Transcript The Medium Access Sublayer

Medium Access Control
Channel Allocation
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Static channel allocation in LANs and
MANs
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FDMA, TDMA, CDMA
Dynamic channel allocation in LANs and
MANs
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MAC protocols: with collisions, polling, token
Static Channel Allocation
• Delay for one fast channel:
1
D
C / E[ L]  
• Delay for multiple FDM slower
channels:
DFDM
1

 NT
C /( NE[ L])   / N
Poisson Process
• Probability of k arrivals in time t:
(  t ) k e  t
P[ A  k ] 
k!
• Probability that interarrival time
exceeds t:
P[no arrivalin t ]  et
M/M/1 Queue
• Queue equations
p0 (t  dt)  (1  dt) p0 (t )  dtp1 (t )
p j (t  dt)  (1  (   )dt) p j (t )  dtpj 1 (t )  dtpj 1 (t )
• Delay for multiple FDM slower
channels:
j
p j  p0 ( /  )
Delay
• Little’s formula
E[Q]
E[ D] 
( /  )
• Delay is:
1
1
E[ D][packets] 

1  /  1 
Static Channel Allocation
• Delay for one fast channel:
1
D
C / E[ L]  
• Delay for multiple FDM slower
channels:
DFDM
1

 NT
C /( NE[ L])   / N
Dynamic Channel Allocation
1.
(a) Single channel
(b) Multiple channel
2.
(a) Collision
(b) Collision-free
3.
(a) Continuous Time.
(b) Slotted Time.
4.
(a) Carrier Sense.
(b) No Carrier Sense.
Multiple Access Protocols
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ALOHA
Carrier Sense Multiple Access (CSMA)
protocols
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CSMA/CD
CSMA/CA
Collision-Free protocols
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Reservation based
Token based
Pure ALOHA
In pure ALOHA, frames are transmitted at completely arbitrary times.
Pure ALOHA
Vulnerable period for the shaded frame.
ALOHA Throughput
• Throughput is S=GP0, where P0 is the probability
of successful transmission.
• The k frames per f frame slots is
( fG ) k e  fG
P[k ] 
k!
P0=e-fG
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For pure ALOHA f=2, for slotted ALOHA f=1, so:
Pure and Slotted ALOHA
Throughput versus offered traffic for ALOHA systems.
Carrier Sense Multiple Access
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1-Persistant CSMA
Nonpersistant CSMA
P-Persistant CSMA
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various
random access protocols.
CSMA with Collision Detection
CSMA/CD can be in one of three states: contention,
transmission, or idle.
Wireless LAN: CSMA with
Collision Avoidance
A wireless LAN. (a) A transmitting. (b) B transmitting.
Wireless LAN: CSMA-CA
The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.
DOCSIS (Data Over Cable Service Interface
Specification
Collision-Free Protocols:Reservations
The basic bit-map protocol.
Collision-Free Protocols: Bidding
The binary countdown protocol. A dash indicates silence.
Collision Free Protocols:
Fiber Distributed Data Interface (FDDI)
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Station transmits only when it has a token
Timers count the time while the token is away
Two timers determine how much data a station
may transmit, so that the token delay is limited
Limited-Contention Protocols
Acquisition probability for a symmetric contention channel.
Adaptive Tree Walk Protocol
The tree for eight stations.
WDMA Networks
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Stations may be connected to the optical star
coupler or onto the optical ring and utilize
multiple wavelengths.
Stations are equipped with the tunable/fixed
transmitters and tunable/fixed receivers.
Stations are tuned to the common control
channel where they compete for the medium.
Ethernet
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Ethernet, IEEE 802.3
10Base (10Mbps)
Fast Ethernet (100Mbps)
Gigabit Ethernet
Ethernet MAC Sublayer Protocol
Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
CSMA with Collision Detection
CSMA/CD can be in one of three states: contention,
transmission, or idle.
Performance of Non-Persistant CSMA/CD
Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990
Performance of 1-Persistant CSMA/CD
Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990
Comparison of CSMA and CSMA/CD
Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990
Ethernet Performance
Collision detection can take as long as 2 .
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
Back-Off Mechanism
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After a collision, user accesses medium
with probability 1/W where W is the
window size.
With each collision W doubles.
10Mbps Ethernet Cabling
The most common kinds of Ethernet cabling.
10Mbps Ethernet Cabling
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
Ethernet Cabling
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
10Mb Ethernet Coding
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
10 Mb Ethernet Collision Detection
10Base5 cabling,
Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998
10 Mb Ethernet Collision Detection
10Base2 and 10BaseT cabling,
Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998
Fast Ethernet
The original fast Ethernet cabling.
Fast Ethernet
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Auto negotiation enables communication
with 10Mb Ethernet
Manchester code → 4B/5B code
Full duplex mode is optional with using
PAUSE command
Switched Ethernet
A simple example of switched Ethernet.
Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.
Gigabit Ethernet
Gigabit Ethernet cabling.
Gigabit Ethernet
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Prioritization of fiber over copper
4B/5B coding → 8B/10B coding
Full duplex mode is preferred with PAUSE
message
Carrier extension, and frame bursting
introduced in half-duplex mode
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
A Sample HFC System
Downstream: 500 MHz shared by ~50,000 (broadcast)
200 MHz by 1200 (narrowcast)
Upstream: ~37 MHz shared by 300
oo
up
b
n
(4n/fiber)
Secondary Hub
oo
oo
HOME
oo
Fiber
Node
oo
oo
oo
RF Spectrum on coax:
return
oo
80 broadcast channels
broadcast
5-42 MHz
Sheryl Woodward, AT&T Labs-Research
oo
30 QAM channels
(~150 video channels)
narrowcast
550 MHz
750 MHz
Justification for Using Shared Medium
• Equivalent circuit rate (ECR) on a cable with
many users is the rate of a dedicated link that would
provide the same e.g. average delay (similar results is
obtained for 90th percentile page delay). By Shankar,
Jiang and Mishra:


