Download Media Access Control #7

Media Access Control
#7
Chapter 6
MAC
1
MAC
2
Outline
Why use MAC protocols
General classes of MAC protocols
Standard LAN protocols
Broadband Access
Deterministic
Random Access
Cable
DSL
Other..
Satellite Networks
Media Access Control:
Protocols provide:
Direct access to the media
Distributed control over
resource allocation
Typically broadcast
(real or virtual)
MAC
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MAC
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Media Access Control:
Advantages
High data rates
(open new applications)
Low cost
Local organizational control
Wireless is a broadcast media and efficient
use of resources is important
Enable sharing of resources
Mobility
via Wireless
Media Access Control
MAC protocols establish a set of rules that govern
who gets to use the shared transmission media in an
efficient manner.
Obstacle to perfect channel utilization
Finite propagation delay means that each users’
knowledge of the state of the system is imperfect
and thus they can not perfectly schedule
transmissions, i.e., some time will be wasted
attempting to learn the state of the system and/or
learning the fate of transmissions.
Lost messages
MAC
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Media Access Control
Perfect Knowledge would lead to FIFO
performance.
Performance of MAC protocols will be
compared to FIFO performance.
Ideal
MAC
Performance
MAC
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Impact of MAC Overhead
MAC Protocol 2
E[T]/E[X]
Transfer Delay
MAC Protocol 1
1
Load
Smax-1
Smax-2 1
ρ
Adapted from: Leon-Garcia & Widjaja: Communication Networks
MAC
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MAC
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Alternative Media Access Control
Strategies
Static Allocation
FDM
TDM
Problems
Management; not easy to add users
– Requires signaling
Wasteful in resources for bursty traffic
Example
A transmission media has a rate of 10 Mb/s and supports
50 users. The system uses static allocation. A user has a 1
Mbyte file to transmit. The file transfer time is:
Alternative Media Access Control
Strategies
Suppose
you send a message to all
the other 49 users saying, ‘I need the
whole channel for about 1sec, do not
use it, please’
As long as the overhead incurred
in sending the message is less than
39 sec. the user will get better
performance.
MAC
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Alternative Media Access Control
Strategies
Deterministic
Polling
Token
networks
Random Access
ALOHA
Carrier
sense multiple access (CSMA)
CSMA with collision detection (CSMA/CD)
MAC
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Alternative Media Access Control
Strategies: Dynamic allocation of resources
Deterministic; Polling, Token Ring &Token
Bus
Advantage: the maximum time
between users chances to transmit is
bounded. (assuming a limit on the
token holding time)
Disadvantage: Time is wasted polling
other users if they have no data to send.
The technology does not scale
MAC
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MAC
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Deterministic Protocols
Roll
Call Polling
Master/slave
arrangement
Master polls each node; Do you
have data to send?
If the polled node has data it is
sent otherwise next node is
polled.
Deterministic Protocols
Node
Node
Master
Node
Node
Maximum token holding time= Maximum time a
station is allowed to transmit before passing on the
permission to transmit, the token.
MAC
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MAC
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Deterministic Protocols
Hub Polling
No master station
Each nodes polls the next node in turn
Node
Node
Node
Node
Node
Deterministic Protocols
Example:
# nodes = 10
Link rate = 1 Mb/s
Packet Size = 1000 bits
Assume Low load no queueing
Assume node interface delay = 0
0.1 ms between nodes (30 km/(3x108m/s) = 0.1ms)
Find the effective transmission rate and efficiency.
–
–
–
–
Repeat for link rate = 10 Mb/s
–
–
–
–
On average destination is 5 nodes away .5 ms
Time to transmit 1000 bits = 0.5 ms + 1 ms = 1.5 ms
Effective transmission rate = 1000 bits/ 1.5 ms = 666Kb/s
Efficiency = (666 Kb/s)/(1000 Kb/s) = 0.66
On average destination is 5 nodes away .5 ms
Time to transmit 1000 bits = 0.5 ms + .1 ms = .6 ms
Effective transmission rate = 1000 bits/ .6 ms = 1.67 Mb/s
Efficiency = (1.67 Mb/s)/(10 Mb/s) = 16.7%
Conclusion Polling does not scale with link rate
MAC
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MAC
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Alternative Media Access Control
Strategies: Random Access
Each node sends data with limited
coordination between users:
No explicit permission to transmit
Total chaos: Send data as soon as ready
Limited chaos: Listen before sending data, if
the channel is busy do not send.
Further Limiting chaos: Listen before sending
data, continue listening after sending and if
collision with another transmission stop
sending.
