, the romantics theory of OSI model
failed
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Similar to object oriented prograaming
TCP/IP: Does not differentiate services, protocols and interfaces
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Stack of independent protocols
Layers with similar functionality
Model arrived without any protocol in mind, none existed at the time
of model development
Ignored inter-networking, e.g., the Internet
TCP/IP: Protcols derived and implemented, then fiollowed by the
TCP/IP ref. model emerged
OSI: both connection-orineted and connectionless supported
TCP/IP, only connectionless supported
Critcique of OSI Model
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Bad timing: Research vs. standardization approach
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Bad Technology
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Unbalanced design some layers are empty, some are full
Difficult to implement
Bad Implementation
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Everybody rushed on TCP/IP due to widespread
implementations and test in academia
Initial implemetnations were huge, slow
TCP/IP’s first emergance as part of Berkley’s UNIX: a
success story
Bad politics
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Governments attempted to push a not-good enough OSI
model implementation to researchers, which did not go
through as expected
Critique of TCP/IP
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Initial versions ignored services, protocols and
interfaces
Not e generic ref. model; only good to explain the
protocol stack for TCP/IP itself
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Link layer, is rather than being a layer, is only an
interface
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For example, it can not explain or fit to many of actual
networking platforms, such as bluetooth
Ignores the difference between physical and data link
layer
Not cerafully thought prtocols
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Only TCP and IP were deliberatly designed
Others just based on trials and errors, some apprentice
job by graduate student work .
Examples 1: Internet
Example 2: Cellular
Example 3: Wi-Fi
RFID and sensor networks
Network Standardization
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Interoperability of products from different
vendors targeted for same applications
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802.11 describes many schemes, but do not specify
when and how to use them
Protocol defined, but the service interface inside the
box is an implementation issue, not specified.
Standard development
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De facto: HTTP by some guy in CERN, Bluetooth by
Ericcson, etc.
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De facto may gradually evolve to De jure
De jure (by-law): By standard bodies
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ITU, ISO, IETF, IEEE, 3GPP
Company politics to push owned patents into the standards
Telecom World
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International interoperability
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ITU: International Telecommunications Union
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ITU-T: Telecom. Standardization Sector: Technical
recommendations for interfaces, then becomes standards
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ITU-R: Recommendations Sector
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ITU-D: Development Sectors
A member of ITU-T
NIST: National Institute of Standards and Technology
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Example: Recommendation H.264 for video
X.509: public key cryptography for web browsing
ISO: International Standardization Organization
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Study groups: e.g., SG15 for DSL development
US Department of Commerce
IEEE: Institute of Electrical and Electronics Engineers
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802.11: WLANs : great success
802.3: Ethernet (LAN) : great success
816: MANs : almost fail
Ex. 802 Working groups
Internet Standardization
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Internet Architecture Board (from the times of ARPANET)
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RFC: Request for Comments, reports
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IRTF: Internet Research Task Force: long term research
IETF: Internet Engineering Task Force: short term
engineering issues
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Many working groups with a fine granular task, e.g, OSI
integration, routing and addressing, security, etc..
Procedure to standardize the outcomes
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www.ietf.org/rfcs
Proposed Standard: RFC is a must
Draft Standard: Requires a 4-month test at 2 independent
sites
Internet Standard: IAB approval
W3C: WWW Constroitum: protocols and guides for longterm growth of Web
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E.g., HTML, Web privacy
Chapter 2: Physical Layer
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Lowest Layer
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Electrical, timing, other inteerfaces in the
‘bit’ level
Physical environment
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Impact on all network performance parameters
Wireless, wire, fiber, space, underwater
Modulation, coding
Multiple access
Basis of Communications
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Signal representations: In terms of sums
of sinusoids at different freqeuencies, e.g,
harmonics
Communication channel: The transmission
media affects different frequencies by
different gains and delays: distortion
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Channel bandwidth
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Wi-Fi: Upto 160 MHz
DVT: 6 MHz
Base-band vs. pass-band
Example: Square-wave
8-bits per character
Rate: b bits per second
T=8/b: Duration of 8 bits for binary
modulation
First harmonic: 1/T = b/8 Hz
Channel bandwidth: 3000 Hz
Chanenel Data Rate
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Nyquist: For channel with B bandwidth,
2B samples is the maximum allowed
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V levels for modulations: 2Blog2(V) bits/sec
Noisy channel: Blog2(1+S/N)
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Increase bandwidth
Increase signal quality (e.g. repeater)
Guided Media
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Magnetic types, removable media
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Twisted pairs
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~gigabyte/half a cent with the existing recording
and shipping costs
Twisting due to avoiding an antenna structure
Telephony system
Coaxial cable
Power lines
Fiber optics
Wireless Media
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Electromagnetic spectrum
Radio Transmission
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Multipath propagation
Wireless Media 2
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Microwave transmission
Infrared transmission
Light transmission
Satellite Communications
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Geostationary satellites
Medium earth orbit satellites
Low-earth orbit satellites
Satellite vs. Fiber
Digital Modulation and
Multiplexing
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Bits to analog-signal mapping
Base-band vs pass-band transmission
Carrier frequency
Multiplexing
Baseband Transmission
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Simple: + voltage for 1, - voltage for 0
Fiber: light for 1, dark for 0
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Receiever side: closest signal detection
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For NRZ: + voltage as 1, - voltage as 0
NRZ: seldom used
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Non-return to 0: NRZ
Bandwidth: B/2 Hz for B bits/s (Recall: Nyquist Rate: 2Blog2(V) =
2B samples)
Need to increase bandwidth for faster transmission
Higher attenuation for higher frequencies
Higher-order modulation: symbol rate or baud rate
Clock recovery: a long run of 1s or 0s: how to distinguish one
bit from other if syncronziation is not very accurate
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Megabits/s speeds requires microsec level accuracy
Expensive clocks!
