Transcript What`s a protocol? - CS Course Webpages

What’s a protocol?
human protocols:
• “what’s the time?”
• “I have a question”
• introductions
… specific msgs sent
… specific actions
taken when msgs
received, or other
events
network protocols:
• machines rather than
humans
• all communication
activity in Internet
governed by protocols
protocols define format, order of
msgs sent and received among
network entities, and actions
taken on msg transmission,
receipt
The Network Core
• mesh of interconnected
routers
• the fundamental question:
how is data transferred
through net?
– circuit switching:
dedicated circuit per call:
telephone net
– packet-switching: data
sent thru net in discrete
“chunks”
Network Core: Circuit Switching
End-end resources
reserved for “call”
• link bandwidth, switch
capacity
• dedicated resources: no
sharing
• circuit-like (guaranteed)
performance
• call setup required
Network Core: Circuit Switching
network resources (e.g.,
bandwidth) divided into
“pieces”
• pieces allocated to calls
• resource piece idle if not
used by owning call (no
sharing)
• dividing link bandwidth into
“pieces”
– frequency division
– time division
Network Core: Packet Switching
each end-end data stream
divided into packets
• user A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed,
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
• aggregate resource
demand can exceed
amount available
• congestion: packets
queue, wait for link use
• store and forward:
packets move one hop
at a time
– transmit over link
– wait turn at next link
Network Core: Circuit Switching
End-end resources
reserved for “call”
• link bandwidth, switch
capacity
• dedicated resources: no
sharing
• circuit-like (guaranteed)
performance
• call setup required
Network Core: Circuit Switching
network resources (e.g.,
bandwidth) divided into
“pieces”
• pieces allocated to calls
• resource piece idle if not
used by owning call (no
sharing)
• dividing link bandwidth into
“pieces”
– frequency division
– time division
Network Core: Packet Switching
each end-end data stream
divided into packets
• user A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed,
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
• aggregate resource
demand can exceed
amount available
• congestion: packets
queue, wait for link use
• store and forward:
packets move one hop
at a time
– transmit over link
– wait turn at next link
Network
Core:
Packet
Switching
10 Mbs
A
Ethernet
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
45 Mbs
E
Packet-switching versus circuit switching: human
restaurant analogy
• other human analogies?
Network Core: Packet Switching
Packet-switching:
store and forward behavior
Packet switching versus circuit switching
Packet switching allows more users to use network!
• 1 Mbit link
• each user:
– 100Kbps when “active”
– active 10% of time
• circuit-switching:
N users
– 10 users
• packet switching:
– with 35 users, probability >
10 active less that .004
1 Mbps link
Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
• Great for bursty data
– resource sharing
– no call setup
• Excessive congestion: packet delay and loss
– protocols needed for reliable data transfer,
congestion control
• Q: How to provide circuit-like behavior?
– bandwidth guarantees needed for audio/video
apps
still an unsolved problem (chapter 6)
Basic Network Concepts
Circuit switching - {basic TELCO service.