[Carrier Sense Multiple Access with Collision Detection
CSMA/CD]
MAC
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MAC
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Alternative Media Access Control
Strategies: Random Access
Advantage: Simple
Disadvantage:
No
guarantee that you will
ever get to send.
The MAC protocol
technology does not scale
Random Access Protocols:
Assumptions
Overlap
in time and space of two
or more transmissions causes a
collision and the destruction of all
packets involved.
[ No capture effects]
One
channel
For analysis no station buffering
MAC
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MAC
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Random Access Protocols:
Assumptions
Time-Alternatives
Synchronization between
users (Slotted
time)
No synchronization between users
(unslotted time)
Knowledge of the channel stateAlternatives
Carrier
sensing (Listen before talk-LBT)
Collision detection
Random Access Protocols
Strategies
ALOHA
Backoff
No coordination between users
Send a PDU, wait for acknowledgment,
if no acknowledgment ASSUME collision then
backoff and try again
Select “random” time to attempt another
transmission
Slotted ALOHA
Same as ALOHA only time is slotted
MAC
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MAC
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Random Access Protocols
Strategies
p-persistent CSMA
Listen to channel, on transition from busy to idle
transmit with probability p
After sending the PDU, wait for
acknowledgment,
if no acknowledgment then backoff and
retransmit
Non-persistent, if channel busy then reschedule
transmission
1-persistent, Transmit as soon as idle
Random Access Protocols
Strategies
CSMA/CD
1-persistent
but continue to sense
the channel, if collision detected
then stop transmission.
CSMA/CD is used in 10, 100
Mb/s, and 1 Gb/s Ethernet
MAC
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MAC
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Limitations on Random
Access Protocols
Distance
Time
to learn channel state Propagation time
Speed
Time to learn channel state Clocking speed
Random Access Protocols
Analysis of ALOHA:
Goal: Find Smax
Protocol Operation
Packet of length L (sec) arrives at station i
– Station i transmits immediately
– Station i starts an acknowledgment timer
If no other station transmits while i is transmitting
then success
Else a collision occurred
Station i learns that a collision occurred if the
acknowledgment timer fires before the
acknowledgment arrives
MAC
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Random Access Protocols
Analysis of Aloha
collision detected then station i
retransmitts at a later time, this time
is pseudo-random and is determined
by a backoff algorithm
If
Design
Issue:
Determine
the maximum normalized
throughput for an Aloha system
MAC
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Random Access Protocols
Analysis of Aloha
Assumptions
1.
= Average number of new message arrival to the system
2.
= Average number arrivals to the system, i.e.,
new arrivals + retransmissions
3. The total arrival process is Poisson
4. Fixed Length Packets
MAC
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Random Access Protocols
Analysis of Aloha
Collision Mechanism
Arrival
Arrival
Arrival
Target Packet
2L
Target packet is vulnerable to collision for 2L Sec.
MAC
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Random Access Protocols: Analysis of Aloha
New Retransmitted
Delay
Load
0.18
MAC
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Random Access Protocols
Analysis of Slotted Aloha
Target Packet
Synchronization reduces the vulnerable period
from 2L to L so the maximum throughput is
increases to 36%
MAC
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Random Access Protocols
Performance of Unslotted and Slotted Aloha
From: “Computer Networks, 3rd Edition, A.S. Tanenbaum.
Prentice Hall, 1996
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Random Access Protocols
CSMA Protocols
Listen to the channel before transmitting to reduce the
vulnerable period
Let D = maximum distance between nodes (m)
Let R = the transmission rate (b/s)
Let c = speed of light = 3 x 108 m/s
The propagation time = D/(kc)=τ (sec)
k is a constant for the physical media:
k = .66 for fiber, k=.88 for coax
Example: 1 km
- Free space propagation time
- Fiber propagation time
- Coax propagation time
= 3.33 us
= 5.05 us
= 3.79 us
MAC
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Random Access Protocols
CSMA Protocols
Assume node A transmits at time t and node
B at t -x, where x 0
(That is, Node B transmits right before it hears A)
If after 2D/kc sec. no collision occurred, then
none will occur
Let a= τ/L=(D/kc)/L = normalized length of
the bus
Remember L(sec) =
(Packet Length [bits])/R [b/s]
As a --> 1, CSMA performance
approaches Aloha performance
MAC
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Random Access Protocols
CSMA Protocols
Limits on a
Want a small to keep vulnerable period
short by having:
– Short bus
a= DR/Xkc
where
– Lower speeds
X= packet length in bits
– Long packets
Lower limit (Minimum) packet length to
upper bound a
Maximum packet length to be fair
Reason for Minimum/Maximum
Packet Size in the Internet
MAC
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Random Access Protocols
Performance
Ideal
From: “Computer Networks, 3rd Edition, A.S. Tanenbaum.
