6 slides on page - Andrzej Duda

15 Pages·2003·0.94 MB·English

LAN
LANs
Computer Networking Our goals: Overview:
ß understand principles ß multiple access protocols
behind LANs: ß example LANs:
ß sharing a broadcast ß Ethernet
Local Area Networks channel: multiple ß 802.11
access ß token ring
ß link layer addressing ß token bus
ß LAN interconnection ß link layer addressing
Prof. Andrzej Duda ß instantiation and ß LAN interconnection
[email protected] implementation of ß hubs, bridges, switches
various LAN
technologies
http://duda.imag.fr
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Characteristics Data link layer in LANs
ß Shared channel
ß multiplexing (TDM, FDM, or CDM)
ß fixed allocation: wasted badwidth if no active sources
ß statistical multiplexing (multiple access)
ß suitable for bursty traffic - channel used at the full capacity
ß Most of LANs
ß no retransmission (up to upper layers)
Metcalfe’s Etheret ß WLANs
ß Short distances (100 m - 1 km) sketch
ß ACK of delivery
ß High bit rate (10 Mb/s, 100 Mb/s, 1 Gb/s)
ß Shared communication channel
ß Used in a distributed environment
ß shared equipment, shared data
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Multiple Access protocols
Multiple Access Protocols
ß single shared communication channel
ß two or more simultaneous transmissions Three broad classes:
by nodes: interference ß Random Access (Ethernet, 802.11)
ß only one node can send successfully at a time ß allow collisions
ß multiple access protocol:
ß “recover” from collisions
ß distributed algorithm that determines how ß Tokens - “Taking turns” (Token Ring, FDDI)
stations share channel, i.e., determine when
station can transmit ß tightly coordinate shared access to avoid collisions
ß communication about channel sharing must ß Distributed Queue (DQDB)
use channel itself! ß use the channel in the arrival order
ß what to look for in multiple access protocols:
ß synchronous or asynchronous ß Goal: efficient, fair, simple, decentralized
ß information needed about other stations
ß robustness (e.g., to channel errors)
ß performance
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LAN
LAN Reference model
LAN technologies
ß Data link layer: LLC 802.2
ß services, multiple access Data link
ß LAN technologies MAC MAC MAC
ß addressing Physical 802.3 802.4 802.5
ß Ethernet, 802.11
ß repeaters, hubs, bridges, switches
ß virtual LANs ß LLC - Logical Link Control: IEEE 802.2 (ISO 8802.2)
ß MAC - Medium Access Control
ß IEEE 802.3 (ISO 8802.3): CSMA/CD
ß IEEE 802.4 (ISO 8802.4): token bus
ß IEEE 802.5 (ISO 8802.5): token ring
ß IEEE 802.11: CSMA/CA
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IEEE 802.3 - Ethernet Coding
host 100 ns time
transceiver
ß Synchronous transmission
ß receiving station locks on 10 MHz - preamble
ß Manchester coding
repeater
terminator
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Random Access protocols CSMA/CD (Collision Detection)
ß CSMA/CD (Carrier Sense Multiple Access/ Collision
ß When node has packet to send
Detection)
ß transmit at full channel data rate R.
ß no a priori coordination among nodes ß carrier sensing, deferral if ongoing transmission
ß two or more transmitting nodes -> “collision”, ß collisions detected within short time
ß colliding transmissions aborted, reducing channel wastage
ß random access protocol specifies:
ß persistent transmission
ß how to detect collisions ß collision detection:
ß hreotwra ntos mreiscsoivoenrs )from collisions (e.g., via delayed ß easy in wired LANs: measure signal strengths, compare
transmitted, received signals
ß Examples of random access protocols:
ß difficult in wireless LANs: receiver shut off while transmitting
ß ALOHA, slotted ALOHA
ß CSMA, CSMA/CD (Ethernet), CSMA/CA (802.11)
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2
LAN
CSMA/CD algorithm CSMA / CD Collision
i = 1
while (i <= maxAttempts) do ß A senses idle
listen until channel is idle channel, starts A B
transmitting
transmit and listen 0
wait until (end of transmission) or ß shortly before T, T
B senses idle
(collision detected)
channel, starts
if collision detected then
transmitting
stop transmitting, send jam bits (32 bits)
else
wait for interframe delay (9.6 ms)
leave
wait random time
increment i
end do
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CSMA / CD Jam Signal Random retransmission interval
ß B senses r = random (0, 2k -1)
collision,
k = min (10, AttemptNb)
continues to
A B
transmit the jam
