ch2
TRANSCRIPT
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COM 360
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Chapter 2
Direct Link Networks
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Network Technologies
1. Point-to-Point Links
2. Carrier Sense Multiple Access ( CSMA) – (for example the Ethernet)
3. Token Rings – (for example IEEE 802.5 and FDDI Fiber Distributed Data Interface)
4. Wireless – (for which 802.11 is the emerging standard)
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ProblemsConnecting computers is a first step.
There are additional problems to solve before they can exchange packets:
• Encoding bits into the transmission medium
• Framing the bits so they can be understood
• Error detection
• Reliable delivery, in spite of occasional errors
• Media access control
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Hardware Building Blocks
• Networks are constructed from nodes and links• Nodes are general purpose computers such
as workstations, multiprocessors or PCs as well as special purpose switches, routers.– Memory – finite – must be managed– Network Adapter (NIC) and its device driver
• Links implemented on physical media, such as twisted pair, coaxial cable, optical fiber
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Nodes
I/O bus
(To network)
CPU
Memory
NetworkadaptorCache
Example workstation architecture
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•The work station’s adaptor component connects the rest of the work stations to link.
•More then just a physical connection , it is an active intermediary between node and link with its own internal processor.
•Its role is to transmit data from the work station on to the link and receiver data from the link, storing it for the workstation.
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Network Adaptor
A network adaptor can be thought of as having two main components.
•A BUS Interface•A Link Interface
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Network Adaptor
•A BUS InterfaceA bus interface that understands how to communicate with host.
•A Link InterfaceA link Interface that understands how to use link.
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Network Adaptor
•The adaptor exports a Control Status Register(CSR) that is readable and writeable from CPU.
•The CSR is typically located at some address in the memory, thereby making it possible for CPU to read /write just like any other memory location.
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Network Adaptor
•S/W on the host – A Device driver – writes to the CSR to instruct it to transmit and/or receive data and reads from the CSR to learn the current status from the CRS.•Notifications• Reception of a FRAME.• The adaptor interrupts the host.
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Block Diagram of a Network Adaptor
Adaptor
Network linkBus
interfaceLink
interface
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Interrupts• The host only pays attention to the network
device when the adaptor interrupts the host, (for example, when a frame has been transmitted or one arrives).
• A procedure is invoked by the operating system, and an interrupt handler is invoked to take the appropriate action.
• While servicing this interrupt, the OS disables other interrupts.
Network Adaptor
• There are TWO basic mechanisms
1.Direct Memory Access(DMA)
2.Programming I/O (PIO)
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Direct Memory Access vs. Programmed I/O
• There are two ways to transfer the bytes from the frame between the adaptor and host memory:
• Direct Memory Access (DMA)- the NIC directly reads/writes to the host’s memory without CPU involvement, using a pair of buffer descriptor lists.
• Programmed I/O (PIO)- network adaptor (NIC) copies message into its own buffer, until CPU can copy it into the host memory.
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Programmed I/O
Host
Adaptor Memory
CPU
Memory
Memory
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Links• Physical media are used to propagate
signals as electromagnetic waves, traveling at the speed of light.
• Properties of EM waves:– Frequency- or oscillations, measured in hertz– Wavelength – distance between adjacent
maxima and minima, measured in meters
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Electromagnetic Waves
• Wavelength = speed / frequency• Voice grade phone lines carry waves ranging
from 300 Hz to 3300 Hz• Voice-grade example: 300Hz in copper wire• Wavelength = Speed in Copper/ Frequency
= 2/3 x 3 x 108 /300 = 667 x 103 meters
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Electromagnetic Spectrum
Radio Infrared UVMicrowave
f(Hz)
FM
Coax
Satellite
TV
AM Terrestrial microwave
Fiber optics
X ray
100
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016
102 106 108 1010 1012 1014 1016 1018 1020 1022 1024104
Gamma ray
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Links
• A link is a physical medium carrying signals in the form of electromagnetic waves.
• Binary data is encoded in the signal.– Lower layer is concerned with modulation,
varying the frequency, amplitude or phase of the signal
– Upper layer is concerned with encoding the data
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Link Attributes
• Another link attribute is how many bit streams can be encoded on it, at a given time.
• One bit stream- connected nodes share access
• Point-to-point – often two bit streams at once– Full duplex - two directions – simultaneously – Half duplex – one direction at a time – Simplex – one direction
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Cables
• Type of cable depends on technology
• Coaxial – ( thick and thin) – within buildings
• Category 5 ( CAT 5) – twisted pair, thicker gauge than telephone wire
• Fiber –plastic or most often glass, more expensive, but used to connect buildings, and transmits light instead of electrical waves.
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Figure 7.3 Twisted-pair cable
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Figure 7.4 UTP and STP cables
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Table 7.1 Categories of unshielded twisted-pair cables
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Figure 7.5 UTP connector
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Figure 7.7 Coaxial cable
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Table 7.2 Categories of coaxial cables
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Figure 7.8 BNC connectors
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Figure 7.10 Bending of light ray
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Figure 7.11 Optical fiber
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Figure 7.12 Propagation modes
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Figure 7.13 Modes
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Figure 7.14 Fiber construction
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Figure 7.15 Fiber-optic cable connectors
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Local Link CablesCable Typical
BandwidthDistances
Cat 5 twisted pair
10-100 Mbps 100 m
Thin-net coax 10-100 Mbps 200 m
Thick-net coax 10-100 Mbps 500 m
Multimode fiber
100 Mbps 2 km
Single-mode fiber
100- 240 Mbps 40 km
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Leased Lines
• To connect nodes on opposite sides of the country, or at great distances, you must lease a dedicated line from the telephone company.
• DS1, DS3, T1, and T3 are relatively old technologies, defined for copper
• STS-N links are for optical fiber (Synchronous Transport Signal), also called OC-N for Optical Carrier
• Originally designed for voice, today can carry data, voice and video
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Common Bandwidths
Services Bandwidth
DS1 or T1 1.544 Mbps
DS3 or T3 44.736 Mbps
STS-1 51.840 Mbps
STS-3 155.250 Mbps
STS-12 622.080 Mbps
STS-48 2488320 Gbps
STS-192 155.250 Mbps
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Last-Mile links• Leased lines range in price from $1000/month to “don’t
ask”• Last mile links span the last mile from the network
service provider to the home or office.• Conventional modem- POTS (plain old telephone
service)• ISDN – (Integrated Services Digital Network) – uses
CODEC ( coder/decoder) to encode analog to digital signal
• xDSL (Digital Subscriber Line)• Cable modem- uses cable television (CATV)
infrastructure, available to 95% of US households
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Common Available ServicesServices Bandwidth
POTS 28.8 - 56 Kbps
ISDN 64 – 128 Kbps
xDSL 16 Kbps – 52.2 Mbps
CATV 20 –40 Mbps
TELEPHONE NETWORKTELEPHONE NETWORK
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Telephone networks use circuit switching. Telephone networks use circuit switching. The telephone network had its beginnings The telephone network had its beginnings in the late 1800s. The entire network, in the late 1800s. The entire network, which is referred to as the which is referred to as the plain old plain old telephone systemtelephone system ( (POTSPOTS), was originally ), was originally an analog system using analog signals to an analog system using analog signals to transmit voice.transmit voice.