Guaranteed response
because resources are guaranteed. Inefficient for some applications}
Virtual-circuit packet-switching - {divide the info into packets to
multiplex}
Datagram packet-switching - {like the US Mail....}
Connectionless vs Connection-oriented
{At the Link layer, do we do acknowledgements? At the network layer,do
all the packets have to follow the same route?}
Multiplexing - {single media, multiple independent 'circuits'}
{putting multiple 'sessions' on a single media}
Packet-switched networks: routing
• Goal: move packets among routers from source to
destination
– we’ll study several path selection algorithms (chapter 4)
• datagram network:
– destination address determines next hop
– routes may change during session
– analogy: driving, asking directions
• virtual circuit network:
– each packet carries tag (virtual circuit ID), tag determines next hop
– fixed path determined at call setup time, remains fixed thru call
– routers maintain per-call state
Physical Media
• physical link:
transmitted data bit
propagates across link
• guided media:
– signals propagate in
solid media: copper,
fiber
• unguided media:
– signals propagate
freelye.g., radio
Twisted Pair (TP)
• two insulated copper
wires
– Category 3: traditional
phone wires, 10 Mbps
ethernet
– Category 5 TP:
100Mbps ethernet
Physical Media: coax, fiber
Coaxial cable:
Fiber optic cable:
• wire (signal carrier)
within a wire (shield)
• glass fiber carrying
light pulses
• high-speed operation:
– baseband: single
channel on cable
– broadband: multiple
channel on cable
• bidirectional
• common use in 10Mbs
Ethernet
– 100Mbps Ethernet
– high-speed point-topoint transmission (e.g.,
5 Gps)
• low error rate
Physical media: radio
• signal carried in
electromagnetic
spectrum
• no physical “wire”
• bidirectional
• propagation
environment effects:
– reflection
– obstruction by objects
– Interference
Radio link types:
• microwave
– e.g. up to 45 Mbps channels
• LAN (e.g., waveLAN)
– 2Mbps, 11Mbps
• wide-area (e.g., cellular)
– e.g. CDPD, 10’s Kbps
• satellite
– up to 50Mbps channel (or
multiple smaller channels)
– 270 Msec end-end delay
– geosynchronous versus LEOS
Physical Layer Issues
• Theoretical Underpinning
– or, Bandwidth 101
• Media Characteristics
–
–
–
–
Optical Fiber
Coax
Copper Wire (Twisted Pair)
Wireless
• Other Useful Ideas
Signals
• Propagation - {how fast does the signal travel in that
•
•
•
•
media, esp. compared to light?}
Frequency - {number of oscillations per second of the
electromagnetic field of the signal}
Bandwidth - {the width/size, in Hz, of the signal -usually defined by where most of the energy is}
Data Rate - {the number of bits per second. Distinct
from, but related to, frequency and bandwidth}
Baud - {Changes per second in the signal. Limited by
bandwidth.}
Freq/BW/DR
BW
Power
FREQ
Frequency
{see Fig 2-1}
1
0
1
1
0
0
0
1
0
0
Time
T
0.50
0.25
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Harmonic number
0.50
0.25
1 harmonic
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Harmonic number
1
0
0.50
0.25
2 harmonics
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Harmonic number
1
0
0.50
0.25
4 harmonics
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Harmonic number
1
0
0.50
0.25
8 harmonics
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Harmonic number
1
0
Maximum Data Rates
• Nyquist:
– DataRate <= 2*BandWidth * log2 V
where ‘V’ is the number of values which are encoded into
the signal. In the On/Off, 0/1 world, V = 2. Your highspeed
modem has V = 16.
• Shannon:
– The real world is noisy, so Nyquist was an optimist.
• Marti:
– Complexity costs money and adds fragility.
So be choosy.
DR ~ 2 * BW {Max by Theory}
DR ~ 1/2 * BW {Practical}
<- In an On/Off world
(V = 2)
Physical Effects
Bandwidth Limits - {Signals consist of many (infinite) different
sine waves, not all of which can be carried by the media}
Dispersion - {Particularly for multimode fiber, different parts of the
signal may move at different speeds, thus changing the shape of
the signal at the receiver}
Jitter - {Imperfect clock synchronization along the transmission path}
Noise - {Unwanted, external energy that may corrupt the signal}
Media
• Optical Fiber
Multimode
Single Mode
• Coax
Broadband
Baseband
• Twisted Pair
Shielded
Unshielded
Cost and Performance
Media Types:
UTP
Coax
Baseband
Broadband
Increasing
Bandwidth
Increasing
Cost
Fiber
Multimode
Single Mode
But remember, cost includes
--material
Biggest part of installation cost
--LABOR
--electronics
Distances
Media Types:
UTP
Typically 100m +/-
Coax
Baseband
Broadband
200m-500m
up to 40km
Fiber
Multimode
Single Mode
depends on power budget;
can be 100s of km*
* Most LANs use 2km between devices
Fiber Facts
Core
Cladding
Protective Coating
Core Cladding
50
125
Multimode (microns)
62.5
125
"
"
8 to 10 n/a
Singlemode
Fiber Facts,
cont.