signal (32-bit) 0 t =r¥51.2ms, rŒ[0, 2k -1]
r
ß A senses T
collision,
continues to ß slot time = 51.2 ms
transmit the jam ß 1st collision, r = 0, 1
signal t2 ß 2nd collision, r = 0, 1, 2, 3
ß 10th, r = 0, 1, …, 1023
ß 15th, stop
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CSMA / CD Retransmission
CSMA/CD performance
A B
0 ß Maximum utilization of Ethernet (approximation)
ß A waits random T
time t1 q ª 1 / ( 1 + C a )
ß B waits random
time t2=slottime t2
< t1 =2*slottime where a = 2Db / L,
ß B senses channel D = propagation delay, b = bit rate,
idle and transmits L = frame size
ß A senses channel
C is a constant:
busy and defers to
B ß C = 3.1 is a pessimistic value;
ß A now waits until ß C = 2.5 is an approximate value based on simulations
channel is idle t1
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LAN
Frame format (Ethernet v.2) Frame format (802.3)
preamble dest sourcelength data pad CRC
preamble dest source type data CRC 8 bytes 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes
8 bytes 6 bytes 6 bytes 2 bytes 46 - 1500 bytes 4 bytes
ß Preamble LLC frame DSAP SSAP control data
• synchronization : 10101010….0101011 1 byte 1 byte 1 byte
• Addresses (xAA) (xAA) (x03)
• unique, unicast and multicast (starts with the first bit 1)
• broadcast: 11111…11111 SNAP frame prot. id type data
• Type 3 bytes
2 bytes
• upper layer protocol (IP, IPX, ARP, etc.) (x00)
ß SNAP (Subnet Access Protocol) used in bridge management
(any length of data: 0 - 1492)
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Addressing Addressing
ß MAC address: 48 bits = adapter identifier ß Data on Ethernet is transmitted least significant bit of
ß sender puts destination MAC address in the frame first byte first (a bug dictated by Intel processors)
ß all stations read all frames; keep only if destination ß Canonical representation thus inverts the order of bits
address matches inside a byte (the first bit of the address is the least
ß all 1 address (FF:FF:FF:FF:FF:FF) = broadcast significant bit of the first byte)
ß examples of addresses:
ß 01:00:5e:02:a6:cf (a group address)
ß 08:00:20:71:0d:d4 (a SUN machine)
B C ß 00:00:c0:3f:6c:a4 (a PC )
ß 00:00:0c:02:78:36 (a CISCO router)
ß FF:FF:FF:FF:FF:FF the broadcast address
MAC address A D
08:00:20:71:0d:d4 00:00:c0:3f:6c:a4
01:00:5e:02:a6:cf (group address)
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Interconnecting LANs Repeaters
ß Function of a simple, 2 port
Why not just one big LAN? repeater:
ß Limited amount of supportable traffic: on single LAN, all stations ß repeat bits received on one port
must share bandwidth to other port
ß limited distance ß if collision sensed on one port,
ß large “collision domain” (can collide with many stations) repeat random bits on other port
ß processing broadcast frames ß One network with repeaters = Repeater
LAN evolution one collision domain
ß increase the bit rate: 10Mb/s, 100Mb/s, 1 Gb/s ß Repeaters perform only
ß from hubs to switches physical layer functions (bit
repeaters)
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LAN
From Repeaters to Hubs 10 BASE T Hubs
Multiport
ß Multiport repeater (n ports),
Repeater
logically equivalent to:
hub
ß n simple repeater
ß connected to one internal
Ethernet segment
hub hub
ß Multi-port repeaters make it
possible to use point-to-point
segments (Ethernet in the Ethernet Hub
box)
S1
ßß efaauslet iosfo lmataionnagement S2 MpuRoletr-it- ß Tßreheu bto (préopléotgeuyr (mstualtri)port)
S3 UTP segment peater ß max. 4 hubs
to other hub
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10 BASE T 10BaseT and 100BaseT
ß 10/100 Mbps rate; latter called “fast ethernet”
hub ß T stands for Twisted Pair
ß Hub to which nodes are connected by twisted pair,
thus “star topology”
ß CSMA/CD supported by hubs
host
ß Two pairs ß Hub - host
ß emission ß straight cable
ß reception ß Hub - hub
ß RJ-45 jack ß inversed cable
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Gigabit Ethernet Gigabit Ethernet
ß use standard Ethernet frame format ß 1000 BASE T
ß allows for point-to-point links and shared broadcast ß over twisted pair (25 m)
channels ß 1000 BASE SX
ß in shared mode, CSMA/CD is used; short distances ß short wavelength (850 nm) over multimode (500 m)
between nodes to be efficient ß 1000 BASE LX
ß Full-Duplex at 1 Gbps for point-to-point links ß long wavelength (1300 nm) over multimode (550 m) and single-
mode fiber (10 km)
ß 1000 BASE LH (Long Haul)
ß greater distance over 10 µm single-mode (500 m)