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Figure 9.1 A telephone system
Endoffices
Local loop
Trunk
Tandemoffices Regional offices
Trunk
• • •
9.43
DIGITAL SUBSCRIBER LINEDIGITAL SUBSCRIBER LINE
After traditional modems reached their After traditional modems reached their peak data rate, telephone companies peak data rate, telephone companies developed another technology, DSL, to developed another technology, DSL, to provide higher-speed access to the provide higher-speed access to the Internet. Internet. Digital subscriber lineDigital subscriber line ( (DSLDSL) ) technology is one of the most promising technology is one of the most promising for supporting high-speed digital for supporting high-speed digital communication over the existing local communication over the existing local loops. loops.
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xDSL
• Collection of technologies, able to transmit data at high speeds over standard twisted pair lines
• ASDL ( Asymmetric Digital Subscriber Line)- different speeds in different directions (upstream and downstream) – called local loop
• VDSL- (Very high rate Digital Subscriber Line)- runs over shorter distances – “fiber to neighborhood”
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ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses.
Note
The existing local loops can handle bandwidths up to 1.1 MHz.
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•ADSL is an adaptive technology. •The system uses a data rate based on the condition of the local loop line.• Bit rate is the number of bits per second.• Baud rate is the number of signalelements per second.
Note
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Figure 9.11 Bandwidth division in ADSL
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Figure 9.12 ADSL modem
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Figure 9.13 DSLAM
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ADSL
1.554─ 8.448 Mbps
16─ 640 Kbps
Local loop
Centraloffice
Subscriberpremises
downstream
upstream
ADSL connects the subscriber to the central office via the local loop.
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VDSL•Very High-bit-rate Digital Subscriber Line similar to ADSL uses coaxial, fiber- optic, or twisted-pair cable for short distance.•It provides a range of bit rate 25 to 55 Mbps for up stream communication at a distance of 3000 to 10,000 ft.•The down stream rate is normally 3.2mbps
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VDSL
STS-N
over fiber
VDSL at 12.96─ 55.2 Mbps
over 1000─ 4500 feet of copperCentraloffice
Subscriberpremises
Neighborhood opticalnetwork unit
VDSL connects the subscriber to the optical network that reaches the neighborhood.
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ISDNISDN
•Integrated Services Digital Network(ISDN)Integrated Services Digital Network(ISDN)•An ISDN connection two 64-kbps channels, An ISDN connection two 64-kbps channels, one that can be used to transmit data and one that can be used to transmit data and another that can be used for digitized voice.another that can be used for digitized voice.•A device that encodes analog voice into a A device that encodes analog voice into a digital ISDN link is called CODEC, for digital ISDN link is called CODEC, for coder/decoder.coder/decoder.•When the voice channel is not in use, it can When the voice channel is not in use, it can be combined with the data channel to be combined with the data channel to support up to 128 Kbps of data bandwidth.support up to 128 Kbps of data bandwidth.
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CABLE TVCABLE TV
Cable companies are now competing with Cable companies are now competing with telephone companies for the residential telephone companies for the residential customer who wants high-speed data customer who wants high-speed data transfer. transfer.
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CATV• A subset of CATV channels are made available for
transmitting digital data• A single CATV channel has a bandwidth of 6 MHz• Like ADSL, CATV is asymmetric with
downstream rates much greater than upstream– 40 Mbps downstream ( 100 Mbps max)– 20 Mbps upstream ( roughly half as much)
• Unlike DSL, bandwidth is shared among all subscribers in a neighborhood.
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Figure 9.16 Division of coaxial cable band by CATV
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The theoretical downstream data rateis 30 Mbps.
Note
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The theoretical upstream data rate is 12 Mbps.
Note
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Figure 9.17 Cable modem (CM)
9.60
Figure 9.18 Cable modem transmission system (CMTS)
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Shannon’s Theorem• Shannon’s theorem gives an upper bound to the
capacity of a link, in terms of bits per second.
C = B log2 (1+S/N)
where C is channel capacity, B is Bandwidth, S is signal power, N is noise and S/N is the signal to noise ratio expressed in decibels, related as:
dB= 10 x log10 (S/N)
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Shannon’s TheoremExample
• dB ratio pf 30 dB• S/N = 1000• Bandwidth = 3000Hz
C = B x log2 ( 1+S/N)C = 3000 x log2 (1001)C = 30 Kbps = roughly the limit of a 28.8
modemHow are 56 Kbps modems possible? See p. 76
3.63
Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as
Example
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This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot receive any data through this channel.
3.65
We can calculate the theoretical highest bit rate of a regular telephone line. A telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is usually 3162. For this channel the capacity is calculated as
Example
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This means that the highest bit rate for a telephone line is 34.860 kbps. If we want to send data faster than this, we can either increase the bandwidth of the line or improve the signal-to-noise ratio.
3.67
Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as
Example
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This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot receive any data through this channel.
3.69
The signal-to-noise ratio is often given in decibels. Assume that SNRdB = 36 and the channel bandwidth is 2 MHz. The theoretical channel capacity can be calculated as
Example
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We have a channel with a 1-MHz bandwidth. The SNR for this channel is 63. What is the theoretical channel capacity?
SolutionFirst, we use the Shannon formula to find the upper limit.
3.72
Example 3.41
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Wireless Links
• AMPS- Advance Mobile Phone System- standard for cellular phones
• PCS- Personal communication Services – digital cellular services in US and Canada
• GSM- Global System for Mobile Communication in the rest of the world.
• They use a system of towers to transmit signals and are moving toward ringing the globe with satellites.
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Local Wireless Links• Radio and infrared portions of the spectrum can
be used over short distances.• Technology- limited to in-building environments• Radio bands at 5.2 GHz and 17 GHz are
allocated to HIPPERLAN in Europe and 2.4 GHz for use with the IEEE 802.11 standard, which supports data rates up to 54 Mbps.
• Bluetooth – radio, operates in the 2.45 GHz band– Used for all devices, printers, PDAs, phones– Networks of these devices are called piconets
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Bit Rates and Baud Rates
• Rate at which the signal changes is called the baud rate.
• When one bit is transmitted on a signal, the bit rate and baud rate may be equal.
• Often multiple bits are encoded onto a signal, where for example with 4 bits per signal, the baud rate may be 4 times the bit rate
4.76
Figure 4.2 Signal element versus data element
4.77
A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?Solution
We assume that the average value of c is 1/2 . The baud rate is then
Example
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Encoding• First step in turning nodes and links into usable
building blocks is to understand how to connect them so that bits can be transmitted.
• Next encode binary data that the source want to send into signals that the links can carry and then decode the data back into the corresponding data at the receiving end.
• The high and low signals correspond to 2 different voltages on a copper based system or 2 different power levels on an optical link.
4.79
Line coding and decoding
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NRZ Encoding• NRZ – non-return to zero, maps the data value 1 to the
high signal and 0 to the low signal• A sequence of several consecutive 1’s means that the
signal stays high for a prolonged period of time.• Two fundamental problems;
– Baseline wander –makes it difficult to detect a significant change in the signal
– Clock recovery needs frequent changes from high to low to be enabled
• Sender and receiver clock must be precisely synchronized.
