Transcript Computer Networks

Computer Networks
Data link layer
Data link layer -- June 2004
1
Overview
 Design issues
 Channel allocation
 Point-to-point links
 Multi access protocols
 Local area Networks
 Ethernet
 Data Link layer Switching
 Logical link control
 Wireless LANs
 Broadband wireless
Data link layer -- June 2004
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LANs: channel allocation
 Allocation?
Who goes next?
o Static:
o Dynamic:
assignment for long duration
stations continuously compete
Data link layer -- June 2004
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LANs: channel allocation
 Static channel allocation
o Scheme: • Request channel
• Data Transfer
• Release channel
o After allocation: • Channel is private
• Can be used for a long time
o Division of channel • FDM: Frequency division multiplexing
• TDM: time division multiplexing
Data link layer -- June 2004
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LANs: channel allocation
 Static channel allocation
o Simple and efficient
• Small & fixed number of users
• Heavy load
o Problems
T
Mean delay
C
Capacity of channel in Mbps

Mean arrival rate in frames/sec
• Loss of bandwidth
1/ 
– If some users are quiescent
– If less users than subchannels
• Poor performance
T
1
=
.C-
Mean frame length in bits
1
TFDM =

Data link layer -- June 2004
C
N
= N.T
-

N
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Overview
 Channel allocation
 Design issues
 Point-to-point links
 Local area Networks
 Data Link layer Switching
 Multi access protocols
o Aloha
o CSMA protocols
o CSMA/CD protocols
o Collision free
o Wavelength division
multiple access protocols
o Wireless LAN protocols
 Ethernet
 Logical link control
 Wireless LANs
 Broadband wireless
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LANs: MA Protocols
Multiple Access protocols
 University of Hawaii Aloha system
o Central computer system
o Terminals spread over 4 islands
o Communication: FM radio
o No station to station communication
o Shared channel for communication from terminals to computer
system
o Transmission strategy:
• Terminal sends data as data comes available
• Collision possible
• Retransmission if no ack received
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LANs: MA Protocols
 Pure Aloha: frames transmitted at arbitrary times
2 frames sent at same time 
collision
both frames destroyed
Data link layer -- June 2004
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LANs: MA Protocols
 Pure Aloha: channel efficiency
o Vulnerable period = 2 x packet time
o Packet time = time required to transmit 1 frame
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LANs: MA Protocols
 Aloha: channel efficiency
o Pure: max 18%
o Slotted (time divided in slots; start sending at start of slot)
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LANs: CSMA protocols
Carrier Sense Multiple Access CSMA
 Aloha type system + ability to test for a carrier i.e. a transmission
 Persistent <> Nonpersistent
o 1-persistent
• If channel is idle, a frame is transmitted
• If channel is busy, the channel is continuously checked
o Nonpersistent
• If channel is idle, a frame is transmitted
• If channel is busy, a random time is waited before channel is sensed again
o p-persistent (slotted channel only)
• If channel is idle a frame is transmitted with probability p
• If channel is busy, next slot is sensed again
Data link layer -- June 2004
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LANs: CSMA protocols
Carrier Sense Multiple Access CSMA
 Performance
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LANs: CSMA/CD protocols
Carrier Sense Multiple Access Collision Detect
 Strategy
o Try to detect collisions asap
o Listen while transmitting
o If collision is detected, abort transmission
 Channel model
Data link layer -- June 2004
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LANs: CSMA/CD protocols
Carrier Sense Multiple Access Collision Detect
 Contention period
o Worst case scenario
o Detection = analog process
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LANs: collision free protocols
 Problem?
o 20 km, 100 Mbps
o Very long, high bandwidth protocols
 = 100 sec or 10.000 bits
o 1 km, 10 Mbps
o 1 km, 10 Gbps
 = 5 sec or 50 bits
 = 5 sec or 50.000 bits
 Bit-Map protocol
o Each contention period has N slots (N = #stations)
o Station k is assigned slot k; is used to indicate if station k has to
send data
o Data transmission proceed without collisions
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LANs: WDMA protocols
Wavelength division multiple access
 Approach:
o Divide channel into subchannels (FDM,…)
o Allocate them as needed
 2 channels/station
o Narrow: used by other stations to signal the station:
o Wide: used by station to output data frames
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LANs: WDMA protocols
Wavelength division multiple access
 2 transmitters & 2 receivers for each station:
o
o
o
o
Fixed-wavelength receiver for its own control channel
Tunable transmitter for sending on other control channels
Fixed wavelength transmitter for its own data channel
Tunable receiver for other data channels
 Support for 3 traffic classes:
o Constant data rate connection oriented traffic
o Variable data rate connection oriented traffic
o Datagram traffic
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LANs: WDMA protocols
Wavelength division multiple access
 Scenario for datagram from A to B:
o A: tunes on data channel of B
o A: waits for status slot & selects free slot on control
channel of B (e.g. slot 4)
o A: sends on control channel of B, slot 4: data on data
channel of A, slot 3
Collision possible!
o B: tunes on data channel of A
o B: accepts data on slot 3
B not ready to accept data
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LANs: WDMA protocols
Wavelength division multiple access
 Scenario for variable data rate connection from A to B:
o A: tunes on data channel of B
o A: waits for status slot & selects free slot on control channel
of B (e.g. slot 4)
o A: sends on control channel of B, slot 4: connection request
o B: announces assignment of slot 4 to A in status slot of its
data channel
o A wants to send data:
• A: sends on control channel of B, slot 4: data on data channel of A,
slot 3
• B: tunes on data channel of A
• B: accepts data on slot 3
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LANs: wireless protocols
 Common configuration for a wireless LAN
o Base stations (access points) wired together
o Notebooks with radio transmitter/receiver
o A receiver within range of 2 active transmitters receives
a garbled signal
o Not all stations are in range of one another
o CSMA does not work: interference at receiver is
important not at sender
o Example …
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LANs: wireless protocols
 A sends to B
 B sends to A
 If C senses medium, it will
 If C senses the medium, it
not hear A
 If C transmits to B, it will
garble the signal at B
may falsely conclude it
cannot send to D
Hidden station problem
Exposed station problem
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LANs: wireless protocols
Multiple Access with Collision Avoidance MACA
 A wants to send to B
 A sends RTS (request to
send) frame (short frame)
 B replies with CTS (clear to
with length of data frame
send) frame, containing
same length
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LANs: wireless protocols
Multiple Access with Collision Avoidance MACA