ß 1000 BASE ZX
ß extended wavelength (1550 nm) over 10 µm single-mode (70 km)
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LAN
Bridges Bridges – interconnection at layer 2
ß Link Layer devices: operate on Ethernet frames,
port 1 port 3 Forwarding Table examining frame header and selectively forwarding
Bridge Dest Port frame based on its destination
A C MAC Nb ß Bridge isolates collision domains since it buffers
Repeater port 2 addr frames
A 1 ß When needs to forward a frame on a segment,
B 2
B C 3 bridge uses CSMA/CD to access the segment and
D D 2 transmit
ß Can connect different type Ethernets, since it is a
buffering device
ß Bridges are intermediate systems, or switches, that ß Two main types of bridges: transparent bridges and
forward MAC frames to destinations based on MAC spanning tree bridges (guarantee no loops)
addresses
ß Transparent bridges: learn the Forwarding Table
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Collision domains
Bridges vs. Routers
ß both store-and-forward devices bridge
ß routers: network layer devices (examine network layer headers)
ß bridges are Link Layer devices (look into MAC headers)
ß routers are more complex
hub hub
ß bridges are plug-and-play
ß Bridges separate collision domains
ß a bridged LAN maybe much larger than a repeated LAN
ß there may be several frames transmitted in parallel in a bridged
LAN
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Repeaters and Bridges in OSI Model
Ethernet Switches – layer 2
Application Application ß layer 2 (frame) forwarding,
5 to 7 Presentation Presentation 5 to 7 filtering using LAN addresses
Session Session
ß Switching: A-to-B and A’-to-
4 Transport Transport B’ simultaneously, no
3 Network Network 4
L2 PDU L2 PDU collisions
2 MLLACC (MAC Frame) MAC (MAC Frame) MLLACC 32 ß large number of interfaces
1 Physical Physical Physical Physical 1 ß often: individual hosts, star-
End System Repeater Bridge End System connected into switch
ß Bridges are layer 2 intermediate systems ß Ethernet, but no
collisions!
ß Repeaters are in layer 1 intermediate systems
ß Routers are layer 3 intermediate systems (IP routers)
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LAN
Ethernet Switches (more)
Switching
Dedicated
ß Store-and-forward
ß receive frame, check if valid, retransmit
ß 50 ms delay for a 64 bytes frame
Shared ß Cut through
ß address read, retransmit
ß 20 ms delay for a 64 bytes frame
ß transmission of non-valid frames
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Full duplex Ethernet Gigabit Ethernet
ß 1000 BASE T
ß A shared medium Ethernet cable is half duplex ß over twisted pair (25 m)
ß Full duplex Ethernet = a point to point cable, used in ß 1000 BASE SX
both directions ß short wavelength (850 nm) over multimode (500 m)
ß no access method, no CSMA/CD ß 1000 BASE LX
ß 100 Mb/s and Gigabit Ethernet switches use full ß long wavelength (1300 nm) over multimode (550 m) and
duplex links to avoid distance limitations and to single-mode fiber (10 km)
guarantee bandwidth for stations ß 1000 BASE LH (Long Haul)
ß Requires full duplex adapters at stations ß greater distance over 10 µm single-mode (500 m)
ß 1000 BASE ZX
ß extended wavelength (1550 nm) over 10 µm single-mode
(70 km)
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Wireless LAN: 802.11b 802.11 - Physical layer
ß 802.11b: wireless LAN
ß nominal bit rate of 11 Mb/s, degraded to 5.5, 2, 1 Mb/s ß 802.11b
ß 6.5 Mb/s at application layer (file transfer) ß frequency band of 2.4 GHz: [2,4 GHz ; 2,48 GHz]
ß shared radio channel, 2.4 GHz band, 13 channels (3 non ß nominal bit rate of 11 Mb/s
overlapping of 22 MHz) ß passes through concrete
ß DSSS (Direct Sequence Spread Spectrum), 1 bit Æ chipping ß 802.11g
sequence
ß frequency band of 2.4 GHz
ß coverage 50m, open air 100m ß nominal bit rate of > 22 Mb/s
ß MAC layer
ß 802.11a
ß DCF (Distributed Coordination Function) ß frequency band of 5 GHz: [5,15 GHz ; 5,825 GHz]
ß CtoS MEtAh/eCrAn e(tC, anrori ecro lSliesinosne dMeuteltciptiloen Access/Collision Avoidance), similar ß nominal bit rate of 54 Mb/s
ß PCF (Point Coordination Function) ß 6, 9, 12, 18, 24, 36, 48, 54 Mb/s, (6, 12, 24 Mb/s mandatory)
ß polling, optional ß LOS - Line-of-Sight (no obstacles)
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LAN
802.11 - Physical layer Channel selection
Europe (ETSI)
channel 1 channel 7 channel 13
2400 2412 2442 2472 2483.5
22 MHz [MHz]
US (FCC)/Canada (IC)
channel 1 channel 6 channel 11