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NRZ Encoding
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
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NRZI Encoding
• NRZI – non-return to zero inverted, addresses the previous problem, by having the sender make a transition from the current signal to encode a 1 and stay at current signal to encode a 0. ( Solves the problem of consecutive 1’s, but not 0’s)
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Manchester Encoding• Merges the clock with the signal by transmitting the
exclusive–OR of the NRZ encoded data.• Results in 0 being encoded as a low-to-high transition
and 1 encoded as a high-to-low transition. Because both 0s and 1 result in a transition, the clock can be recovered at the receiver.
• Problem: doubles the rate at which transitions are made on the link, which gives receiver half the time to detect them.
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Encoding Strategies
Bits
NRZ
Clock
Manchester
NRZI
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
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4B/5B Encoding
• Attempts to address the inefficiency of Manchester encoding.
• It inserts extra bits into the bit stream to break up long sequences of 0s and 1s:
– Every 4 bits of data are encoded in a 5 bit code
(See table 4B/5B encoding on p. 79)
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4B/5B Encoding
• The 5-bit codes are selected in such a why that each one has no more than one leading 0 and no more than two trailing 0s.
• No pair 5-bit codes results in more three consecutive 0s being transmitted.
• the resulting 5-bit codes are then transmitted using the NRZI encoding
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4B/5B Encoding
• 5bits are enough to encode 32 different codes ad we are using only 16 of these data.
• Of these code 11111 is used when line is Idle.
• Code 00000 is used when the line is dead.
• 00100 is interpreted to mean halt.
• Of the remaining 13 codes 7 of them are not valid because they violate the “one leading 0’ , “two trailing 0s”
4.88
4B/5B mapping codes
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Packets and Frames
Packet is ``generic'' term that refers to a small block of data.
Each hardware technology uses a different packet format.
Frame or hardware frame denotes a packet of a specific format used on a specific hardware technology.
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FRAMINGFRAMING
The data link layer needs to pack bits into The data link layer needs to pack bits into framesframes, so that each frame is , so that each frame is distinguishable from another. Our postal distinguishable from another. Our postal system practices a type of framing. The system practices a type of framing. The simple act of inserting a letter into an simple act of inserting a letter into an envelope separates one piece of envelope separates one piece of information from another; the envelope information from another; the envelope serves as the delimiter. serves as the delimiter.
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Framing• Blocks of data (frames), not bit streams, are
exchanged between nodes.
• The network adapter (NIC) enables the nodes to exchange frames.
• Recognizing what set of bits constitutes a frame, and where the frame begins and ends, is the challenge faced by the network adapter.
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Frame Format Need to define a standard format for data
to indicate the beginning and end of the frame
Header and trailer used to ``frame'' the data (SOH and EOT)
Can choose two unused data values for framing for example, if data is limited to printable ASCII characters, you can use
``start of header'' (soh) ``end of text'' (eot)
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Frame Format
• Framing in Practice
Incurs extra overhead - soh and eot take time to transmit, but carry no data
Accommodates transmission problems:
Missing eot indicates sending computer crashed
Missing soh indicates receiving computer missed beginning of message
Bad frame is discarded
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Frame Format
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Framing
• Suppose A wishes to transmit a frame to B
• It tells adapter to transmit a frame from the node’s memory
• A sequence of bits is sent over the link
• The adapter on B then collects the sequence of bits arriving on the link and deposits them in B’s memory.
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Framing
Frames
Bits
Node A Node BAdaptor Adaptor
Bits flow between adaptors, frames between hosts
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Framing
• There are several approaches to the framing problem:
• Byte-Oriented Protocol (PPP)– Sentinel Approach (frame start and end)– Byte counting
• Bit –Oriented Approach (HDLC)
• Clock-based framing (SONET)
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A frame in a character-oriented protocol
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Byte-Oriented protocols• One of the oldest approaches to framing is to view each
frame as a collection of bytes (characters) rather than bits.
• BISYNC (Binary Synchronous Communication) protocol is a byte-oriented approach developed by IBM in 1960’s
• DDCMP ( Digital Data Communication Message Protocol) was used in Digital Equipment’s DECNET.
• These are examples of the sentinel approach and the byte counting approach.
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Sentinel Approach• A packet is a sequence of labeled fields.• Above each field is a number indicating the number of
bits in the field.• Packets are transmitted beginning with the leftmost
field. The beginning of the frame is the SYN (synchronization) character.
• Data is contained between sentinel characters – STX (start of text) and ETX (end of text).
• The header begins with a SOH (start of header) field.• It ends with a CRC (cyclic redundancy check) field.
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BISYNC Frame Format
Header Body
8 8 8 8 168
CRC
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Framing problem• ETX character may appear in the data.
• BISYNC overcomes this by using byte-stuffing or character-stuffing by preceding the ETX character with an escape character or DLE (data link escape– (similar to \n or \t in programming)
• CRC (cyclic redundancy check) is used to detect transmission errors.
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Point-To-Point Connection
The first computer communication systems were connected by communication channels that connected exactly two computers.
Called a mesh or point-to-point network
Had three useful properties:– 1. Each connection was independent and different hardware could be
used. (bandwidth, modems, etc. did not have to be the same)• Allow for greater flexibility.
– 2. The connected computers have exclusive access and could decide how to send data across the connection. The can determine the frame format and size, error detection mechanism, etc.
– 3. Since only two computers share the channel it is private and secure.
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Disadvantages of Point-To-Point
• 1. Number of wires grows as the number of computers increases
• 2. The total number of connections exceeds the number of computers being connected.
• The number of connections needed is proportional to the square of the number of computers, since the new computer must have a connection to each of the existing computers. So to add the Nth computer requires N-1 new connections.
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Disadvantages of Point-To-Point• For N computers: • Connections = (N2 - N)
2
Computers Connections
2 1
3 3
4 6
5 10
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Point-to-Point Protocol
• Point-to-Point Protocol (PPP) is run over dialup modem links and is similar to BISYNC.
• Flag denotes the start-of-text character, address and control fields contain default values.
• The protocol is the high level protocol, such as IP or IPX.
• Payload size is usually 1500 bytes.• Checksum field is either 2 or 4 bytes long.
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PPP Frame Format
ProtocolControlAddressFlag Payload
88 816168
FlagChecksum
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PPP Framing• PPP framing is unusual in that several of the field
sizes are negotiable rather than fixed.• The negotiation is conducted by the LCP (Link
Control Protocol) Protocol.• PPP and LCP work in tandem:
– LCP sends control messages encapsulated in PPP frames denoted by an LCP identifier
– Changes PP’s frame format based on the information contained in the control messages.
– LCP also establishes a link between the peers when both sides detect the carrier signal.
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Byte-Counting Approach
• The alternative to detecting the end of a file with a sentinel value is to include the number of items in the file at its beginning.
• This is true in framing- the number of bytes in a frame can be included in the header.
• DDCMP protocol uses this approach and the COUNT field specifies the number of bytes in the frame’s body.