A wants to send to B
 A sends RTS (request to send) frame (short
frame) with length of data frame

 Stations hearing RTS
 Stations hearing CTS
should remain silent to not
interfere with CTS
B replies with CTS (clear to send) frame,
containing same length
should remain silent to not
interfere with data frame
Collisions possible!
Wait random time
Binary exponential backoff!
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LANs: wireless protocols
MACAW = MACA for Wireless
 Optimisations for MACA
o Introduce ACK from receiver of data frame to sender:
detect loss in DL iso network/transport layer
o Add CSMA: avoid sending a RTS by a station close to
a station sending to the same destination
o Run binary exponential backoff for each destination
iso each station
o Exchange of information between stations about
congestion
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Overview
 Design issues
 Channel allocation
 Point-to-point links
 Multi access protocols
 Local area Networks
 Ethernet
 Data Link layer Switching
 Logical link control
 Wireless LANs
 Broadband wireless
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LANs: IEEE 802.3 or Ethernet
 Overview
o 1-persistent CSMA/CD
•
•
•
•
When a station wants to transmit, it listens to the cable
If idle, it transmits immediately
If busy, it waits until the cable goes idle
If collision, it waits a random time
o History
•
•
•
•
Real start: Aloha
3 Mbps experiment at Xerox  ethernet
Agreement between Intel, DEC, Xerox
Base for IEEE 802.3
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LANs: IEEE 802.3 or Ethernet
 Cabling
Name
Cable
Max
Length
Nodes/
segment
Advantages
10Base5
Thick coax
500m
10Base2
Thin coax
200m
30 Cheapest system
10Base-T
Twisted pair
100m
1024 Easy maintenance
10Base-F
Fiber optics
2000m
Data link layer -- June 2004
100 ??
1024 Best between buildings
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LANs: IEEE 802.3 or Ethernet
Cabling
 Thick Ethernet
o Vampire taps, 2.5 m apart
o Segments up to 500m + repeaters
o Transceiver at tap
 Twisted pair
o Cables to central hub
o Net = box
o Easy maintenance
 Thin Ethernet
o Industry standard BNC
connectors
o Easier to install, more reliable,
cheaper
o Up to 200m, 30 systems per
segment
o Transceiver on controller board
 Fiber
o Expensive due to cost of
connectors
o Excellent noise immunity
o Preference for connections
between buildings
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LANs: IEEE 802.3 or Ethernet
 MAC frame
o Preamble: 7 bytes with 1010101010
o Start of frame delimiter: 1010101011
o Address:
• For 10 Mbps: 6 byte addresses
• Assigned by IEEE; globally unique
• All 1s: broadcast
o Pad: to ensure minimum length of 64 bytes for frame; minimum
frame must take 51.2 sec
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LANs: IEEE 802.3 or Ethernet
 MAC frame: 2 definitions
o DIX ethernet (original proposal of DEC, Intel, Xerox)
o IEEE 802.3
 Differences:
o Type  length: All defined types > 1500
o SOF: for compatibility with 802.4 & 802.5
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LANs: IEEE 802.3 or Ethernet
 Binary exponential Backoff Algorithm
o Randomisation in case of collisions?
o After collision time is divided in discrete slots of 51.2 sec
o After 1 collision: station waits 0..1 slots before trying to send again
o After 2 collisions: station waits 0..3 slots before …
o After i collisions: station waits 0 .. 2i – 1 slots before …
o After 10 collisions: interval is frozen at 1024 slots
o After 16 collisions: failure reported
o Why not always 1024 slots?
o Fair?
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LANs: IEEE 802.3 or Ethernet
 Performance
o Assumptions
• Heavy load, k stations always ready to transmit
• Constant retransmission probability (not exponential Backoff)
o Channel efficiency
P
Time to transmit a frame