2400 2412 2437 2462 2483.5
22 MHz [MHz]
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Infrastructure vs. ad-hoc 802.11 - infrastructure
ß Station (STA)
infrastructure 802.11 LAN 802.x LAN ß ttoer tmhien awl irweiltehs sa cmceesdsiu mme acnhdan riasdmios
network
AP: Access Point contact to the access point
AP wirAePd network AP STA1 BSS1Access Portal ß ßBagrsaircdo iuSop ef rroevfq icsuteea ntSicoeynts ( uBsSinSg) the same
Point
ß Access Point
Distribution System
ß station integrated into the
Access wireless LAN and the distribution
ESS Point system
ad-hoc network
BSS2 ß Portal
ß bridge to other (wired) networks
ß Distribution System
STA2 802.11 LAN STA3 ß ionntee rlcoognicnaelc ntieotnw noerktwork to form
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802.11 802.11 DCF - CSMA/CA
ß Inter-frame spacing contention window
ß SIFS (Short Inter Frame Spacing) DIFS DIFS (mraencdhoamniiszmed) back-off
ß 10 ms, for ACK, CTS, polling response
ß PIFS (PCF IFS) medium busy next frame
ß for time-bounded service using PCF direct access if t
ß DIFS (DCF IFS) medium is free ≥ DIFS slot time
ß 50 ms, for contention access ß Channel idle during DIFS, transmit frame
ß If the medium is busy, wait for a free DIFS and a
random back-off time (collision avoidance, multiple of
DIFS DIFS slot-time)
PIFS
medium busy SIFS contention next frame ß If another station uses the medium during the back-off
time of the station, the back-off timer stops (fairness)
t
direct access if
medium is free ≥ DIFS
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LAN
CSMA/CA (Collision Avoidance)
802.11 - CSMA/CA
A B ß Sending unicast packets
ß Channel idle during
ß station has to wait for DIFS before sending data
DIFS, transmit frame DIFS ß receivers acknowledge at once (after waiting for SIFS) if the
ß Frame received packet was received correctly (CRC)
correctly, wait SIFS, and ß automatic retransmission of data packets in case of
send ACK transmission errors
data
DIFS
data
sender
SIFS SIFS
receiver ACK
ACK other DIFS data
stations t
waiting time contention
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CSMA/CA (Collision Avoidance)
Contention
A B
ß If channel busy, defer.
T(N)
DIFS SLOT SIFS Then, if idle during DIFS, DIFS
wait random interval
data ACK (multiple of the slot) and contention
t transmit window
backoff time
ß If channel busy, wait again
ß Backoff time - random interval until medium idle for at slot
ß Contention Window: uniform distribution [0, CW] * SLOT least DIFS
ß CW: CWmin = 31, CWmax = 1023 ß Contention window doubles data
ß SLOT = 20 ms with each collision -
ß T(N) should also include time wasted in collisions exponential back-off
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Hidden Terminal effect
802.11 - contention
ß Hidden terminals: A and B cannot hear each other
DIFS DIFS DIFS DIFS because of obstacles or signal attenuation; so, their
station1 busy packets collide at B
busy
station2
exponential
busy backoff
station3
busy
station4 collision
busy
station5 t
elapsed backoff time
busy medium busy
residual backoff time
packet arrival at MAC
shortest backoff time 53 54
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LAN
RTS/CTS Extension Register to Access Point
A B
ß CTS (Clear To Send)
“freezes” stations within DIFS
range of receiver (hidden
from transmitter); this RTS Mobile
Sign-on (Addr)
prevents collisions by SIFS
hidden station during data OK (NWID) Beacon
CTS
transfer
ß RTS (Request To Send) and SIFS
CTS are very short:
collisions are very unlikely data Access point Access point
(the end result is similar to Ethernet
Collision Detection) SIFS addressport
ACK Addr Wireless
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Hand-off Bluetooth
ß Replaces cables
ß short range (10m), low power, cheap
ß 2.4 GHz band
Hand-off Mobile ß FHSS (Frequency Hopping Spread Spectrum)
ß piconet
ß all devices share the same hopping sequence
OK (NWID) ß one master, seven slaves
ß bit rate: around 1 Mb/s
ß symmetric connections - 432.6 kb/s
ß asymmetric - 721 kb/s, 57.6 Kb/s
Access point Hand-off Access point ß access method: polling, reservation
Ethernet
addressport
Addr Wireless
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IEEE 802.4
Physical layer
ß Token Bus
ß industrial LAN 1 0
ß Physical layer
ß modulation (broadband)
ß coaxial cable 75 W
ß 1, 5, 10 Mb/s bit rate
ß Access method code violation
ß token on a virtual ring
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