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DDCMP Frame Format
Header Body
8 8 4214 168
CRCCount
Tree types of messages:•DATA Message ( 81 )•Control messages ( 05)•Maintenance ( 90)
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Framing Errors
• A transmission error could corrupt the COUNT field and the end of the frame would be incorrectly detected.
• A similar problem exists with the ETX field being corrupt.
• This is called a framing error.• The receiver waits for the next SYN character to
collect data for the next frame. • A framing error may cause back-to-back frames to
be incorrectly received.
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Bit-Oriented Protocols
• Bit-oriented protocols are not concerned with byte boundaries. It views the frame as a collection of bits.
• Synchronous Data Link Control ( SDLC), developed by IBM is a bit-oriented protocol, later standardized as the High Level Data Link Control (HDLC).
• Uses bit sequence 01111110 to denote beginning and end of a frame.
• It is also transmitted when the link is idle.
11.113
A frame in a bit-oriented protocol
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High-level Data Link Control (HDLC)High-level Data Link Control (HDLC) is a is a bit-orientedbit-oriented protocol for communication protocol for communication over point-to-point and multipoint links. It over point-to-point and multipoint links. It implements the ARQ mechanisms we implements the ARQ mechanisms we discussed in this chapter.discussed in this chapter.
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HDLC Frame Format
Header Body
8 16 16 8
CRCBeginningsequence
Endingsequence
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Data Stuffing• Networks usually insert extra bits or bytes
to change data for transmission and this is called Data Stuffing
Bit stuffing and byte stuffing are two techniques for inserting extra data to encode reserved bytes
Byte stuffing translates each reserved byte into two unreserved bytes
11.117
Byte stuffing is the process of adding 1 extra byte whenever there is a flag or
escape character in the text.
Note
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Byte Stuffing
Can use esc as prefix, followed by x for soh, y for eot and z for esc:
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Byte Stuffing Sender translates each reserved byte into
the appropriate encoding pair of bytes Receiver interprets pairs of bytes and
stores encoded byte in buffer• Data still framed by soh and eot
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Byte Stuffing
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Bit Stuffing
• Anytime 5 consecutive 1’s are transmitted, the sender inserts a 0 before sending the next bit. On the receiving side.
• When the receiver detects 5 consecutive 1’s, it assumes the next 0 was “stuffed” and removes it.
• If the next bit is a 1, either this is the end of frame marker or an error has occurred.
• Size of the frame is dependent on the data being sent in the frame payload.
11.122
Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s
follow a 0 in the data, so that the receiver does not mistake
the pattern 0111110 for a flag.
Note
11.123
Bit stuffing and unstuffing
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Clock-Based Framing
• Third approach to framing is the Synchronous Optical Network (SONET) standard, called clock-based framing.
• SONET was proposed by Bell Communications Research (Bellcore) for digital transmission over an optical fiber.
• Addresses the framing and encoding problems as well as multiplexing low speed links onto a high speed link.
• More complex protocol
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SONET Framing
• SONET Frame has special information that indicates where the frame starts and ends.
• No bit stuffing is used• How does receiver know where the frame starts and
ends? • Frame consists of 9 rows of 90 bytes each.
– First 3 bytes of each row are overhead. – First two bytes of frame contain special bit pattern – Use of overhead bytes is complex
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SONET STS-1 Frame
Overhead Payload
90 columns
9 rows
First two bytes of the frame contain a special bit pattern that indicates the start of the frame
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Find the data rate of an STS-1 signal.
STS-1, like other STS signals, sends 8000 frames per second. Each STS-1 frame is made of 9 by (1 × 90) bytes. Each byte is made of 8 bits. The data rate is
128
Find the data rate of an STS-3 signal.
SolutionSTS-3, like other STS signals, sends 8000 frames per second. Each STS-3 frame is made of 9 by (3 × 90) bytes. Each byte is made of 8 bits. The data rate is
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NoteNote
In SONET, the data rate of an STS-nsignal is n times the data rate
of an STS-1 signal.
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What is the duration of an STS-1 frame? STS-3 frame? STS-n frame?
SolutionIn SONET, 8000 frames are sent per second. This means that the duration of an STS-1, STS-3, or STS-n frame is the same and equal to 1/8000 s, or 125 μs.
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Note
In SONET, the duration of any frame is 125 μs.
17.132
STS-1 frame overheads
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STS-1 Multiplexing
STS-1 STS-1 STS-1
STS-3cHdr
Three STS-1 frames are multiplexed onto one STS-3 frame.
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Error Detection
• Bit errors occur in frames due to electrical interference or thermal noise.
• Detecting errors is one part of the problem; correcting errors is the other.
• What happens when an error is detected?• Two basic approaches:
– Notify the sender that message is corrupt so the sender can retransmit it; ( most often used in every day applications)
– Use an error-correcting code to reconstruct the correct message
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Transmission Errors
External electromagnetic signals can cause incorrect delivery of data Data can be received incorrectly Data can be lost Unwanted data can be generated
Any of these problems are called transmission errors
10.136
In a single-bit error, only 1 bit in the data unit has changed.
Note
10.137
Single-bit error
10.138
A burst error means that 2 or more bits in the data unit have changed.
Note
10.139
Burst error of length 8
10.140
To detect or correct errors, we need to send extra (redundant) bits with data.
Note
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Error Detection
• Detecting Transmission Errors: basic idea is to add redundant information to a frame that can determine if errors have been introduced.
• Two-dimensional parity – based on a simple parity bit added to balance the number of 1’s
• Checksums – code created based on addition
• Cyclic Redundancy Check (CRC) – based on a complex mathematical algorithm and used in nearly all link level protocols.
142
Parity
Parity refers to the number of bits set to 1 in the data item
Even parity - an even number of bits are 1
Odd parity - an odd number of bits are 1
A parity bit is an extra bit transmitted with a data item,chose to give the resulting bits even or odd parity
Even parity - data: 10010001, parity bit 1• Odd parity - data: 10010111, parity bit 0
143
Parity and Error Detection
If noise or other interference introduces an error, one of the bits in the data will be changed from a 1 to a 0 or from a 0 to a 1
Parity of resulting bits will be wrong
Original data and parity: 10010001+1 (even parity)
Incorrect data: 10110001+1 (odd number of 1’s)
Transmitter and receiver agree on which parity to use
Receiver detects error in data with incorrect parity
144
Limitations of Parity Checking
Parity can only detect errors that change an odd number of bits
Original data and parity: 10010001+1 (even parity)
Incorrect data: 10110011+1 (even parity!)
Parity usually used to catch one-bit errors
145
Two-Dimensional Parity
• Two-dimensional parity involves adding on extra bit to balance the number of 1’s in each byte (making the total either even or odd).
• Two-dimensional parity does a similar calculation for each bit position across all the bytes in the frame, resulting in adding an extra parity byte for the frame as well as an additional parity bit for each byte.
• Two-dimensional parity catches all the one, two and 3 bit errors and most 4 bit errors.
146
Two-Dimensional Parity
1011110 1
1101001 0
0101001 1
1011111 0
0110100 1
0001110 1
1111011 0
Paritybits
Paritybyte
Data
147
Probability and Error Detection All error detection methods are approximate
and aim at a low probability of accepting corrupted data.