Prentice Hall, 1996
MAC
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Random Access Protocols
Performance: Nonpersistent CSMA
S
0.81
0.01
0.51
0.14
0.1
G
1
From: Leon-Garcia & Widjaja: Communication Networks
MAC
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Polling vs Random Access Performance
α= a
N=
# Nodes
MAC
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MAC
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Random Access Protocols
CSMA Protocols: States
From: “Computer Networks, 3rd Edition, A.S. Tanenbaum.
Prentice Hall, 1996
Collision Free Protocols
Collision free protocols establish rules
to determine which stations sends after
a successful transmission.
Assume there are N stations with
unique addresses 0 to N-1.
A contention interval is a period after a
successful transmission that is divided
into N time slots, one for each station.
MAC
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Collision Free Protocols
If a station has a PDU to send it sets a
bit to 1 in its time slot in the contention
interval.
At the end of the contention interval all
nodes know who has data to send and
the order in which it will be sent.
MAC
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Collision Free Protocols
Problems:
Fairness
Flexibility
Many
systems use the basic
approach of collision free
protocols
MAC
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Random Access and Reservations
Distributed systems: Stations implement a decentralized algorithm to
determine transmission order, e.g., reservation Aloha
Centralized systems: A central controller accepts requests from stations
and issues grants to transmit
Frequency Division Duplex (FDD): Separate frequency bands for
uplink & downlink
Time-Division Duplex (TDD): Uplink & downlink time-share the same
frequency channel
The centralized system is used in many access technologies, e.g.,
DOCSIS
IEEE 802.16 (WiMAX)
Wideband code division Multiple access (WCDMA)
High Speed Data Packet Access (HSDPA) - Evolution from GSM
Long Term Evolution (LTE)
CDMA2000
Evolution Data optimized EV-DO – Evolution from CDMA
Adapted from: Leon-Garcia & Widjaja: Communication Networks
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Reservation System
System Characteristics
Asymmetric
– Upstream
Minislots with requests for resources
Access Minislots via random access protocol
– Downstream
Accepts minislots and includes grants for
transmission
Grants control the flow on the upstream link
Order of grants established via a “scheduling”
algroithm
MAC
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Reservation Systems
Reservation
interval
r
d
Frame
transmissions
d
d
Upstream Transmissions
r
d
Cycle n
d
d
Time
Cycle (n + 1)
Minislot
r
= 1 2
3
M
Transmissions organized into cycles (or frames)
Cycle: reservation interval + frame transmissions
Reservation interval has a minislot for each station to request
reservations for frame transmissions; minislot can carry other
information, e.g., number of frame to TX, station backlog, channel
quality indicator (CQI)
Adapted from: Leon-Garcia & Widjaja: Communication Networks
MAC
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Central controller issues
grants to transmit
Algorithms to grant permission to UE
permission to transmit on upstream can
be based on:
number
of frame to TX,
station backlog,
channel quality indicator (CQI)
– Leads to Opportunistic Scheduling
Other…..
MAC
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MAC
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Throughput
Let
Assume
R = Link rate (b/s)
L = packet size (bits)
v= minislot size (sec)
M = Number of stations
X = L/R (sec)
Propagation delay < X Access network
Heavy load stations have packets to send
All requests are granted
One minislot needed for each packet/station
Time to transmit M packets = Mv+MX
Adapted from: Leon-Garcia & Widjaja: Communication Networks
Throughput
If k frame transmissions can be reserved with ONE
reservation message and if there are M stations, as
many as Mk frames can be transmitted in XM(k+v)
seconds
Adapted from: Leon-Garcia & Widjaja: Communication Networks
MAC
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Throughput: with random access
contention for Minislots
Real systems have too many nodes for each to
get a fixed minislot.
Therefore a random access protocol is used to
transmit in a minslot.
A station attempts to obtain a grant by
transmitting in a minslot in the upstream
direction.
If successful the station will get the grant on the
down stream
If unsuccessful then assume collision, backoff and
retry.
MAC
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Throughput: with random access
contention for Minislots
Assume slotted Aloha is used for
contention for minislots.
