1 an overview of networks outline contemporary networks networks: single-link network to an...
TRANSCRIPT
1
An overview of networks
• Outline• Contemporary networks• Networks:
• single-link network to an internetwork• For each type of network: tasks & layers• Mechanisms for each task• Protocols and Protocol reference models• Standards
Malathi Veeraraghavan Univ. of Virginia
Updated: Aug. 29, 2013
Contemporary networks1. Ethernet switched networks2. Wireless LANs: IEEE 802.11
3. Cellular networks: 3G and 4G (WiMAX and LTE)
4. Residential access networks, such as Passive Optical Networks (PONs) and cable (DOCSIS)
5. Data center networks, such as InfiniBand and FibreChannel
6. Wireless sensor networks: Zigbee, IEEE 802.15.4
7. Vehicular networks: IEEE 1609.3 over 802.11p
8. Satellite networks and disruption tolerant networks
9. Supervisory Control and Data Acquisition (SCADA) networks for electric grid, water/sewage, etc.
10. Backbone circuit-switched networks (SONET, WDM)
11. Dynamic circuit networks12. Public-Switched Telephone Network (PSTN)
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Netw
ork
s at
the
edges
Where/what is the Internet?
• In this listing of contemporary networks– No mention of the Internet
• Internet is “THE” global internetwork – Connects enterprise, home, provider networks
• How? Using gateways called IP routers– IP routers are gateways that interconnect
networks that are owned and operated by different enterprises, homes, and providers
3
4
Increasingly complex networks
• Unshared single-link network• Shared single-link network• Multiple-link network (with
switches)• Internetwork (with gateways)
5
Single-link network with one sender and one receiver
Host Host
Simplest type of network:Unshared single-link
network
e.g., private road
Hosts are data sources and sinks
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Shared single-link network
Single-link network with multiple senders and multiple receivers
e.g., shared road
Symbol for wireless link
Host
Host
Host
http://en.wikipedia.org/wiki/File:Wireless-icon.png
7Courtesy: http://mars.gmu.edu/dspace/bitstream/1920/2497/1/pca_608_23_16n.jpg
Analogy of a shared single link: a shared road with multiple sources of traffic (cars) - multiple driveways connected to single shared road
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Host Host
HostHost
Switch Switch
One network – same type of switches
Multiple-link network
e.g., roadways network (an intersection is comparable to a switch)
Switch
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Analogous to a switch on roadways network
Courtesy: http://en.wikipedia.org/wiki/Image:Cloverleaf.jpgRoad intersection with traffic lights
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Host
Host
Switch Switch
Network 1
Host
Host
Switch Switch
Network 2Gateway
An internetwork
Internetwork:Multiple networks
e.g. roadways network
e.g. airport
e.g. airlines network
Network 3
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Tasks
• What tasks are required for successful communication in each of these types of networks?– where should the hardware or
software carrying out these tasks be implemented?• in hosts?• in switches?• in gateways?
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Tasks in an unshared single-link network• What tasks are required for
successful communication across one link, with one sender and one receiver?
• Where are these tasks executed?– Only possibility: Hosts
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Tasks in an unshared single-link network
• Three sets of tasks:– Tasks specific to the application programs
generating data and receiving data• Example?
– Transmit and receive data bits across a link – Error control and flow control
• Error control: Detect and recover from errors– bit errors caused by interference and electrical
noise• Flow control: Handle mismatches in speed
– between receiving application program and sending application program
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Outline
• Outline• Contemporary networks• Networks• For unshared single-link network
• TasksLayers
• Mechanisms for each task• Protocols and Protocol reference models• Standards
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Layer
• What is a layer?– A grouping of a set of tasks– Hardware/software that implements this
grouping of tasks
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Layers in an unshared single-link network
Tasks Layer
Application-specific tasks, e.g. email, web
Application layer
Error control and flow control
Data-link layer(DLL)
Send/receive data bits Physical layer (PHY)
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Why the name "layers?"
• Because each layer uses the services of the layer below it
Host
Data link layer
Physicallayer
Applicationlayer
Host
Data link layer
Physicallayer
Applicationlayer
1
23
4
5
Draw in response arrows 6 through 10
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Increasingly complex networks
• Unshared single-link networkShared single-link network
What additional tasks are required?How are these grouped into additional
layers?