• Parity can detect a single bit error, but not all possible errors, especially where two bits ( or an even number of bits) are changed.
Many alternative schemes exist: Detect multi-bit errors Correct errors through redundant information• Checksum and CRC are two widely used
techniques
148
Internet Checksum Algorithm
• Simple idea: add up all the words that are to be transmitted and then transmit the sum, called the checksum, with the data.
• The receiver performs the same calculation and compares it to the checksum received. If they do not match, an error has occurred.
• Does not detect all errors…• Algorithm is easy to implement ( See p. 94)
149
Checksum Sum of data in message treated as array of
integers
Can be 8-,16- or 32-bit integers
Typically use 1s-complement arithmetic
Example -16-bit checksum with 1's complement arithmetic
10.150
Sender site:1. The message is divided into 16-bit words.2. The value of the checksum word is set to 0.3. All words including the checksum are added using one’s complement addition.4. The sum is complemented and becomes the checksum.5. The checksum is sent with the data.
Note
10.151
Receiver site:1. The message (including checksum) is divided into 16-bit words.2. All words are added using one’s complement addition.3. The sum is complemented and becomes the new checksum.4. If the value of checksum is 0, the message is accepted; otherwise, it is rejected.
Note
10.152
Example
153
154
Advantages of Checksum
Fastest implementations of 16-bit checksum use 32-bit arithmetic and add carries in at end
Easy to do - uses only addition Small size of checksum means cost of transmitting it is small.• Ease of computation to create and verify
checksum.
155
Checksum Limitations
Does not detect all common errors (like reversed bits)
156
Cyclic Redundancy Check (CRC)
• CRC uses powerful mathematics ( finite fields) to give strong protection against common bit errors in messages that are thousands of bytes long.
157
Detecting Errors with Cyclic
Redundancy Checks Consider data in message as coefficients of
a polynomial
Divide that coefficient set by a known
polynomial
Transmit remainder as CRC
Good error detection properties
Easy to implement in hardware
158
CRC Hardware The hardware used to computer a CRC is a shift
register, which act like a tunnel through which bits move in a single file from right to left.
The shift register holds a fixed number of bits so when a new bit moves in, another bit moves out.
The output gives the value of the leftmost bit. When a bit changes, the output changes. The shift register has two operations: initialize and
shift. Initialize sets all bits to zero Shift moves all bits one position to the left.
159
CRC Hardware
• X-Or and Shift Registers
160
CRC Hardware• CRC Hardware consists of 3 shift registers
connected with X-Or units.• Output from the leftmost unit goes to 3 places
simultaneously - the • 3 X-Or units.• To compute the CRC values in all registers are
initialized and the bits are shifted one at a time.• One bit of the message is applied to the input unit
and all three registers perform a shift. This repeats for each bit of the message.
161
CRC Calculation using Shift Registers
x0 x1XOR gate x2
Message
162
CRC Computation
• After an entire message has been input, the shift registers contain the 16 bit CRC for the message.
163
CRC Computation
• A CRC can compute more errors that a simple checksum because:
An input bit is shifted through 3 registers; The hardware uses feedback so that the
effect from a single bit cycles through the shift registers more than once.
• Mathematically a CRC uses a polynomial to divide the message:
• P(X) = x 16 + X 12 + X 5 + 1
164
Example CRC
• A message is treated as a long binary polynomial, P. • Before transmitting, the data link layer divides P, by a fixed polynomial function G(x), resulting in a whole quotient Q and a remainder R/G. The remainder is appended to the message and transmitted. • It is checked by the receiver to see if R agrees with
the locally generated value for R. (See Tanenbaum p.208-210 for analysis)
165
Example CRC
• Frame: P: 1101011011• Generating function G(x): 10011• Message after appending 4 zero bits:
11010110110000• Divide P by G to get remainder R:• 1100001010 with R = 1110 • 10011 | 11010110110000 • Transmitted frame with remainder R appended:
11010110111110
166
Accuracy of CRC
• CRC actually adds 8, 16, 24, or 32 bits to the message.
• This method detects up to 99.969% of errors with CRC-8 and nearly 99.9% with CRC-16 or CRC-24.
167
Another CRC Example
Generator 11011111100110011010000 Message1101
10011101
10001101
10111101
11001101
10001101101 Remainder
See text. Pp. 94-95
168
Error Correction or Error Detection?
• When error is detected, frame is discarded and resent, using bandwidth and causing latency, waiting for its arrival.
• Error correction requires additional bit to be sent with every frame.
• Correction is useful when – 1) errors are probable or – 2) the cost of retransmission is too high
169
Reliable Transportation
• A data link level protocol that wants to deliver frames reliably must recover from discarded ( lost) frames.
• Acknowledgements - (ack) is a small control frame that a protocol sends back to report that it has received the frame. If the sender does not receive a frame in a reasonable amount of time, it retransmits.
• Timeouts -waiting a reasonable mount of time is called a timeout
170
Automatic Repeat Request
• Using acknowledgements and timeout to implement reliable delivery is called automatic repeat request (ARQ).
• The simplest ARQ scheme is the Stop and Wait algorithm.
171
Stop and Wait
• After transmitting one frame the sender waits for an ACK before transmitting the next frame.
• If it does not arrive in a reasonable time, the sender retransmits the original frame.
172
Stop and Wait AlgorithmSender Receiver
Frame
ACK
Sender Receiver
Frame
ACK
Frame
ACK
Sender Receiver
Frame
ACK
Frame
ACK
Sender Receiver
Frame
Frame
ACK
(a) (c)
(b) (d)
a) Arrives
b) Frame lost
c)ACK lost
d) Timeout too soon
173
Duplicate Frames
• If a frame is late arriving another frame might be retransmitted, resulting in duplicate frames.
• To correct this, a header usually contains a sequence number (0,1), which is used for alternate frames.
• When sender retransmits frame 0, the receiver can see that it is a second copy of frame 0, not frame 1.
174
Timeline for Stop and WaitSender Receiver
Frame 0
ACK 0
Frame 1
ACK 1
Frame 0
ACK 0
175
Sliding Window Protocol
Allows sender to transmit multiple packets before receiving an acknowledgment
Number of packets that can be sent is defined by the protocol and called the window
As acknowledgments arrive from the receiver, the window is moved along the data packets; hence ``sliding window''
Sliding window protocol can increase throughput dramatically
176
Sliding Window Protocol• Sliding window algorithm allows the transmission of
a frame at about the same time as the ACK arrives.• Sender assigns a sequence number (SeqNum) to
each frame and maintains 3 variables:– Send window size (SWS) - # of unacknowledged frames
that sender can transmit
– Last acknowledgement received (LAR)
– Last frame sent (LFS)
– LFS - LAR <= SWS
177
Sliding Window
178
Timeline for Sliding WindowSender Receiver
179
Sliding Window• When ACK arrives, the sender moves LAR to the right,
allowing the sender to transmit another frame.• Sender buffers up to SWS (send window size) frames (in
case they need to be retransmitted).• It also associates a timer with each frame it transmits, so
it can retransmit if an ACK is not received in time.• LAR – Last Acknowledgement Received• LFS – Last Frame Sent• See pp. 105-115 for details and for interactive demo see• http://www2.rad.com/networks/2004/sliding_window/demo.html
180
Sliding Window on Sender
< SWS
LAR LFS
■ ■ ■ ■ ■ ■ ─
181
Sliding window
• The receiver maintains 3 variables:– The receive window size ( RWS) – the upper
bound on the number of out of order frames that the receiver can accept.
