Download C01-Overview

Review: MAC
Link Layer: Introduction
Terminology:
hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links

wired links
 wireless links
 LANs
layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a link
Data Link Layer
5-2
Link layer: context
datagram transferred by
different link protocols over
different links:

e.g., Ethernet on first link,
frame relay on intermediate
links, 802.11 on last link
each link protocol provides
different services

e.g., may or may not provide
rdt over link
transportation analogy
trip from Princeton to Lausanne
 limo: Princeton to JFK
 plane: JFK to Geneva
 train: Geneva to Lausanne
tourist = datagram
transport segment =
communication link
transportation mode = link
layer protocol
travel agent = routing
algorithm
Data Link Layer
5-3
Link Layer Services
framing, link access:


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encapsulate datagram into frame, adding header, trailer
channel access if shared medium
“MAC” addresses used in frame headers to identify source,
dest

different from IP address!
reliable delivery between adjacent nodes



we learned how to do this already (chapter 3)!
seldom used on low bit-error link (fiber, some twisted pair)
wireless links: high error rates

Q: why both link-level and end-end reliability?
Data Link Layer
5-4
Link Layer Services (more)
flow control:

pacing between adjacent sending and receiving nodes
error detection:


errors caused by signal attenuation, noise.
receiver detects presence of errors:

signals sender for retransmission or drops frame
error correction:

receiver identifies and corrects bit error(s) without resorting
to retransmission
half-duplex and full-duplex

with half duplex, nodes at both ends of link can transmit, but
not at same time
Data Link Layer
5-5
Multiple Access Links and Protocols
Two types of “links”:
point-to-point


PPP for dial-up access
point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)



old-fashioned Ethernet
upstream HFC
802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
Data Link Layer
5-6
Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes:
interference

collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
communication about channel sharing must use channel
itself!

no out-of-band channel for coordination
Data Link Layer
5-7
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average
rate R/M
3. fully decentralized:


no special node to coordinate transmissions
no synchronization of clocks, slots
4. simple
Data Link Layer
5-8
MAC Protocols: a taxonomy
Three broad classes:
Channel Partitioning


divide channel into smaller “pieces” (time slots, frequency,
code)
allocate piece to node for exclusive use
Random Access


channel not divided, allow collisions
“recover” from collisions
“Taking turns”

nodes take turns, but nodes with more to send can take
longer turns
Data Link Layer
5-9
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt trans
time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
6-slot
frame
1
3
4
1
3
4
Data Link Layer
5-10
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
FDM cable
frequency bands
channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands
2,5,6 idle
Data Link Layer
5-11
Random Access Protocols
When node has packet to send


transmit at full channel data rate R.
no a priori coordination among nodes
two or more transmitting nodes ➜ “collision”,
random access MAC protocol specifies:


how to detect collisions
how to recover from collisions (e.g., via delayed retransmissions)
Examples of random access MAC protocols:



slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA
Data Link Layer
5-12
Slotted ALOHA
Assumptions:
all frames same size
time divided into equal size
slots (time to transmit 1
frame)
nodes start to transmit only
slot beginning
nodes are synchronized
if 2 or more nodes transmit
in slot, all nodes detect
collision
Operation:
when node obtains fresh
frame, transmits in next slot
 if no collision: node can
send new frame in next
slot
 if collision: node
retransmits frame in
each subsequent slot
with prob. p until
success
Data Link Layer
5-13
Slotted ALOHA
Pros
single active node can
continuously transmit at
full rate of channel
highly decentralized: only
slots in nodes need to be
in sync
simple
Cons
collisions, wasting slots
idle slots
nodes may be able to
detect collision in less
than time to transmit
packet
clock synchronization
Data Link Layer
5-14
Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization
when frame first arrives

transmit immediately
collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1,t0+1]
Data Link Layer
5-15
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
human analogy: don’t interrupt others!
Data Link Layer
5-16
CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
Data Link Layer
5-17
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA


collisions detected within short time
colliding transmissions aborted, reducing channel wastage
collision detection:

easy in wired LANs: measure signal strengths, compare transmitted,
received signals
 difficult in wireless LANs: received signal strength overwhelmed by local
transmission strength
human analogy: the polite conversationalist
Data Link Layer
5-18
CSMA/CD collision detection
Data Link Layer
5-19
“Taking Turns” MAC protocols
channel partitioning MAC protocols:

share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access, 1/N bandwidth allocated
even if only 1 active node!
random access MAC protocols

efficient at low load: single node can fully utilize channel
 high load: collision overhead
“taking turns” protocols
look for best of both worlds!
Data Link Layer
5-20
“Taking Turns” MAC protocols
Polling:
master node “invites”
slave nodes to
transmit in turn
typically used with
“dumb” slave devices
concerns:



polling overhead
latency
single point of failure
(master)
data
poll
master
data
slaves
Data Link Layer
5-21
“Taking Turns” MAC protocols
Token passing:
 control token passed
from one node to next
sequentially.
 token message
 concerns:
 token overhead
 latency
 single point of failure
(token)
T
(nothing
to send)
T
data
Data Link Layer
5-22
Summary of MAC protocols
channel partitioning, by time, frequency or code

Time Division, Frequency Division
random access (dynamic),




ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technologies (wire), hard in
others (wireless)
CSMA/CD used in Ethernet
CSMA/CA used in 802.11
taking turns


polling from central site, token passing
Bluetooth, FDDI, IBM Token Ring
Data Link Layer
5-23
Introduction: Wireless/Mobile
Computers for the next decades?
Computers are integrated

small, cheap, portable, replaceable - no more separate devices
Technology is in the background
computer are aware of their environment and adapt (“location awareness”)
 computer recognize the location of the user and react appropriately (e.g.,
call forwarding, fax forwarding, “context awareness”))