On average, each reservation takes at
least e = 2.71 minislot attempts
1
Smax = 1 + 2.71v/X
Effect is just to make the minislots seem
longer
MAC
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MAC
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Standard LANs
IEEE 802.3 [10 Mb/s]
IEEE 802.4 [up to 10 Mb/s]
Token Bus
Modulated
IEEE 802.5 [4 and 16 Mb/s]
CSMA/CD
Baseband
Token ring
Baseband
IEEE 802.11 Wireless LAN (more later)
Standard LANs
FDDI (CDDI) [100 Mb/s]
Token Ring
IEEE 802.12 [100 Mb/s]
Fiber Distributed Data Interface
Copper Distributed Data Interface
Baseband
Demand priority
IEEE 802.16
(Wireless local loop or Wireless MAN)
Other IEEE 802.xx Protocols
Cable Modem Data over Cable Service Interface
Specification (DOCSIS)
MAC
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Standard LANs
Network Layer
IEEE 802.2
Medium Access Control
Physical Layer
IEEE 802.2 = Logical Link Control
Protocol
IEEE 802.3
IEEE 802.4
IEEE 802.5
IEEE 802.6
IEEE 802.11
IEEE 802.12
IEEE 802.16
Others…..
MAC
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Random Access Protocols
CSMA Protocols
Example: Ethernet
Rate = 10 Mb/s
Minimum packet size = 512 bits
Maximum packet size = 12144 bits
D (max per segment) = 500 m
a --> [0.001, 0.03]
CSMA networks do not scale
Increase D performance degrades
Increase R performance degrades
MAC
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MAC
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Ethernet
Unslotted 1-persistent CSMA/CD
Procedure
Frame_to_transmit and idle
– wait interframe_gap
– transmit
Listen
to channel and if collision
– stop sending
– send jam signal
– schedule retransmission
Ethernet: Schedule retansmission
(the binary exponential backoff algorithm)
N= N+1 where N = number of retransmission
attempts
If N > attempt limit then trash the PDU
else
calculate the backoff time
– k = Min[N, backoff_limit]
– R ~ Uniform [0, 2k]
– retransmit at time tnow +
R*slot_time
MAC
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MAC
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Ethernet: Parameters
Slot time = 512 bit times
Interframe gap = 9.6us
Attempt limit = 16
Backoff limit = 10
Jam time = 32 bits/rate
Maximum frame size = 8*(1518) bits
Minimum frame size = 8*(64) bits
Ethernet
Packet structure
802.3 MAC Frame
1518 Bytes
7
1
Preamble
SD
Synch
6
Destination
Address
Start
frame
6
Source
Address
2
4
Length Information Pad
1A-23-F9-CD-06-9B
FCS
46 to 1500 bytes
Preamble: 7 bytes of 10101010
Start Of Frame: 1 byte, thus: 10101011
Source and Destination Address: Each 6 bytes and are globally unique.
Type field:
In 802.3 this field gives the length (in bytes) of the data field.
Data field: From 46 to 1500 bytes.
MAC
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MAC
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Ethernet:
Typical Configurations
Started here
NIC
NIC
NIC
Bus
Hub
Evolved to
NIC
NIC
NIC
NIC =
Network
Interface
Card
Switched Ethernet
AHigh Speed Interface, GigE
From: “Computer Networks, 3rd Edition, A.S. Tanenbaum.
Prentice Hall, 1996
MAC
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Gigabit Ethernet
Allows half- and full-duplex operation at speeds of
1000 Mb/s
Uses the 802.3 Ethernet frame format
Can use the CSMA/CD access method with support
for one repeater per collision domain: Half-duplex
Common use is with Gigabit Ethernet switches in
Full-duplex mode
→ Νo CSMA/CD
Addresses compatibility with 10 Mb/s, 100 Mb/s,
and 10-Gigabit Ethernet technologies
From: Whitepaper: Gigabit Ethernet Overview,
Gigabit Ethernet Alliance, May, 1997, http://www.gigabit ethernet.org/technology/whitepapers
MAC
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Gigabit Ethernet: Distance
Multimode fiber-optic link with a maximum
length of 550 meters
Single-mode fiber-optic link with a
maximum length of 3 kilometers
Optical links based on Fiber channel
technology
Copper based link with a maximum length
of at least 25 meters.