• Multiple-link network (with switches)• Internetwork (with gateways)
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Sharing analogy
• How are roads shared?– multiple lanes
• frequency-based sharing - radio AM/FM stations
– back-to-back cars on a single lane• time-based sharing
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Shared single-link network
Single-link network with multiple senders and multiple receivers
Let’s call thisScenario 1for creatinga shared single link
Wireless link:what is sent by onecan be heard by all
Host
Host
Host
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Other scenarios for shared single-link networks
Host HostHost
Shared single wired link
App. 1 App. 2 App. 1 App. 2
Multiple applications
Host 1 Host 2Scenario 2
Scenario 3
Multipoint repeater
HubMultiple links, correct?So, why is it labeled "single" link?
Because only one host can sendat a time
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What is a hub?
• Multipoint repeater or hub– Simple physical-layer device that
forwards all packets received on one link to all other links
– So how does a host receiving a packet know whether the packet is meant for it?
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“Sharing” Need for addresses
• On a Shared single-link network– A sending host needs to indicate destination
for its packet– How is this done?
• By adding an address to the packet header.– What is a packet? A set of data bits– What is a header? A few bits tagged on at the front
of the packet
• Example: Ethernet (MAC) address – 6 bytes
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Hub: multipoint repeatingsend to all links except input link
• Packet carries destination address “B” in its header; example packet is carrying the data bits 1000001 (which is ASCII for ‘A’)
Hub or multipoint repeater
Host B Host DHost A Host C
packet10
0000
1
Step 3: All three hosts compare their address to that in the arriving packet header; only B finds a match and delivers data to higher layer; Rest drop the packet
step 1 step 2
step 2
step 2B
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• If hosts A and D simultaneously send packets, they will collide, even if they are headed to different destinations because all packets are sent on all other links
• Bits cannot be deciphered: both packets are “lost”• No buffers in the hub to hold one while processing
the other
Hub or multipoint repeater
Host B Host D Host A Host C
packet
Cost of no buffers
1000
001
B1100001
C
Differences between hub and switch
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Buffers to hold packets
Forwards packets to
Hub No all links except input link
Switch Yes specific link based on forwarding table
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What a switch does instead(recall the term "switch" was used in
multiple-link networks)
• Packet carries destination address “B” in its header; example packet is carrying the data bits 1000001 (which is ASCII for ‘A’)
Host B Host D Host A Host C
packet
Step 2: Switch looks up forwarding table
step 1 step 3
I II III IVSWITCH
Destination Output port
B II
Forwarding table
Step 3: Switch sends the packet on only port II unlike the hub which transmits it on all ports
1000
001
B
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Single link vs. multiple links
• Hub:– Single-link network because at a time
only one sender can send data
• Switch:– Multiple-link network because multiple
senders can simultaneously send data • even if two packets are destined to the
same host, the switch can buffer one packet (hold in memory) while sending the first
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Coming back to our tasks/layers thread
• Recap: Having understood the three scenarios for shared single-link networks:– Wireless link– Two applications between hosts– Single wired link with a hub
• What are the additional tasks and layers needed in these shared single-link networks?
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What additional tasks are needed in a shared single-link network?
• Tasks related to sharing, e.g., decide which sender gets to send next
– a.k.a (also known as) • multiplexing schemes• Medium Access Control (MAC) schemes
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What additional layers are needed in shared single-link networks?
• MAC tasks are implemented in a– sub-layer within the data-link layer
• In other words, data-link layer in a shared single-link network consists of
– error control– flow control– multiplexing/MAC
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Layers in an shared single-link network
Tasks Layer
Application-specific tasks, e.g. email, web
Application layer
Error control, flow control, and multiplexing/MAC
Data-link layer (with MAC sublayer)
Send/receive data bits Physical layer(PHY)
Compare this slide with slide 15
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Where are these layers implemented?
• All three layers at the hosts– For all three scenarios
• In scenario 3:– Hubs implement only the
physical layer•which consists of the tasks of ........
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Increasing complexityof networks
• Unshared single-link network• Shared single-link networkMultiple-link network (with
switches)• Additional tasks?• Additional layers?