– The sequence number of the largest acceptable frame (LAF)
182
Sliding Window on Receiver
RWS
LFR LAF
■ ■ ■ ■ ■ ■
< ─
183
Sliding Window Algorithm
• When frame with sequence number SeqNum arrives, the receiver does the following:
• If SeqNum <= LFR or SeqNum >LAF then frame is outside the window and is discarded.
• If LFR < SeqNum <= LAF, then it is accepted.• SeqNumToAck is largest not yet acknowledged• Receiver acknowledges receipt of SeqNumToAck and
sets LFR = SeqNumToAck• LAF=LFR +RWS
184
Comparison of Sliding Window and Stop & Wait
185
Frame Order and Flow Control
• Sliding Window can be used for:– To reliably deliver frames on an unreliable link;– To preserve the order in which the frames are
transmitted, using the sequence numbers;– To support flow control- a feedback mechanism
by which the receiver is able to throttle the sender to keep it from overrunning the sender.
186
Concurrent Logical Channels
• ARPANET Data Link protocol, or concurrent logical channels, is an alternative to sliding window protocol and can keep pipe full while using the simple stop and wait protocol.
• It multiplexes several logical channels onto a single point-to-point link and runs the stop and wait protocol on each.
187
Ethernet (802.3)• The Ethernet is the most successful local area
networking technology.• 1973- Developed at Xerox Park by Bob Metcalfe
and David Boggs, it is a general form of the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) technology.
• Based on Aloha, early packet network developed at the University of Hawaii to support communication across the islands.
Bob Metcalfe
188
•Developed the Ethernet with David Boggs• 1979 Founded 3COM Corporation, which makes wirelesss access points• Founded Infoworld•Authored numerous books and articles•Recipient of many awards including the National Medal of Technology (2005) and induction into the National Inventors Hall of Fame for his contributions to the “welfare of mankind”.•Spoke at the CCSCE Conference at SJC, October, 2007 “ETHERNET IS THE ANSWER; WHAT IS THE QUESTION?”
189
Ethernet (802.3)
• Digital Equipment Corporation (DEC), Intel and Xerox joined to form the 10 Mbps Ethernet standard in 1978.
• This standard formed the basis of the IEEE standard 802.3
• It has recently been extended to include a 100 Mbps version, called Fast Ethernet and a 1000 Mbps version called Gigabit Ethernet.
190
Ethernet (802.3)
• The Ethernet is a multiple-access network meaning that a set of nodes send and receive frames over a shared link.
• The “carrier sense” means that the nodes can distinguish between a busy and idle link.
• “Collision detect” means that a node listens as it transmits and can detect when a transmitting frame has interfered (collided) with a frame transmitted by another node.
• When a collision occurs, both nodes back off, wait a random amount of time and then attempt to send again.
191
Physical Properties
• An Ethernet is typically implemented on coaxial cables of up to 500 meters.
• (On older versions, called thick-net or 10Base5, a transceiver connected hosts to the cable and then to the network adapter or NIC card.)
• Newer versions, 10Base2, connect directly through the NIC, where all the logic is contained.
• 10BaseT, for twisted pair, uses Cat 5 cable and is limited to 100meter.
• “Base” refers to the baseband system.
192
Ethernet Transceiver and Adapter
Transceiver
Ethernet cable
Adaptor
Host
193
Thick Ethernet Wiring Uses thick coax cable AUI cable (or transceiver or drop cable connects from NIC to transceiver AUI cable carries digital signal from NIC to transceiver Transceiver generates analog signal on coax • Wires in AUI cable carry digital signals, power and other control signals
194
Ethernet Wiring Uses thin coax that is cheaper and easier to install than thick Ethernet coax Transceiver electronics built into NIC; NIC connects directly to network medium • Coax cable uses BNC connector
195
Ethernet Wiring Coax runs directly to back of each connected computer
• T connector attaches directly to NIC
Useful when many computers are located close to each other
May be unreliable - any disconnection disrupts entire net
196
Twisted Pair Ethernet• Variously called 10Base-T, twisted pair or TP
Ethernet • Replaces AUI cable with twisted pair cable
• Replaces thick coax with hub
197
Physical Properties• Multiple Ethernet segments are joined by repeaters,
which forward a digital signal.• No more than 4 repeaters may be connected to any pair
of hosts, limiting an Ethernet to a maximum of 2500 meters.
• An Ethernet can support a maximum of 1024 hosts.• Any signal placed on the Ethernet is broadcast to all
hosts.• Terminators are attached to the end of each segment to
absorb the signal.• The Ethernet uses Manchester encoding.
198
Ethernet repeaters
Repeater
Host
■ ■ ■
■ ■ ■
■ ■ ■
■ ■ ■
199
Ethernet Hubs
Hub Hub
The common 10BaseT configuration is to have several point-to-point segments connected to a hub or switch. This is also true for 100Mbps Ethernet, but not for Gigabit Ethernet.
200
HUBS
201
Access Protocol• On an Ethernet, all hosts are competing for
access to the same shared link.
• The media access control (MAC) algorithm controls access to the link.
• It is implemented in hardware on the network adapter.
202
Network Adapter Cards (NIC)
CPU can't process data at network speeds Computer systems use special purpose
hardware for network connection Typically a separate card in the backplane Network adapter card or network interface card
(NIC) Connector at back of computer then accepts
cable to physical network
203
Network Interface Hardware
204
NIC Cards
The sockets for the NIC cards are usually located near the back of the cabinet and a network cable attaches to the end of the NIC.
205
NIC Cards and Wiring
NICS can provide all three connection technologies
206
Ethernet Frame Format
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
•Taken from the Digital-Intel-Xerox Ethernet Standard
•Each Ethernet frame is defined by the following format where the preamble allows the receiver to synchronize with the signal.
•Both source and destination hosts are identified by addresses
•Packet type identifies the protocol
•Each packet can contain up to 1500 bytes of data (46bytes minimum)
•32-bit CRC for error detection
207
Addresses
• Each host on an Ethernet has a unique Ethernet address.
• Technically the address belongs to the adaptor, not to the host and is usually burned into the NIC card ROM.
• Each NIC card has a unique prefix and makes sure it assigns unique addresses
• Addresses can be assigned statically, dynamically or can be configurable and assigned by the network administrator
208
Assigning AddressesStatic: Hardware manufacturer assigns permanent
address to each interface - doesn't change
Manufacturer must ensure every interface has a unique address
Configurable: Address can be set by end user, either through switches or jumpers, or electronically ( EPROM) or through software
System administrators must coordinate to avoid conflict
Dynamic: Interface automatically assigns hardware address each time it is powered up (tries random number)
Automatic scheme must be reliable to prevent conflicts
209
Addressing Scheme Comparison
Static:
Ease of use and permanence
Less flexibility
Configurable:
Small permanent addresses hardware easily replaced
Requires initial configuration
Dynamic:
Smaller addresses and Vendors do not have to coordinate to assign
them
Lack of permanence and potential conflict
Addressing Scheme Advantages Disadvantages
210
Address Types• Each frame on an Ethernet is received by every
connected adaptor.• Each adaptor recognizes the frames addressed to it and
passes those frames to it host. These are unicast addresses.