tON
tON  tOFF  ECR 
ECR  r  1 
M   M 
1 

tON 
r 
 tON  tOFF 
where tON is the transmission tim, and tOFF is the think
time, r is the channel rate, tON/(tON+tOFF)<<1, on
periods have an exponential distribution.
Justification for Using Shared Medium
• Let’s calculate how many users can be allocated one
DOCSIS channel of 32Mbps to get the same experience
as DSL user with dedicated rate of 2Mbps. According to
traffic statistics page size is 68KB on average, and tOFF is
14.5s on average,
M
68KB / 32Mbps  14.5s 
2
1   
68KB / 32Mbps
 32 
0.017  14.5

 800
0.017
which is much more than 32/2=16 users. Price: high user
speed.
DOCSIS MAC Protocol
• Traffic that is transmitted downstream to the users is
controlled by CMTS (cable modem termination system)
in headend. It polices and shapes the traffic, and perform
algorithms such are WFQ and RED.
• Users requests are resolved at headend, and they are
informed about the resolution through the downstream
channel. If there is a collision of requests, users repeat
their requests according to exponential back-off
mechanism, otherwise they send data in specified time
slot(s).
QoS in DOCSIS
Service
QoS parameters
Access Mode
Applications
UGS
Unsolicited grant size, interval,
jitter
Isonchronous
Videoconferencing,
VoD, VoIP
UGS-AD
Unsolicited grant size, interval,
jitter;polling interval, jitter
Isonchronous, periodic
request polling
VoIP with silence
supression
rtPS
Polling interval, jitter
Periodic request polling,
piggybacking
reservation
VoIP
QoS in DOCSIS
Service
QoS parameters
Access Mode
Applications
nrtPS
Polling interval, min reserved
rate, max sustained rate, priority
Periodic request
polling, piggybacking
reservation, immediate
access
Demanding FTP
BE
min reserved rate, max sustained Normal, piggybacking
rate, priority
reservation, immediate
access
Telnet, FTP, WWW
CIR
Unspecified
Unspecified
Unspecified
Performance for BE service in DOCSIS
• Assume that requests form a Poisson process with rate
g, T is time slot duration, and Tp is a packet duration. The
throughput equals S=Tp/(Tp+I), where I is the average
time between packet transmissions.
• The probability of a packet transmission is equal to the
probability that there is only one request in some previous
time slot which is gTe-gT.
• The average time between transmissions is
I  T  i(1  Ps ) Ps  T / Ps
i 1
i
Performance for BE service in DOCSIS
• The throughput is
 e 