• Internetwork (with gateways)
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Host Host
HostHost
Switch Switch
One network – same type of switches
Multiple-link network
e.g., roadways network (with road intersection comparable to a switch)
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What additional tasks are needed in multiple-link networks?
• Switching: forward data units (groups of bits) from one link to another
• End-to-end error control and flow control– Why?
• Switches can drop data due to buffer overflows
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Additional layers in an multiple-link network
Tasks Layer
Switching Network layer
End-to-end error control and flow control
Transport layer
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Where are these layers implemented?
Host Host
HostHost
Switch Switch
Data link layer
Physicallayer
Networklayer
Transportlayer
Applicationlayer
Data link layer
Physicallayer
Networklayer
Transportlayer
Applicationlayer
Data link layer
Networklayer
Data link layer
Physicallayer
Data link layer
Physicallayer
Networklayer
Data link layer
Physical layer
Physical layer
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Why is "network layer" shown at the end hosts?
• Answer:– Network-layer (NL) implementation at a host supports the
network-layer switching task implemented at the switch– at the sending host: NL adds destination address to packet
headers• this allows the switch to forward packets to different output
ports (links) based on destination addresses• analogy: who writes the address on an envelope sent through
USPS?
– at the receiving host: NL checks if destination address in packet header corresponds to host's address
• receive the packet it there is a match• otherwise, drop the packet
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Interesting to note
• That the network-layer tasks implemented at the switch are different from the network-layer tasks implemented at end hosts
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Layers in a multiple-link network
Tasks LayerApplication-specific tasks, e.g. email, web Application layer
End-to-end error control and flow control
Transport layer
Switching Network layer
Error control, flow control, multiplexing/MAC
Data-link layer(with MAC sublayer)
Send/receive data bits Physical layer
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Increasing complexityof networks
• Unshared single-link network• Shared single-link network• Multiple-link network (with
switches)Internetwork (with gateways)
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Host
Host
Switch Switch
Network 1
Host
Host
Switch Switch
Network 2Gateway
An internetwork
Internetwork:Multiple networks
e.g. roadways network e.g. airport
e.g. airlines network
Gateways are the "switches" of the internetwork forwarding data units between networks
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Layers in an internetwork
• Complex in a general context• Example: the Internet
– TCP/IP model– Gateway: IP router
• Another example: VoIP– Public Switched Telephone Network
(PSTN) connected to the Internet– Media Gateway (with controller)
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Putting together the four “types” of networks introduced in lesson
• Four types of (increasingly complex) networks described in this lesson was just for purposes of understanding all the various tasks involved in communications
• In reality, most hosts are connected to the Internet (which is an internetwork)
• These "four types" are just types of communication instances rather than networks
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Host
Host
Switch Switch
Network 1
Host
Host
Switch Switch
Network 2Gateway
An internetwork
Communication instances
Hub
Host
Comm. instance 3:multiple-link
Comm. instance 2:shared single-link
Comm. instance 4:inter-network
Comm. instance 1: unshared single-linkleft out of the figure because it is usually only in labs that two hosts are directly connected to each other by an unshared link
Host
Network management tasks
• FCAPS:– Fault management– Configuration management– Accounting– Performance monitoring– Security
48
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Outline
• Outline• Contemporary networks• Networks:
• from a simple single-link network to an internetwork
• TasksLayers
Two ends of a layer• Mechanisms for each task• Protocols and Protocol reference models• Standards
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Are the implementations of a layer at the two communicating ends the
same?• Sometimes yes.• Sometimes no.