Category 5 unshielded twisted pair (UTP)
wiring link 100 meters
From: Whitepaper: Gigabit Ethernet Overview,
Gigabit Ethernet Alliance, May, 1997, http://www.gigabit ethernet.org/technology/whitepapers
MAC
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MAC
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Gigabit Ethernet:
Possible Application
From: Whitepaper: Gigabit Ethernet Overview,
Gigabit Ethernet Alliance, May, 1997, http://www.gigabit ethernet.org/technology/whitepapers
Evolution of Ethernet Technology
PMD=Physical Media Dependent
From: Mike Salzman, "10 GbE requirements & architecture,"
http://grouper.ieee.org/groups/802/3/10G_study/public/june99/salzman_1_0699.pdf
MAC
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MAC
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Ethernet
40 Gb/s Ethernet 40GbE
100 Gb/s Ethernet
100GbE
Provide support for
optical transport
network(OTN)
Carrier Grade Ethernet
Virtual LAN
Ethernet services to end
customers
Ethernet technology in
carrier networks
Adapted from: Leon-Garcia & Widjaja: Communication Networks
Vision: Ethernet End-to-End
Carrier CO
PoP
Carrier CO
PoP
GbE
Location A
GbE
GbE
Optical
Transport
MAN
10GbE
MAN
GbE
Optical
Transport
10GbE
Core DWDM Optical
Network
GbE
Optical
Transport
GbE
Campus LAN
Location B
MAC
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MAC
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From:10 Gigabit Ethernet Alliance Networld+Interop Las Vegas May, 2000
Definitions of Network Elements
Repeaters
Bridges
Switches
Routers
Routing
Forwarding
Repeaters & Bridges
Repeaters forwards electrical signals from one
Ethernet to another
Bridges (sometimes called hubs)
Interconnects multiple access LANs to form
Extended LANs
Only forwards frames destined for other LANs
Link layer forwarding based on MAC address
Bridges are devices that forward link-level frames
from one physical LAN to another
Bridging forwarding rules prevent loops
MAC
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Switches & Routers
Switches
– Forwards packets
– Star topology
– New hosts can be added without degrading the
performance of the existing hosts
– Scales by adding additional switches
(if supported by the switch fabric)
Routers & Gateways
– Forwards packets
– Interconnects two or more networks (maybe owned by
different organizations)
– Forwarding take packet from input port then use
routing table to find an output port then forward packet
to that port.
– Routing process of filling in the routing table
MAC
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Switches & Routers
Layer 2 switching is performed by
looking at a destination MAC address
within a frame. Layer 2 switching
builds and maintains a switching table
that keeps track of which MAC
addresses belong to each port or
interface.
Ethernet
switches
ATM Switches
MAC
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Switches & Routers
Layer 3 switching (routers) operates at the
network layer. It examines packet
information and forwards packets based on
their network-layer destination addresses.
IP Switches
Layer 4 Switching operates at the transport
layer makes forwarding decision based on IP
address and TCP/UDP application port
Gateways, Firewalls, IPSec, Policy Based
Networks (PBN), Directory Enabled Networks
(DEN)
MAC
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Wireless Networking
Some general problems
Noise
(likelihood of bit errors)
Hidden Terminal
(Removes advantage of carrier sensing)
– B Hears A
– B Hears C
– C can not hear A
Terminal A
Terminal B
Physical
obstruction
Terminal C
MAC
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MAC
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Wireless Networking
(Some problems continued)
Average received signal strength
Falls off with distance between tx and rec
Is a function of
– Reflection
– Diffraction
– Scattering
Changing received signal strength
Signal fading
– Mobility
– Weather
Propagation mechanisms
R=Reflection
S=Scattering
D=Diffraction
From: Anderson, J., Rappaport, and Yoshida, S. “Propagation Measurements and Models for
Wireless Communications Channels,” IEEE Communications Magazine, Jan 1995
MAC
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Propagation Effects
Signal strength at a
distance varies due to
Multipath
Large scale models
predicts the average
signal strength
Red line
Modified from: W. Stallings, Wireless Communications & Networks, Pearson 2005
Small scale (fading)
models characterize the
short term fluctuations
Black line
MAC
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IEEE 802.11
Form of a packet radio network
Must deal with the hidden terminal
problem
Physical layer
Frequency Hopping Spread Spectrum
Direct Sequence Spread Spectrum
Infrared
MAC
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IEEE 802.11
MAC- Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)
Sense channel if idle the sendRequest to Send (RTS) PDU
Receiver responds with a Clear to Send (CTS) PDU
If receive CTS then all other nodes know the channel is
captured and will not send, original sources sends without
collision
If RTS PDUs collide then use random access backoff
algorithm
RTS/CTS deals with the hidden terminal problem
Access Points (AP) are the wireless/wired gateways:
Infrastructure mode
Ad hoc mode
MAC
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802.11a
802.11a supports mandatory data rates of 6, 12, and 24 Mbps
Optional data rates of 9, 18, 36, and 54 Mbps
Operates in 5-GHz UNII (Unlicensed National Information
Infrastructure) band
Occupies 300 MHz in three different bandwidths of 100 MHz
each:5.150 to 5.250 GHz (U-NII lower band)5.250 to 5.350 GHz
(U-NII middle band)5.725 to 5.825 GHz (U-NII upper
band)802.11a provides 12 channels, each channel being 20 MHz
wide, and each centered at 20 MHz intervals (beginning at 5.180
GHz and ending at 5.320 GHz for the upper and middle U-NII
bands, beginning at 5.745 GHz and ending at 5.805 GHz for the
upper U-NII band). It is important to note that none of these
channels overlap.