Clock Recovery with Line Codes
XOR clock signal with
data signal: Manchester
coding
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Clock twice the bit rate
b xor b’ = 1,
b xor b = 0
Low-2-high: 0, high-2-low: 1
Ethernet
Twice bandwidth
NRZI: non-return to -0 inverted
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1: transition, 0: no transition
USB ports (Universal serial bus): Long runs 1 is ok.
Long runs of 0: e.g., T1: no more than 15 consecutive 0s
4B/5B code: every 4 bits to 5 bits
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No more than 3 consecutive 0s
Scrambling: pseudo random sequence via random number
generator
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No guarantee for long-runs od b’s
Killer packets: e.g. IP over SONET
Balanced signals
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Similar amount of + voltages and –
voltages
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Average DC 0: DC is attenuated over
cables
Bipolar encoding (a.k.a Alternative Mark
Inversion - AMI)
Passband Transmission
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Baseband signals shifted to desired carrier
frequency retaining the bandwidth
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ASK: Amplitude shift keying
FSK: Frequency shift keying
PSK: Phase shift keying
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BPSK: Binary PSK
QPSK: Quadrature PSK
QAM: Quadrature Amplitude Modulation
Constellation diagram
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Bit-to-symbol mapping: Gray encoding
Modulation
Multiplexing
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Frequency Division Multiplexing
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Band pass signals share the spectrum
Split into smaller bands, e.g, bandwidth of
baseband signal
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Guard bands to avoid inerference
OFDM: orthogonal frequency division
multiplexing
FDM and OFDM example
Ex: Telephone network,
cellular networks
802.11,802.16,LTE, etc.
Time Division Multiplexing
Time slot
 Guard time
 Telephone and cellular networks
 Not confuse with
Statistical TDM: packet switching
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Code Division Multiplexing
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Spread spectrum
Allows transmission all the time over all
the spectrum
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Chips: short intervals
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TDM: sharing, FDM: low/high pitch
CDMA: different languages
64 bit/128 bit chip sequence
E.g., to send 0: sends a schp sequence of –
1,1,-1,1,-1,-1,-1,1,1...-1,1 (64 bits), 1 with
antoher set of -1s and 1s
Orthogonal seqeunces
Networks and PHY layer
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PSTN: Public switched telephone network
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Originally developed to carry voice traffic
Reduce the last mile as much as possible via
fiber and digital technology deployment
Structure of PSTN
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2 copper wire
End office: switch
Local loop: end office to subscriber
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Modems, ADSL, Fiber
Typical circuit route
Local loops: Twisted pair to switches
Trunks: Fibers between switches
Switching offices
Digital Data over PSTN
Telephone modem
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ADSL modem: reuse local loop
3100 Hz bandwidth !!!: Nyquist: 6000 baud
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Trellis coded modulation: allows higher order modulation,
Bandwidth efficient
V32: 32 constellation points: 9600 bps
V34….. Finally: v90: 56 kbps
Higher bit rates
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ADSL: Asymetric digital subscriber line
Trick: remove the switch filter optimized
for human voice
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New bandwidth: 1.1 MHz
Discrete multitone (DMT)
Fiber
Multiplexing and Trunks
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TDM: Time Division Multiplexing
FDM: Frequency DM
Digital data
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Voice digitization: Pulse Code Modulation
Companding: mu-law or A-law
125 mico sec blocks
T1
T1 multiplexed