Modes == Different paths thru core
"photons"
Since the photons travel at the same speed, but for different distances,
the energy is spread out, or dispersed, at the receiver
Fiber is specified as XX Mhz-km. So a specification of 800Mhz-km
means you could have a bandwidth of 400Mhz over a 2km distance
or 1.6Ghz over a 0.5km distance.
Dispersion has two components: modal and material
Traditional Baseband
Transceiver
Coax
Terminator
Transceiver Cable
Host
CATV Systems
Translator
Headend
"Forward" Signal
"Return" Signal
Splitter
Amplifier
Network
Interface
Unit
Twisted Pair
• Just copper wire where each two wires
(“pairs”) have been twisted around each
other in the cable. {Phone wire}
• Rejects common mode noise
• Minimizes antenna characteristics
• Shielded or Unshielded refers to a ground
sheath around the whole cable.
• Cat 3 vs Cat 4 vs Cat 5
Design
Distribution
Backbone
Daisy Chain
Home Run
Riser Systems
Bus
Modulation
"Modification of a transmitted signal to encode information (bits)"
ASK - Amplitude Shift Keying {varying signal strength}
FSK - Frequency Shift Keying {varying signal frequency}
PSK - Phase Shift Keying {don't ask!}
{NB the above three methods are usually applied to signal carriers}
PCM/PWM - Pulse Code Modulation/ Pulse Width Modulation
{good for fiber}
Others
Switching
• Circuit Switching
– Guaranteed resource
– No size limit on information sent
• Packet Switching - Divides the information into
packets; restricts sizes; also sharing of resources
– Virtual Circuit // Connect-oriented
– Datagram // Connection-less
Multiplexing
TDM - {time division multiplexing}
{low overhead, inefficient}
FDM - {frequency division multiplexing}
STDM - { statistical time division multiplexing}
{some overhead, more efficient, may FAIL}
Multiplexing Examples
2400
TDM
A
B
C
D
2400
2400
BADCBADCBADCBAD
9600
A
B
C
D
STDM
BACACBDCBDBABBD
4800
2400
Multiplexing w/ Packets
Like STDM, except NO "ROUND ROBIN"
Framing & Synchronization
{Here synchronization refers to the sender's and receiver's clocks}
{Frames are packets added signal needed to transmit them on Physical Layer}
Synchronous - Sender and receiver somehow share a common clock.
good for longer runs of data; more efficient but requires the clock signal
somehow be sent along with the data
Asynchronous - Sender and receiver use different clocks so data runs
have to be shorter. Doesn't require the extra clock signal
Synchronous vs Asynchronous - Framing & clocks
Isochronous vs Aperiodic - Characteristics of traffic {beware of
confusion as each writer may mix terms}
Specific Framing
Bit Stuffing - Used to ensure special framing and/or control
characters are not sent in the data. A problem because the 'clock' is
usually continuous but data may not be there, so we have to know when
the line is idle and when a frame starts {frame delimiter}
Manchester Encoding - Example of combining clock with data to
form a single signal -- no separate line is required. It does require twice
the bandwidth of the original signal
Manchester
Encoding
Ensures for each bit there is a clock transition. Data values (0 or 1)
are encoded by positive or negative clock transitions in the middle of
the bit time. Transitions are made at bit edges if needed so that the
correct transition can be made in the middle of the bit.
TELCO Architecture
Trunks
CO
CO
Users
CO
Users
CO
Users
Users
TELCO Trunking
Older, Analog World: Frequency
Multiplexing
Current World: Digitizing & Time Multiplexing