S
 1 
Tp  I  gTp 
gT
Tp
1
• It tends to 0 when g increases . Exercise: Find the
mapximum value of S in terms of T and Tp, and plot
graphs of S versus g, and different T/Tp.
• Protocol is unstable like ALOHA.
S

1
2
g
Wireless LANs
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Distributed coordination function (DCF)
Point coordination function (PCF)
The 802.11 Protocol Stack
Part of the 802.11 protocol stack.
The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.
The 802.11 MAC Sublayer Protocol
The use of virtual channel sensing using CSMA/CA.
The 802.11 MAC Sublayer Protocol
A fragment burst.
The 802.11 MAC Sublayer Protocol
Interframe spacing in 802.11.
The 802.11 Frame Structure
The 802.11 data frame.
802.11 Services
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Association
Disassociation
Reassociation
Distribution
Integration
Privacy
Data delivery
Broadband Wireless
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Fixed wireless
Larger bandwidth
The 802.16 Protocol Stack
The 802.16 Protocol Stack.
The 802.16 Physical Layer
The 802.16 transmission environment.
The 802.16 Physical Layer
Frames and time slots for time division duplexing.
The 802.16 MAC Sublayer Protocol
Service Classes
• Constant bit rate service
• Real-time variable bit rate service
• Non-real-time variable bit rate service
• Best efforts service
The 802.16 Frame Structure
(a) A generic frame.
(b) A bandwidth request frame.
Bluetooth
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Used for communication of the equipment
in the house, office
Interferes with IEEE 802.11
Bluetooth Architecture
Two piconets can be connected to form a scatternet.
Bluetooth Applications
The Bluetooth profiles.
The Bluetooth Protocol Stack
The 802.15 version of the Bluetooth protocol architecture.
The Bluetooth Frame Structure
A typical Bluetooth data frame.
Data Link Layer Switching
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Bridges from 802.x to 802.y
Local Internetworking
Spanning Tree Bridges
Remote Bridges
Repeaters, Hubs, Bridges, Switches, Routers, Gateways
Virtual LANs
Data Link Layer Switching
Multiple LANs connected by a backbone to handle a total load higher
than the capacity of a single LAN.
Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
Bridges from 802.x to 802.y (2)
The IEEE 802 frame formats. The drawing is not to scale.
Local Internetworking
A configuration with four LANs and two bridges.
Spanning Tree Bridges
Two parallel transparent bridges.
Spanning Tree Bridges (2)
(a) Interconnected LANs. (b) A spanning tree covering the
LANs. The dotted lines are not part of the spanning tree.
Remote Bridges
Remote bridges can be used to interconnect distant LANs.
Repeaters, Hubs, Bridges, Switches,
Routers and Gateways
(a) Which device is in which layer.
(b) Frames, packets, and headers.
Repeaters, Hubs, Bridges, Switches,
Routers and Gateways (2)
(a) A hub. (b) A bridge. (c) a switch.
Virtual LANs
A building with centralized wiring using hubs and a switch.
Virtual LANs (2)
(a) Four physical LANs organized into two VLANs, gray and white,
by two bridges. (b) The same 15 machines organized into two
VLANs by switches.
The IEEE 802.1Q Standard
Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded
symbols are VLAN aware. The empty ones are not.
The IEEE 802.1Q Standard (2)
The 802.3 (legacy) and 802.1Q Ethernet frame formats.
Summary
Channel allocation methods and systems for a common channel.