802.11a uses OFDM (Orthogonal Frequency Division
Multiplexing) with multiple carriers (aka "subcarriers") per
channel
MAC
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802.11b
Supports data rates of 1, 2, 5.5, and 11 Mb/s.
Operates in the 2.4-GHz ISM (Industrial, Scientific, and
Medical) band.
Occupies 83.5 MHz (for North America) from 2.4000 GHz to
2.4835 GHz.
802.11b provides 11 channels (for North America), each channel
being 22 MHz in width, and each channel centered at 5 MHz
intervals (beginning at 2.412 GHz and ending at 2.462 GHz).
This means that there are only 3 channels which do not overlap
(channels 1, 6, 11).
802.11b uses DSSS (Direct Sequence Spread Spectrum) with a
single carrier per channel.
Both 802.11b and 802.11a use the 802.11 MAC, both use
CSMA/CA to "listen" before transmitting on a given channel (to
avoid colliding with another transmitter by sensing if the
channel is occupied).
MAC
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802.11g
A physical layer standard for WLANs in the 2.4GHz and 5GHz
radio band.
The maximum link rate is 54-Mbps per channel--compared with
11 Mbps for 11b.
802.11g standard uses orthogonal frequency-division
multiplexing (OFDM) modulation but, for backward
compatibility with 11b, it also supports complementary code
keying (CCK) modulation and, as an option for faster link rates,
allows packet binary convolutional coding (PBCC) modulation.
Speeds similar to 11a and backward compatibility may appear
attractive
Advantage-provides ability for dual-mode 2.4GHz and 5GHz
products, in that using OFDM for both modes to reduce silicon
cost.
MAC
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Other IEEE 802.11 standards
IEEE 802.11n
IEEE 802.11ac
Increase rate to 600 Mb/s
Used multiple antennas Multiple-input multiple-output (MIMO)
Frame aggregation, effectively increase packet size to reduce
overhead
Increase rate to 1 Gb/s
80MHz and 160 MHz channels
Higher order modulation
IEEE 802.11af (White-Fi) proposal for Wi-Fi using the TV White
spaces using cognitive radio technology.
IEEE 802.11ad (WiGig) enables data rates up to 7 Gb/s (uses 60
GHz spectrum)
MAC
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802.11e
Supplementary to the MAC layer to provide CoS
support for LAN applications.
Applies to 802.11 physical standards a, b and g.
802.11e provides some features for differentiating
data traffic streams.
MAC
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Broadband Access Technologies:
Cable Modems
FDM on the Cable
From: “Computer Networks, 3rd Edition, A.S.
Tanenbaum. Prentice Hall, 1996
MAC
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Generic Hybird Fiber Coax (HFC)
Distribution Plant
From: Data Over Cable Service Interface
Specifications (DOCSIS) Manish Mangal, Sprint, TP&I
MAC
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Broadband Access Technologies:
Cable Modems - Terms
CMTS: Cable Modem Termination System. Central device for
connecting the cable TV network to a data network like the
internet. Normally placed in the headend of the cable TV
system. Downstream:
Headend: Central distribution point for a CATV system. Video
signals are received here from satellites and maybe other
sources, frequency converted to the appropriate channels,
combined with locally originated signals, and rebroadcast onto
the HFC plant.
Upstream: The data flowing from the Cable Modem to the
CMTS.
Downstream: The data flowing from the CMTS to the cable
modem.
MAC
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Data Rates: DOCSIS 1.0/1.1
Carriers limit down and upstream rates, e.g., New Zealand operator TelstraClear
provides (as of Sp 2007):
- downstream speeds of 10Mbit/s and 2Mbit/s
- upstream speed of 2Mbit/s.