B Bandwidth



P
=
P + 5.4  
1



BL
1 + 5.4 
cF
L
Cable length
F
Frame length
c
Signal propagation speed
Average length of collision period
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LANs: IEEE 802.3 or Ethernet
 Performance
Why so poor?
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LANs: IEEE 802.3 or Ethernet
 From hubs to switches:
o Collision domain = box  hub
= card  #collisions reduced
= line  no collisions
o Internal forwarding?
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LANs: IEEE 802.3 or Ethernet
 Fast Ethernet:
10 Mbps  100 Mbps
 Implications on performance?
P
=
P + 5.4  
 B +
o L
o F
1
1 + 5.4 
BL
cF
same efficiency:
Twisted pair cabling: 100m * 2
<> 2500m for coax cable
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Fast Ethernet
 Faster 802.3, no other changes
o Need for backward compatibility with existing LANs
o New protocol  unforeseen problems?
o Get it done before technology changes
 Simple basic idea:
o Reduce bit time from 100 nsec to 10 nsec
o Allow only hubs/switches
 Cabling
o 100Base-T4
o 100Base-TX
o 100Base-F
UTP3, 25 Mhz signaling
4 twisted pairs (host to hub, hub to host, 2 switchable)
Ternary signals, 3 wires  27 symbols  4 bits per cycle
UTP5, 125 Mhz signaling
2 twisted pairs
Coding scheme: 4B/5B 4 bits in 5 clock periods
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Gigabit Ethernet
 Goal of standards committee
o 10 times faster  1000 Mbps or 1 Gbps
o Remain backward compatible! Same frame format
 Implications on performance?
Same minimum/maximum frame size
Same addressing scheme
P
=
P + 5.4  
 B +
o L
o F
1
BL
1 + 5.4 
cF
same efficiency:
2500m for Ethernet 
25m for gigabit ethernet
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Gigabit Ethernet
 Solution? Limitation on configurations
o Only point-to-point line
o Use hubs or switches
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Gigabit Ethernet
 Full-duplex operation mode  switches
o Collisions impossible: CSMA/CD not used
o Maximum cable length determined by signal strength
 Half -duplex operation mode  hubs
o Collisions possible: CSMA/CD required
o Cable length not to be reduced: 200m!
o Solutions:
• Carrier extension: hardware padding of frames to minimum of
512 bytes
• Frame bursting: transmit concatenated sequence of frames in a
single transmission
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Gigabit Ethernet
 Need for flow control
o 1ms  1.000.000bits or 1953 frames of minimal length
 Control frames
o Type = 0x8808
 PAUSE control frame
o Wait for x time units
o Time unit = 512 nsec
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Gigabit Ethernet
 10-gigabit Ethernet
o IEEE 802.3ae: standard approved in 2002
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Overview
 Design issues
 Channel allocation
 Point-to-point links
 Multi access protocols
 Local area Networks
 Ethernet
 Data Link layer Switching
 Logical link control
 Wireless LANs
 Broadband wireless
Data link layer -- June 2004
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LANs: 802
 LLC: Logical Link Control
o Services: ~ data link
o Header: ~ HDLC
o Identical for all LANs
 MAC: Medium Access Control
o Access to medium specific layer
o Service
• Datagram
• Datagram with some ack
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Overview
 Design issues
 Channel allocation
 Point-to-point links
 Multi access protocols
 Local area Networks
 Ethernet
 Data Link layer Switching
 Logical link control
 Wireless LANs
 Broadband wireless
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Wireless LANs – 802.11
 2 modes:
o In the presence of a wired base station – access point
o In the absence of a base station – ad hoc networking
 Make 802.11 compatible with ethernet
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Wireless LANs – 802.11