Advances in technology





more computing power in smaller devices
flat, lightweight displays with low power consumption
new user interfaces due to small dimensions
more bandwidth per cubic meter
multiple wireless interfaces: wireless LANs, wireless WANs, regional
wireless telecommunication networks etc. („overlay networks“)
Mobile communication
Two aspects of mobility:
user mobility: users communicate (wireless) “anytime, anywhere, with
anyone”
 device portability: devices can be connected anytime, anywhere to the
network

Wireless vs. mobile
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
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Examples
stationary computer
notebook in a hotel
wireless LANs in historic buildings
Personal Digital Assistant (PDA)
The demand for mobile communication creates the need for
integration of wireless networks into existing fixed networks:

local area networks: standardization of IEEE 802.11,
ETSI (HIPERLAN)
 Internet: Mobile IP extension of the internet protocol IP
 wide area networks: e.g., internetworking of GSM and ISDN
Applications I
Vehicles





transmission of news, road condition, weather, music via DAB
personal communication using GSM
position via GPS
local ad-hoc network with vehicles close-by to prevent accidents, guidance
system, redundancy
vehicle data (e.g., from busses, high-speed trains) can be transmitted in
advance for maintenance
Emergencies

early transmission of patient data to the hospital, current status, first
diagnosis
 replacement of a fixed infrastructure in case of earthquakes, hurricanes,
fire etc.
 crisis, war, ...
Typical application: road traffic
UMTS, WLAN,
DAB, DVB, GSM,
cdma2000, TETRA, ...
Personal Travel Assistant,
PDA, Laptop,
GSM, UMTS, WLAN,
Bluetooth, ...
Mobile and wireless services – Always Best Connected
DSL/ WLAN
3 Mbit/s
GSM/GPRS 53 kbit/s
Bluetooth 500 kbit/s
UMTS, GSM
115 kbit/s
LAN
100 Mbit/s,
WLAN
54 Mbit/s
UMTS
2 Mbit/s
GSM/EDGE 384 kbit/s,
DSL/WLAN 3 Mbit/s
GSM 115 kbit/s,
WLAN 11 Mbit/s
UMTS, GSM
384 kbit/s
Applications II
Travelling salesmen

direct access to customer files stored in a central location
 consistent databases for all agents
 mobile office
Replacement of fixed networks

remote sensors, e.g., weather, earth activities
 flexibility for trade shows
 LANs in historic buildings
Entertainment, education, ...

outdoor Internet access
 intelligent travel guide with up-to-date
location dependent information
 ad-hoc networks for
multi user games
Location dependent services
Location aware services

what services, e.g., printer, fax, phone, server etc. exist in the local
environment
Follow-on services

automatic call-forwarding, transmission of the actual workspace to the
current location
Information services
„push“: e.g., current special offers in the supermarket
 „pull“: e.g., where is the Black Forrest Cherry Cake?

Support services

caches, intermediate results, state information etc. „follow“ the mobile
device through the fixed network
Privacy

who should gain knowledge about the location
Mobile devices
Pager
• receive only
• tiny displays
• simple text
messages
PDA
• graphical displays
• character recognition
• simplified WWW
Laptop/Notebook
• fully functional
• standard applications
Sensors,
embedded
controllers
Mobile phones
• voice, data
• simple graphical displays
www.scatterweb.net
performance
Palmtop
• tiny keyboard
• simple versions
of standard applications
Effects of device portability
Power consumption

limited computing power, low quality displays, small disks due to
limited battery capacity
 CPU: power consumption ~ CV2f

C: internal capacity, reduced by integration
 V: supply voltage, can be reduced to a certain limit
 f: clock frequency, can be reduced temporally
Loss of data

higher probability, has to be included in advance into the design
(e.g., defects, theft)
Limited user interfaces

compromise between size of fingers and portability
 integration of character/voice recognition, abstract symbols
Limited memory

limited value of mass memories with moving parts
 flash-memory or ? as alternative
Wireless networks in comparison to fixed networks
Higher loss-rates due to interference

emissions of, e.g., engines, lightning
Restrictive regulations of frequencies

frequencies have to be coordinated, useful frequencies are almost all
occupied
Low transmission rates

local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS
Higher delays, higher jitter

connection setup time with GSM in the second range, several hundred
milliseconds for other wireless systems
Lower security, simpler active attacking

radio interface accessible for everyone, base station can be simulated,
thus attracting calls from mobile phones
Always shared medium

secure access mechanisms important
Areas of research in mobile communication
Wireless Communication

transmission quality (bandwidth, error rate, delay)
 modulation, coding, interference
 media access, regulations
 ...
Mobility

location dependent services
 location transparency
 quality of service support (delay, jitter, security)
 ...
Portability

power consumption
 limited computing power, sizes of display, ...
 usability
 ...
Simple reference model used here
Application
Application
Transport
Transport
Network
Network
Data Link
Physical
Radio
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Medium
Influence of mobile communication to the layer model
Application layer
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
Transport layer

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Network layer
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Data link layer




Physical layer

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service location
new applications, multimedia
adaptive applications
congestion and flow control
quality of service
addressing, routing,
device location
hand-over
authentication
media access
multiplexing
media access control
encryption
modulation
interference
attenuation
frequency
Overlay Networks - the global goal
integration of heterogeneous fixed and
mobile networks with varying
transmission characteristics
regional
vertical
handover
metropolitan area
campus-based
in-house
horizontal
handover