• Physical layer example• Application layer example• Network layer example: already shown
to be different at hosts and switches
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Physical layer example
• Bidirectional links (half-duplex or duplex)– both ends have transmit and receive
capabilities– Same layer-1 implementation at both ends
• Unidirectional link (simplex link)– one end has transmit capability– other end has receive capability
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Half duplex
Transmitter (Tx) Transmitter (Tx)
Receiver (Rx) Receiver (Rx)
Transmitter (Tx) Transmitter (Tx)
Receiver (Rx) Receiver (Rx)
One way at a time
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Full duplex
Transmitter (Tx) Transmitter (Tx)
Receiver (Rx) Receiver (Rx)
Example optical fiberRecall the two connectors (blue and red) at each end
Simultaneous
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Application layer
• Example in which the required tasks differ at the two ends:– Web browsing
• The web server program waits for clients to connect to it and responds to requests by serving out files
• The web client program receives inputs from human users and sends corresponding requests to the web server
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At the application layer
• Two modes of interaction:– Client-server
• the application-layer subroutine at the server is different from that at the client
– Peer-to-peer• the application-layer subroutines are the
same at both end hosts
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Client-server mode
• The server only serves out files while the client only requests files; example: web server access
Courtesy: A. Tanenbaum & Prentice Hall
57
Peer-to-peer mode
• Both ends have the same implementation; example: telephony
Courtesy: A. Tanenbaum & Prentice Hall
58
Outline
• Outline• Contemporary networks• Networks:
• from a simple single-link network to an internetwork
• Tasks and LayersMechanisms for each task• Protocols and Protocol reference models• Standards
59
Application layer:Source coding
• Block-oriented – Text: ASCII (7bits for each character) and EBCDIC; extended ASCII uses 8
bits per character • Compression techniques: "the" "e" occur a lot
– Images: • Fax of an 8" by 10" page with 400 by 400 pixels per sq. inch results in
38400000bytes if three bytes are used per pixel, one each to represent R, G, and B. – Using 1MB = 220 bytes, this is equal to 36.62MB
• GIF: lossless compression • JPEG: lossy compression
• Stream-oriented – Voice: PCM (Pulse Code Modulation); 8000 samples/sec; with 8 bits/sample,
it results in 64kbps signal• Compression techniques:
– ADPCM Adaptive Differential Pulse Code Modulation - 32 kbps – Residual excited linear predictive coding - 8-16 kbps
– Audio (music): needs 32-384kbps – Video:
• H.261 coding: 176 by 144 or 352 by 258 frames at 10-30 frame/sec • Full motion MPEG-2 • HDTV - 1920 by 1080 frames at 30 frames/sec (aspect ratio is important 16:9 vs.
4:3)
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Memory vs. transmission rate
Memory Expressed in Megabytes, Gigabytes, Terabytes
Transmission rate
kilobits/sec, Megabits/sec, Gigabits/sec
With main memory or RAM capacity, gigabyte means 1073741824 bytes;To avoid confusion, let’s call this GibiBytes (see next slide)
capitals for above 1 and small for below m: millibut M for Mega; kbps: small k is an exception
Multiples of bytes
SI decimal prefixes IEC binary prefixes
Name(Symbol)
Value Name(Symbol)
Value
kilobyte (kB) 103 kibibyte (KiB) 210
megabyte (MB) 106 mebibyte (MiB) 220
gigabyte (GB) 109 gibibyte (GiB) 230
terabyte (TB) 1012 tebibyte (TiB) 240
petabyte (PB) 1015 pebibyte (PiB) 250
Exabyte (EB) 1018 exbibyte (EiB) 260
1-62
SI: International System of unitsIEC: International Electrotechnical Commissionsource: http://en.wikipedia.org/wiki/Gibibyte
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Physical layer
• Properties of communication channels– Bandwidth – Amplitude response function – Phase shift function – Attenuation – Speed of light in the medium Shannon's channel capacity
64
Shannon’s channel capacity
• Shannon's channel capacity, C: The maximum rate of a noisy channel whose bandwidth is H Hz is given by – C=H log2(1+S/N) bits/sec
• S/N is the signal to noise fraction at the receiver
– log2x = (log10x)/log102• If 2y = x, take log10 of both sides
– then y log102 = log10x.
Units: The term log2(1+S/N) has a unit of bits, which can be seen in a derivation (not included in this course) of Shannon's equation.Therefore the unit of C is bits/sec, since the unit of H is Hz or /sec
65
Physical layer• Group of functions needed to move bits across a link:
“communications”
Channel (line)coding
Modulator
DemodulatorChannel (line)coding
Data bits
Channel1011000...
Channel (line) coding: is a method for converting a binary information sequence (1s and 0s) into a digital signal in a digital communications systems
Modulation: is a method for carrying an information signal on a carrier signal
Data bits
1011000...