Physical Layer
Too slow!!
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Wireless LANs – 802.11
 802.11a - OFDM
Orthogonal Frequency Division Multiplexing
o 54 Mbps in 5 GHz ISM band
o Different frequencies used (48 data + 4 synchronisation
o Form of spread spectrum
 802.11b - HR-DSS
High rate direct sequence spread spectrum
o up to 11 Mbps in 2.4 GHz ISM band
o Supports 1, 2, 5.5, 11 Mbps
 802.11g – OFDM
o 54 Mbps in 2.4 GHz ISM band
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Wireless LANs – 802.11
MAC Sublayer Protocol
 2 modes of operation:
o DCF – Distributed coordination function
• CSMA/CA: physical + virtual channel sensing
• 2 methods of operation
– Physical channel sensing only
– Physical + virtual channel sensing  MACAW
o PCF – Point coordination function
• Base station controls all activity in cell
• Polling mode + DCF
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Wireless LANs – 802.11
MAC Sublayer Protocol
 DCF – Distributed coordination function
o Physical channel sensing only
• Channel idle: station starts transmission
busy: sense till idle
• No listening during transmission
• Collision: wait random time – binary exponential backoff
o Physical + virtual channel sensing  MACAW
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Wireless LANs – 802.11
MAC Sublayer Protocol
 DCF – Distributed coordination function
o Physical channel sensing only
o Physical + virtual channel sensing  MACAW
NAV = Network Allocation Vector or kind of virtual channel busy
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Wireless LANs – 802.11
MAC Sublayer Protocol
 PCF– Point coordination function
o Polling: base station broadcasts a beacon frame
periodically (10 – 100 times/sec)
• Beacon frame contains system parameters
• Invites new stations to sign up for polling service
o Once signed up for polling at a certain rate  guaranteed
fraction of bandwidth
o Supports battery management: forces a station to go into
sleep state until awakened by base station or user; base
station will buffer arriving frames
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Wireless LANs – 802.11
MAC Sublayer Protocol
 PCF & DCF combined
o Central control (polling) combided with distributed
control by using 4 timers InterFrame Spacing
o Some dead time after every frame: 4 intervals
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Wireless LANs – 802.11
MAC Sublayer Protocol
 Transmission errors
o Wireless  noisy & unreliable
o Long frames  high chance for errors
 Solution: fragmentation (+ Checksum, seq number, stop&wait)
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Wireless LANs – 802.11
Frame Structure
 3 classes of frames: data, control, management
Data frame
Duration time to transmit frame + ack
Address 1 destination
Address 2 source
Seq.
Supports fragment numbering:
12 bits frame + 4 bits fragment
Address 3 destination base station
Address 4 source base station
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Wireless LANs – 802.11
Frame Structure
Data frame
Version
Support for different versions of protocol
Type
Data, control, management
Subtype
e.g. RTS, CTS
To DS
Frame to intercell distribution system
From DS
Frame from intercell distribution system
MF
More fragments to follow
Retry
Indicates retransmission of frame
Pwr
Used by base station to put receiver into sleep state or to wake it up
More
Sender has additional frames for receiver
W
Frame body encrypted using
WEP
Privacy)
Data
link(Wired
layer Equivalent
-- June 2004
O
Sequence of frames must be processed strictly in order
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Wireless LANs – 802.11
 Services:
o 5 distribution services:
•
•
•
•
•