• A broadcast address, consisting of all 1’s, is recognized by all NIC cards.
• A multicast address, with first bit set to 1, is recognized by a subset of NIC cards.
• Running in promiscuous mode, means that a NIC card will pass all messages to its host.
211
Ethernet Address Summary
• An Ethernet adaptor receives all frames and accepts:– Frames addressed to its own address– Frames addresses to the broadcast address– Frames addressed to a multicast address, if it is
part of that subset– All frames if it is in promiscuous mode
212
Transmitter Algorithm• Receiver side is simple.• Sender side implements Ethernet protocol.• When NIC has frame to send and the line is busy, it waits
for the line to become idle.• Because there is no centralized control, two (or more)
adaptors may send at once, causing a collision. When a collision is detected, a jamming sequence is sent to stop transmission.
• The adaptors wait a random amount of time before trying again.
• Each time there is a collision, the delay interval doubles – called exponential backoff.
213
Worst Case Scenario
(a)
(b)
(c)
A B
A B
A B
A B
(d)
214
Success of the Ethernet
• Extremely easy to administer, no switches to fail, no routing or configuration tables
• Easy to add additional hosts
• It is inexpensive, since cables are relatively cheap.
• Most new LAN switching technology is based on the Ethernet
215
Token Rings (802.5, FDDI, RPR)
• Token Rings are the other significant class of shared media networks.
• IBM Token Ring, was the original – followed by the IEEE 802.5 standard, which was nearly identical, and finally the newer FDDI (Fiber Distributed Data Interface) Standard, which is declining in use.
• Resilient Packet Ring or RPR (802.17) is nearly standardized.
216
Token Ring• Token ring Network consists of a set of nodes connected
in a ring.• Data flows in a particular direction around the ring so
that each node receives a packet from its upstream neighbor and forwards it to its downstream neighbor.
• Similar to Ethernet in that it involves an algorithm which controls when a node can transmit, and all nodes see all frames.
• Sending a message differs from that of the Ethernet.
217
Token Ring Network
218
Implementing a Token Ring
219
Tokens• Access to the network is controlled by a token.• A ‘token” is a special sequence of bits, which
circulates around the ring.• Each node receives the token, and when it has
the token, that node may send a packet and then forward the token to the next node in a round-robin fashion.
• This is fair, since each node gets a turn to send.
220
Physical Properties
• Any link or node failure makes the whole network useless.
• When relay is open, the station is included in the ring; if the relay closes, the ring bypasses the node.
• Several relays are packed into a single multi-station access unit ( MSAU) – required by IBM token ring.
• Data rate is 4 or 16 Mbps and uses Manchester differential encoding
• Twisted pair is required for IBM and not specified for 802.5
221
Relay on Token Ring
Host
From previoushost
To nexthost
Relay
(a)
Host
Host Host
From previoushost
To nexthost
Relay
(b)
a) Relay open – host active b) Relay closed-host bypassed
222
Multimedia Access Unit
Host
Host
Host
Host
From previousMSAU
To nextMSAU
MSAU
Used only in electrical rings to compensate for node failure.
223
Token Ring Media Access Control
• Network adapter contains a receiver, transmitter, and one or more bits of data storage.
• When no node is sending, the token circulates.• A sending station, “seizes” the token and sends data.
Token holding time (THT) is the time the node can hold the token. Default THT = 10ms.
• 802.5 also supports a strict priority scheme• Sending node can reinsert token immediately following
its frame (early) or after the frame circles the ring and is removed (delayed) release.
224
Token Release
Token
Fram
eToken Frame
(a) (b)
a) early b) delayed
225
Token Ring Maintenance• Token rings have a station designated as the monitor.• Procedures are defined to elect a monitor when the ring is
first connected or when the monitor fails.• Monitor must make sure there is always a toke in the ring
and that there is sufficient delay.• It also checks for corrupted or orphaned frames.• It also checks for “dead” stations.
226
Token Ring Frame Format
Body ChecksumSrcaddr
Variable48
Destaddr
48 32
Enddelimiter
8
Framestatus
8
Framecontrol
8
Accesscontrol
8
Startdelimiter
8
•Uses differential encoding codes in start and end delimiters.
•Access control byte includes the frame priority
•Frame control byte identifies the higher-level protocol
•Like Ethernet, addresses are 48 bytes long
•Includes a 32- bit CRC and A and C bits for reliable delivery
227
FDDI
• Fiber Distributed Data Interface (FDDI) is similar to 802.5 and IBM token ring.
• Significant differences are that it runs on fiber, not copper and makes use of some newer innovations
• It is usually a dual ring – where each ring transmits in the opposite direction.
• The second ring is only used if the primary ring fails and there is a “loop back” toform a complete ring.
• Instead of a monitor all nodes participate equally in maintaining the ring.
228
Dual Fiber Ring
(a) (b)
a) Normal operation b) Failure of primary ring
229
Physical Properties• FDDI network consists of a dual ring- two rings that
transmit data in opposite directions. The second ring is only used if the primary ring fails.
• Nodes attach to the ring with a single cable called single attachment stations (SAS). A concentrator attaches several SASs to the ring.
• FDDI is a 100 Mbps network and is limited to 500 hosts.• FDDI uses 4B/5B encoding• Token holding algorithms are more complex than 802.5
230
FDDI Frame Format
Control
8 8 8 24
CRCStart offrame
End offrame
Destaddr Body
4848
Srcaddr Status
32
•Similar to 802.5 with these exceptions:
•Uses 4B/5B encoding instead of Manchester
•Has a bit in the header to distinguish synchronous from asynchronous traffic
•Lacks the access control bits present in 802.5
231
Resilient Packet Ring (RPR)• Relatively recent technology – IEEE (802.17)• Resiliency- the ability to recover quickly from a
link or node failure was its key design goal.• Other goals were bandwidth efficiency and
Quality of Service (QoS) support.• Like FDDI it uses 2 rings, but unlike FDDI ,
both are used for normal service.• Uses buffer insertion instead of a token.• Used in MAN’s but “metro Ethernet” is
coming…
232
Wireless
• Wireless is the rapidly evolving technology for connecting communication devices:– Bluetooth – Wi-Fi -802.11 – Wi-MAX – 802.16 – and 3G cellular wireless
• They differ in how much bandwidth they can provide, how far apart nodes can be and which part of the electromagnetic spectrum they use.
233
Wireless TechnologiesBlueTooth Wi-Fi WiMAX 3G Cell
Link Length
10m 100m 10 km
Bandwidth 2.1 Mbps
Shared
54 Mbps
Shared
70 Mbps
Shared
384+Kbps
Per connection
Use/link To notebook
Notebook to base
Building to tower
Phone to tower
Anology USB Ethernet Coaxial DSL
234
Wireless
• The most widely used wireless links are asymmetric – the two endpoints are different kinds of nodes
• One endpoint acts as a base station and has no mobility and is wired to the Internet or other network.