MAC
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DOCSIS Protocol Stack
Convergence Sublayer:
Interface between the MAC layer and
Physical Media Dependent Layer (PMD) or PHY
Can also be Ethernet between the
Host CPE and external CM
NSI=Network
Side Interface
RFI=Radio Frequency Interface
MAC
Modified from: www.cablemodem.com/downloads/specs/SP-CMCI-I09-030730.pdf
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Broadband Access Technologies:
Cable Modems - Terms
Headend controls all transmissions
on the downstream and schedules upstream
Request for use of upstream made using random
access (contention) in specific upstream time slots
From: “Computer Networks, 3rd Edition, A.S.
Tanenbaum. Prentice Hall, 1996
MAC
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Broadband Access Technologies:
Cable Modems
From: http://www.cable-modems.org/tutorial
MAC
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Broadband Access Technologies:
Cable Modems
From: http://www.cable-modems.org/tutorial
MAC
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Broadband Access Technologies:
Cable Modems
Aggregate Data Rates shared by 500 to
2000 homes today
From: http://www.cable-modems.org/tutorial
MAC
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DOCSIS MAC
Data Over Cable Service Interface Specification
(DOCSIS)
A stream of mini-slots in upstream
(6.25 us containing 8-32 Bytes)
Dynamic mix of contention- and reservation-based
upstream transmit opportunities
Quality of service including:
Support of Bandwidth and Latency guarantees
Packet classification
Dynamic service establishment
Extensions provided for security at data link layer
Support of wide range of data rates
MAC
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DOCSIS MAC
Upstream Bandwidth Allocation Protocol
MAP is a MAC management message transmitted by the
CMTS in the downstream which describes the use of the
next upstream mini-slots (up to 4096)
A MAP describes some slots as grants for particular stations
to transmit data in, some for contention transmission and
some as an opportunity for new CMs to join the link
MAC
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DOCSIS MAC
From: Data Over Cable Service Interface
Specifications (DOCSIS) Manish Mangal, Sprint, TP&I
MAC
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MAC
94
DOCSIS MAC
PMD=Physical Media Dependent
From: http://www.jlsnet.co.uk/index.php?page=projects_docsis_chap3c
DOCSIS MAC -- Best Effort
DOCSIS 1.0
From: Data Over Cable Service Interface
Specifications (DOCSIS) Manish Mangal, Sprint, TP&I
MAC
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MAC
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DOCSIS MAC -- DOCSIS 1.1
Real Time Polling Mode
From: Data Over Cable Service Interface
Specifications (DOCSIS) Manish Mangal, Sprint, TP&I
DOCSIS MAC -- DOCSIS 1.1
Circuit Switched Emulation “Unsolicited Grant
Mode”
From: Data Over Cable Service Interface
Specifications (DOCSIS) Manish Mangal, Sprint, TP&I
MAC
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MAC
98
Evolution of DOCSIS
Embedded DOCSIS: eDOCSIS device
contains
CM
One or more embedded Service/Application
Functional Entities (eSAFEs)
–
–
–
–
–
Telephone
Video
Audio
Digital video recorders
Multiple TV tuners
DOCIS 3.0
From: http://www.fttxtra.com/hfc/docsis/docsis-3-0-tutorial/
MAC
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DOCSIS 3.0: Channel Bonding
From: http://www.fttxtra.com/hfc/docsis/docsis-3-0-tutorial/
MAC
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Wireless broadband Internet
IEEE 802.16 (WiMax)
IEEE 802.16 protocol based on DOCSIS
IEEE 802.16e addresses mobility
MAC
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Digital Subscriber Line: Physical topology
NID =
Network
Interface
Device
DSLAM=
Digital
Subscriber
Line Access
Multiplexer
Modified from: Tanenbaum, A. Computer
Networks, Prentice Hall, 4th ED 2003
Telephone Lines
MAC
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Broadband Access Technologies:
DSL - digital subscriber line
Not shared media
Point-to-point connection to network
ADSL - Asymmetric DSL
Downstream -
1.5 Mb/s - 9 Mb/s
Upstream - 16 kb/s - 640 kb/s
HDSL - High data rate DSL
Downstream -
1.5 Mb/s - 2 Mb/s
Upstream - 1.5 Mb/s - 2 Mb/s
MAC
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Broadband Access Technologies:
DSL - digital subscriber line
VDSL - Very high data rate DSL
G.lite officially ITU-T standard G-992.2
Downstream - 1.544 Mb/s to 6 Mb/s
Upstream - 128 Kbps to 384 Kbps
Does not require splitting of the line at the user end
Downstream - 13 Mb/s - 52 Mb/s
Upstream - 1.5 Mb/s - 2.3 Mb/s
DSLAM - digital subscriber line access multiplexer
Speed depends on distance from DSLAM (max
18,000 ft)
MAC
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Powerline Communications Access
What is it:
It is a data communication
technology that operates over
the electricity supply.