66
What are the components of physical-layer delay on a single link?
• Physical-layer delay incurred in moving a data unit (file or packet) of size S bits across an error-free communication link– Propagation delay – Emission (transmission) delay
DELAY = TIME Usage of the word: "I was delayed getting to the airport" impliesthat there was an expectation of the time needed to drive there andthe speaker somehow got "delayed," perhaps due to heavy traffic. In networking, we use the term "delay" to represent the time takento move the file. There is a component called "queuing delay" torepresent additional delay incurred by having to wait.
67
Propagation delay
• Propagation delay = L/v, where L is the length of the link and v is the speed of light in the physical medium (v) of the link
1 bitpropagation delay
bit travels at the speedof light in the medium
hence the dependence onlength of the link
68
Emission (transmission) delay
• Emission (transmission) delay = S/r, where S is the size of the data unit being transmitted in bits, and r is the transmission rate in bits/sec
1 data unit of S bits:
a file or packet
emission (transmission) delay:time to emit (transmit) the data unit on to the link
Let’s say the link transmitter canemit out 10 million bits/sec; this is r,the transmission rate of the link. Hence the size of the data unit, S, and the transmitter rate, r,determine the emission delay
69
Physical-layer delay to move a data unit of size S
bits• Physical-layer delay = emission
delay + propagation delay• Why do we not need to multiply
propagation delay by the number of bits?
70
Packetization (AL) delay(only for streamed data)
• Packetization delay is the time taken to create data for the payload of a packet– Packetization delay = S/rcodec
– rcodec: codec rate; S = packet payload size
– Packet consists of Header and Payload – Payload is the user-generated data
• Example: ADPCM voice codec fills packets with a 32-byte payload size– what is the packetization delay to fill one packet
payload?
codec: coder/decoder
71
Examples of physical media
• Types of media: – Twisted pair – Coaxial cable – Optical fiber – Wireless
• Radio frequencies (RF)• Infra-red (IR)
73
Layers in an unshared single-link network
Tasks Layer
Application-specific tasks, e.g. email, web
Application layer
Error control and flow control
Data-link layer(DLL)
Send/receive data bits Physical layer (PHY)
74
Data link layer:Error control
• Error detection– Parity check– Cyclic Redundancy Code (CRC) or
polynomial codes• Error correction
– Automatic Repeat reQuest (ARQ)– Forward Error Correction (FEC)
75
Error correction:Different ARQ schemes
• Stop-and-Wait• Sliding window
– Go-Back N– Selective repeat
76
ARQ error/loss detection and recovery
• Send a frame• Hold frame in a retransmission buffer at the sender so that if
there is a loss/error, the frame can be resent• Wait for Acknowledgment (ACK) from receiver• If a received frame had errors, the receiver detects the
presence of errors using CRC, and then sends a notification – sender resends errored frame
• But what happens if the frame itself was lost or the receiver's notification of an error is lost?
• Solution:– Start a timer at the sender upon sending a frame– If timer times out before ACK arrives, retransmit frame
77
Flow control problem
• Rates of the transmitter and receiver at the physical layer are matched. • The flow control problem arises because the layer above the DLL at the
receiver does not deplete the buffer at the same rate at which data is being passed to the DLL at the sender (Rsnd Rrcv)
Data-link layer (DLL) Data-link layer (DLL)
Physical layerPhysical layer
Host Switch or host
Data units Receivebuffer
H
T
T H
H
T
Rsnd Rrcv
transmission rate: r
78
Different rates
• Rsnd: Rate at which the higher layer passes data to the DLL at the sender
• Rrcv: Rate at which the higher layer removes data from the DLL buffer at the receiver
• r: physical-layer transmission rate
79
Techniques for flow control
• Flow control mechanisms prevent buffer overflow by regulating the rate at which source is allowed to send information– Stop-and-wait flow control– ON-OFF flow control– Sliding window flow control– Rate based flow control (skip for this
class)
80
Layers in an shared single-link network
Tasks Layer
Application-specific tasks, e.g. email, web
Application layer
Error control, flow control, and multiplexing/MAC
Data-link layer (with MAC sublayer)
Send/receive data bits Physical layer(PHY)
MAC
• MAC: Medium Access Control or Media Access Control– Set of functions to support the sharing of a
single link by multiple endpoints
• MAC vs. Multiplexing– The term "MAC" is used to describe sharing
techniques on multi-access links– The term "multiplexing" is used to describe
sharing techniques on point-to-point links
81
Types of links
• Multi-access links– Typically used to connect multiple hosts to a
switch – Cheaper than point-to-point links– Mostly used in wireless networks– Sometimes in wired networks through hub
• Point-to-point links– Typically used between switches– Increasing typical between hosts and switches
in wired networks
Host Host Host......