Association
Disassociation
Reassociation
Distribution
Integration
o 4 station services:
•
•
•
•
Authentication
De-authentication
Privacy
Data delivery
Association:
 Connect to base station
 Announce identity & capabilities
 Data rates supported
 Need for PCF services
 Power management requirements
Determines how to route frames sent
to base station
Handles translation to format of
destination network
Data link layer -- June 2004
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Overview
 Design issues
 Channel allocation
 Point-to-point links
 Multi access protocols
 Local area Networks
 Ethernet
 Data Link layer Switching
 Logical link control
 Wireless LANs
 Broadband wireless
Data link layer -- June 2004
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Broadband wireless
 802.16 or
o Wireless MAN
o Wireless local loop
 Competition for last mile
o Cable
o Telephone system
Designed for
802.11
mobile ethernet
802.16
mobile cable television
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802.16 - Broadband wireless
 Comparison with 802.11
o Provide service to buildings and buildings are not mobile
o Better radios can be used
o Security & privacy are essential and mandatory
o Users need more bandwidth  operate in 10 – 66 GHz
o Waves are strongly absorbed by water  error handling
more important (error correction)
o Support for real-time traffic (telephony, television,…)
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802.16 - Broadband wireless
 Protocol stack
Replaces LLC: aims for integration with
Location of main protocols & channel management
 Connection oriented
 Controlled by base station
 Datagram protocols: PPP, IP, Ethernet
 ATM
Encryption
Decryption
Key management
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802.16 - Broadband wireless
 Physical layer
o Waves travel in straight lines
o Multiple antenna’s possible
o Signal strength falls off
sharply with distance
o 3 different modulation
schemes
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802.16 - Broadband wireless
 Physical layer
o Efficient use of available bandwidth: 2 schemes
• FDD: frequency division duplexing
• TDD: time division duplexing
• first 2 subframes: upstream & downstream maps
» What is in which slot?
» Which time slots are free?
–
–
–
–
–
–
Base station sends out frames
Each frame contains time slots
Variable # slots for upstream & downstream traffic
Base station maps downstream traffic to slots
Upstream slot allocation done by MAC sublayer
Error correction
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802.16 - Broadband wireless
 MAC protocol
o MAC frame
• Occupy an integral number of physical layer time slots
o Allocation of upstream slots
• Depends on service requested
• Connection-oriented
o Services:
•
•
•
•
Constant bit rate
Real-time variable bit rate
Non-real-time variable bit rate
Best-efforts rate
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802.16 - Broadband wireless
 MAC protocol
o Services:
• Constant bit rate
– Slots are reserved when connection is set up
• Real-time variable bit rate
– Polling at fixed rate to know how much bandwidth is
needed
• Non-real-time variable bit rate
– Polling (not at fixed rate)
– No response for k times  multicast group
• Best-efforts rate
– Requests done in time slots available for contention
– Success noted in downstream map
– No success: retry + binary exponential backoff
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802.16 - Broadband wireless
 Generic frame
 EC: payload encrypted?
 Type: frame type
 Bandwidth request frame
 CI: final CRC present?
 EK: which encryption key used
 Length
 Connection ID
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Overview
 Design issues
 Point-to-point links
 Local area Networks
 Data Link layer Switching
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Computer Networks
Data link layer
Data link layer -- June 2004
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