• The client not is often mobile and relies on its link to the base station to communicate with other nodes.
235
236
Wireless
• Notice that wireless naturally supports point to multipoint communications becaues radiio waves sent out by one device can be simultaneously received by many devices
• However communication between client nodes is routed through the base node
237
Example Wireless Network
A B C D
238
Levels of Mobility
• No mobility- when a receiver must be in a fixed location to receive a directional transmission from a base station (true of the initial WiMAX)
• Mobility within the range of a base as in the case of Bluetooth
• Mobility between bases as is the case with cell phones and Wi-Fi
239
Mesh or Ad hoc Network
• A wireless mesh is an alternative topology• Nodes are peers ( there is no base station)• Messages are forwarded through a chian of
peers as long as each peer is within range of the preceeding node.
• This allows a wireless portion of a network to extend beyond the limited range of a single radio.
240
241
Bluetooth• Bluetooth provides very short range communication
between mobile phones, PDAs, notebook computers and other peripheral devices.
• It is a convenient alternative to connecting with a wire.• It has a range of only 10 m and operates at 2.45 GHz • Because devices usually blong to an individual or
group it is often called a PAN ( personal area network)
• Network connects up to 7 devices to a master and is called a piconet.
242
Bluetooth piconet
243
Wireless ( 802.11)
• Like Ethernet and Token Ring, 802.11 is designed for use in a limited geographical area (homes, office buildings, campuses).
• Primary challenge is to mediated shared access through space.
• 802.11 supports additional features (time-bound services, power management and security)
244
Physical Properties
• 802.11 was designed to run over three different media- two based on spread spectra and one based on diffused infrared.
• The radio based versions run at 11 and 54 Mbps.• A chipping sequence spreads the signal over a wide
frequency using a random sequence and makes the signal look like noise to any receiver that does not know the sequence.
• Infrared signals are diffused so the sender and receiver do not need to be aimed at each other, but must be within buildings.
245
4-Bit Chipping Sequence
Random sequence: 0100101101011001
Data stream: 1010
XOR of the two: 1011101110101001
0
0
0
1
1
1
246
Collision Avoidance
• The protocol is more complex than Ethernet, since all nodes are not always within reach.
• Consider 4 nodes A,B,C,D that are able to send to a node to its immediate left or right,so B can reach A and C but not D.
• If A and C both send to B they collide, but are unaware of each other and are called hidden nodes.
247
Hidden node problem
A & C can collide at B
248
Collision Avoidance
• Another related problem is the exposed node problem.
• Suppose B is sending to A and C is aware of this. It is a mistake for C to think it cannot transmit.
• It is not a problem for C to transmit to D because it will not interfere with A’s ability to receive from C
249
Exposed node problem
B can transmit to A and C can transmit to D
250
Collision Avoidance• 802.11 addresses these two problems with a Multiple
Access Collision Avoidance algorithm. (MACA)• Sender and receiver exchange control frames before
transmitting data• Sender sends a request to transmit (RTS) frame.• Receiver relies with a clear to send (CTS) frame.• Receiver also sends an ACK after successfully receiving
the frame. All nodes must wait for this before trying to transmit.
• CTS frames can collide and both must wait before transmitting, similar to Ethernet backoff.
251
Distribution System• Since an advantage of a wireless system is that
nodes are free to move around, reachable nodes may change over time.
• Some nodes may roam and some, called Access Points (AP) are connected to the network infrastructure by a distribution system.
• Distribution system runs at layer 2 of the ISO architecture and does not depend on higher layers.
252
Access Points connected to a Distribution Network
BH
A
F
G
D
AP-2
AP-3AP-1
C E
Distribution system
Each node associates itself with one access point.
253
Communication Example
• For node A to communicate with node E
• A first sends a frame to its access point AP-1, which forwards the frame across the distribution system to AP-3, which finally transmits the frame to E
254
Selecting an APTechnique called scanning:1. The node sends a Probe frame2. All APs within reach reply with a Probe Response
Frame3. The nodes selects one of the access points and sends
that AP an Association Request Frame.4. The AP replies with an Association Response Frame
A node uses this when it joins the network and when it becomes unhappy with current AP ( weak signal, etc.)
255
Active and Passive Scanning
• After a node has probed the network, it associates itself with an Access Point. This is called active scanning.
• APs also periodically send a Beacon Frame that advertise the capabilities of the Access Point, including transmission rates. A node can change to this point by sending an Association Request Frame to the access Access Point . This is called passive scanning.
256
Node Mobility
BH
A
F
G
D
AP-2
AP-3AP-1
EC
C
Distribution system
257
802.11 Frame Format
Addr4 PayloadSeqCtrlAddr3Addr2Addr1 CRC
0Ð18,4964816 32484848
Duration
16
Control
16
48 bit Source and Destination addresses ( addr1, addr2)
Two additional address fields depends on the ToDS, From DS settings
Control fields: Type, ToDS, FromDS
Type fields indicates whether the frame is RTS, CTS or data
CRC for error detection
258
802.11 Frame Format
259
WiMAX (802.16)
• WiMAX stands for Worldwide Interoperability for Microwave Access
• It is a metropolitan area network(MAN) with a range of 1-30 miles.
• It odes not yet inclued mobility, but that will be added as 802.16e
• Its clients are multiplexers for a building and to adapt to different frequencies it uses different physical layer protocols.
260
Cell Phone Technologies• Frequency bands vary around the world:
– Europe 900 and 1800 MHz bands– North America 850 and 1900 MHz bands
• Cost is high to users because of licensed spectrum
• Incompatible cell phone standards
• Phones designed to carry voice now carry video, and audio which require high bandwidth
261
Cell Phone Technologies
• Relies on use of base stations that are part of a wired network
• Geographic area served by the base station’s antenna is called a cell
• Cells overlap and a base station can serve more than one cell using multiple antennae.
262
Handoff
• As a phone begins to leave a cell, it moves into an area of overlap with other cells
• The current base station senses the weakening signal and give control to whichever base station is receiving the stronger signal from it.
• If a phone is receiving a call, the call must be transferred over to the new base station in what is called a handoff.
263
Cell Phone Generations
• 1G – analog• 2G – digital -most of the current technology, some
are referred to as 2.5 G – not quite third generation, but more advanced. These are GSM- Global System for Mobile Communications
• 3G – Based on CDMA (code division Multiple Access
• Satphones- class of phones that are not cellular, but are satellite phones
264
Summary• Five key problems so that links can exchange information:• Encoding problem for physical links carrying signals;• The framing problem determines how to package bits into
frames;• The error detection problem using CRC, parity, and
checksums• Problem of recovering lost frames discarded because of
errors• Problem of mediating access on shared media (Ethernet,
token ring and wireless)
265
Further Reading
• Metcalfe, Robert. and Boggs, David, “Ethernet: Distributed Packet Switching For Local Computer Networks”, Communications of the ACM, 19(7):395-403, July, 1976
• http://standards.ieee.org/ for status of IEEE standards
• See p. 145 for more complete list…