Switch
HostSwitch Host
Host
82
Classification of Multiplexing/MAC techniques
Multiplexing/MAC techniques
Circuit-based multiplexing
Position based: • space (reuse: cellular)• time • frequency
Each multiplexed data stream occupies a different position
Packet-based multiplexing
Packet header based:• header carries destination address
Each multiplexed data stream consists of packets with headers carrying corresponding destination addresses 83
Packet-based multiplexing
• For point-to-point links– Scheduling techniques
• For multi-access links– Random access schemes
84
Examples
Circuit-based multiplexing
Packet-based multiplexing
Multi-access wireless link Cellular (FDMA/TDMA)
IEEE 802.11 (WiFi)
Multi-access wired link Ethernet hub
Point-to-point switch-to-switch link
PDH, SONET, WDM Ethernet switch
Point-to-point endpoint-to-switch link
Plain Old Telephone Service (POTS)(space division multiplexing)
Ethernet link
Multiplexing/MAC schemesTypes of links
Phone links from residences carry only one phone call andhence it is space-division multiplexing; DSL: new technology for multiplexing data with voice
85
86
Layers in a multiple-link network
Tasks Layer
Application-specific tasks, e.g. email, web Application layer
End-to-end error control and flow control Transport layer
Switching Network layer
Error control, flow control, multiplexing/MAC
Data-link layer(with MAC sublayer)
Send/receive data bits Physical layer
Controller
1
2
3
P
Line card
Line card
Line card
Line card
Spa
ce s
witc
h
Line card
Line card
Line card
Line card
1
2
3
P
Input ports Output ports
Data path Control path
…………
Generic switch architecture(circuit or packet)
• Line cards– Packet switch: header based
mux/demux– Circuit switch: position based
mux/demux
• Space switch– Crossbar, Clos
• Controller– Processor: routing protocols
and signaling protocols
87
Types of switches
Line card
(multiplexing)
Controller
(admission
control or not)
Circuit-switch (CS)(position-based)
Packet-switch (PS)
(header-based)
Connectionless (CL)(no admission control)
e.g., datagram: IP routers; Ethernet switches
Connection-oriented (CO)(admission control)
e.g., telephone network circuit switches, SONET switches
Virtual-circuit switches: MPLS
• Routing: Required in controller for all three types of switches• Signaling: Admission control – hence required only for connection-
oriented switches (not included in this course) 88
A network of connectionless packet
switches• Control path
– Switch controllers exchange routing information and create forwarding tables
• Data path– Packets carrying user data and
destination addresses in headers are switched from input link to appropriate output link based on forwarding table entry for destination address
89
90
Layers in a multiple-link network
Tasks LayerApplication-specific tasks, e.g. email, web Application layer
End-to-end error control and flow control
Transport layer
Switching Network layer
Error control, flow control, multiplexing/MAC
Data-link layer(with MAC sublayer)
Send/receive data bits Physical layer
Mechanisms considered under DLL
Other functions of transport protocols
• Transport protocols include functions that augment the services offered by underlying network layers– port-multiplexing to carry data from
many processes at the end hosts– if network layer is connectionless
packet switched• transport layer may include congestion
control to handle losses in switch buffers
91
92
• Outline• Contemporary networks• Networks:
• from a simple single-link network to an internetwork
• Tasks and Layers• Mechanisms for each task Protocols and Protocol reference models• Standards
Status check
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
Kurose and Ross 93
What’s a protocol?
a human protocol a computer network protocol
Hi
Hi
Got thetime?