Rates Mb/s
Last hop:
– Power line
– Wireless 802.11
For more information see:
http://www.adaptivenetworks.com/
MAC
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Bluetooth
Bluetooth- Harald Blaatand (Bluetooth) II
(940-981) the Viking King who unified
Denmark and Norway
Original Goal: desk top cable replacement
Used to describe the protocol of a short range
(10 meter) frequency-hopping radio link
between devices.
devices are then termed Bluetooth - enabled.
Documentation on Bluetooth is split sections,
– Bluetooth Specification
– Bluetooth Profiles.
MAC
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Bluetooth
The Specification describes how the
technology works (i.e the Bluetooth protocol
architecture),
The Profiles describe how the technology is
used (i.e how different parts of the
specification can be used to fulfill a desired
function for a Bluetooth device)
Uses frequency hopping in 79 hops displaced
by 1 MHz, starting at 2.402GHz and finishing
at 2.480GHz
Data Rate < 1Mb/s
For more details see:
http://www.palowireless.com/infotooth/tutorial.asp
MAC
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MAC
108
RFID
Radio Frequency Identification Tags
Replacement for Barcode
Types
Active (like Kansas Turnpike K-tag)
Passive
Need MAC when reading a group of
products at “once”
Issues
Cost
Privacy
Satellite Networks: GEO
Geostationary
earth orbit
37,700
km
Power--> increased cost
Propagation delay
Limited orbital slots ~ 180
(2 degrees)
MAC
109
MAC
110
Satellite Networks: MEO
Medium earth orbit
5,000 - 15,000 km
Period ~ 4-9 hours
Fewer satellites
Satellite Networks: LEO
Low earth orbit
1500 km
Periods up to 2 hours
More satellites
Cheaper to launch
Odyssey Telecommunications International Inc.:
Worldwide Personal Communications
MAC
Satellite Services
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Satellite Networks
Properties
Large delay ~ 270 ms (Geosynchronous)
Up and down links are on different frequencies
( full duplex)
CSMA/CD will not work because of long
delay
Token networks are not applicable because of
long delays, e.g., 100 nodes will have a 27 sec.
token return time.
MAC
112
Satellite Networks:
Reservation ALOHA
Consider a slotted system with N slots per frame
(TDM like)
Each slot can be in one of three states:
– Empty, i.e., not is use
– Mine, i.e., in use by me
– Other, i.e., in use by another node
Protocol
– If state is mine then continue to use it
– If state is other then do not send in that time slot
– If state is empty then contend for that slot using Aloha
MAC
113
Satellite Networks
At low loads the network performs like
a random access systems, i.e., no
waiting for permission to send.
At high loads the systems performs like
a TDM system.
This scheme has a problem with
fairness
Distributed assignment of time slots
MAC
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Satellite Networks
TDMA - Time Division Multiple
Access
Up stream stations request bandwidth, time
slots.
Central controller grants requests for
bandwidth
Central controller has global view of all
requests
Centralized assignment of time slots
MAC
115
MAC
116
Satellite Access
What it is: Satellite access
Typically Asymmetric
DirecWay
Features
Basic
Plus
Starband
Starband
360
Starband
481
Upload speed
50 Kbps
100 kbps
50 Kbps
100 Kbps
Download
speed
500 Kbps
500 Kbps
500 Kbps
500 Kbps
Equipment cost
$600
$600
$700
$700
Installation cost
$400
$400
$400
$400
Monthly cost
$60
$100
$60
$90
Contract term
2 yr
2 yr
1 yr
1 yr
IP address
NAT
Static
NAT
Static
Installation
training
and cert.
1 day training in Ohio
conducted last
Saturday of every
month. Cost $500
Training and test available
online, free of charge
As of 2005
Nelson Environmental Study Area (NESA) Wireless Data Link
NESA HQ – Flux Tower Network
MAC
117
Satellite Link Bandwidth Tests
Microsoft Remote Desktop was used to access the data server PC located in the NESA HQ building, then
internet access bandwidth tests were initiated. Remote Desktop has a significant impact on the available
bandwidth reported. Typical numbers directly from the NESA site are 1440 kbps up / 700 kbps down.
MAC
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