2:00
TCP connectionresponse
Get http://www.awl.com/kurose-ross
<file>time
TCP connectionrequest
Kurose and Ross 94
95
Example of a protocol
• Parity bit added in data-link layer trailer for error detection• Protocol: (1) parity bit is added (2) sent at end (3) even or odd?
– Agreement on these aspects is required at the data-link layer implementations at the two ends: sending and receiving
– these rules constitute the data-link layer protocol
Packet(110010100...)
Packet(110010100...)
Parity
Packet(110010100...)
Packet(110010100...)
Parity
Data-link layer (layer 2) Data-link layer (layer 2)
Physical layer (layer 1)Physical layer (layer 1)
Host Switch or host
Adds parity Checks parity
96
Layers, protocols, and interfaces
Physical layer
Data-link layer
Network layer
Transport layer
Application layer
Courtesy: A. Tanenbaum & Prentice Hall
sourceapplicatio
ntransportnetwork
linkphysical
HtHn M
segment Ht
datagram
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage M
Ht M
Hn
frame
Kurose and Ross 97
99
What is a protocol reference model?
• A grouping of layers• Recall that we defined a layer as a
grouping of tasks.
100
Two protocol reference models
• OSI (Open Systems Interconnection) reference model – has two more layers, presentation and
session
• The TCP/IP reference model– Used in the Internet
101
OSI protocol reference model(lists tasks for layers)
• Application layer: – Implements some application involving communications
• Presentation layer: – Allow applications to interpret meaning of data – Examples: e.g., encryption, compression, machine-specific conventions
(Endian)• Session layer:
– Dialog control (track whose turn it is to send)– Token management (avoid same critical operation at the same time)– Synchronization (checkpoint long transmissions and continue after a crash)
• Transport layer: – End-to-end error control, flow control
• Network layer:– Switching
• Data link layer: – Error control, flow control– Additionally for shared links: multiplexing/MAC
• Physical layer: – Transmitting and receiving data bits over a communication link
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TCP/IP Protocol Stack (Protocols in each layer)
• Application layer protocols: – web: Mozilla client; Apache server; http (hypertext transfer protocol) – email: Outlook client; mail server; smtp (simple mail transfer
protocol)– file transfers: SecureFX client; SFTP server; ftp (file transfer protocol)– remote login: telnet, SecureCRT
• Transport layer protocols:– TCP (Transmission Control Protocol)– UDP (User Datagram Protocol)– RTP (Real-time Transfer Protocol)
• IP (Internet Protocol):– IP (Internet Protocol)– Provides packet forwarding between networks
• Link-layer/Physical-layer protocols:– all the protocol layers necessary for communication across a specific
network
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Layers in the TCP/IP model
Application Layer
TCP (Layer 4)
IP (Layer 3)
Layer 2 + Layer 1 (industry)
This usage of the term “Layer 2” for anything below IP layer has led to industry usage of the term “Layer 2 switch” to describe a switch within a subnetwork, e.g., Ethernet switch
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• Outline• Contemporary networks• Networks:
• from a simple single-link network to an internetwork
• Tasks and Layers• Mechanisms for each task • Protocols and Protocol reference modelsStandards
Status check
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Standards
• IETF: Internet Engineering Task Force http://www.ietf.org
• ITU-T: International Telecommunications Union – Telecommunications (part of the United Nations)
• ANSI: American National Standards Institute
• IEEE: Institute of Electrical and Electronics Engineers
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Role of standards bodies
• To define protocol specifications, which includes– message formats– parameter formats
• Goal: allow protocol implementations from two different vendors to communicate– analogy: two people speaking the same language
have to obey the rules of the language
• To allow for product differentiation– implementation details are not standardized
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IEEE 802 Standards
The 802 working groups. The important ones are marked with *. The ones marked with are hibernating. The one marked with † was given up.
Courtesy: A. Tanenbaum & Prentice Hall
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Summary
• Key points of this lesson:– List the various tasks required for different
communication instances (across a unshared single link, shared single link, across a multiple-link path via a switch)
– How are these tasks grouped into layers?– List two modes used in application-layer
implementations– List mechanisms for each task type– What is the purpose of protocols? Why is a protocol
needed for each layer?– What are the two main protocol reference models?– Name the main standards bodies. Why are standards
bodies important in this field?