Datalink Layer: Examples

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Datalink Layer Examples

• 4/21/2008

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Recap Summary of MAC Protocols

• How do you access a shared media?
• channel partitioning, by time, frequency or code
• random access,
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• taking-turns
• polling
• token passing

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Recap Aloha Protocol

• Behaviors of Aloha on a LAN
• a total of m stations
• fixed transmission rate p for a backlogged
  station to transmit in a slot
• pa for each un-backlogged station

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Outline

• Admin. and recap
• MAC Examples

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Example MAC Protocols

• Example MAC protocols
• GSM
• Ethernet
• Wireless LAN
• Bluetooth
• There are many more link technologies
• e.g., ATM, DOCSIS, FDDI, Frame relay, IEEE 802.5
  Token Ring, PPP, WiMax, X.25, xDSL
• if you are interested, please see schedule page
  for a link to a set of optional slides
• Key factors traffic services

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Outline

• Admin. and recap
• MAC Examples
• GSM

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http//wireless.fcc.gov/uls/index.htm?jobhome
GSM - TDMA/FDMA
935-960 MHz 124 channels (200 kHz) downlink
frequency
890-915 MHz 124 channels (200 kHz) uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
guard space
guard space
tail
user data
Training
S
S
user data
tail
57 bits
1
1
3
3 bits
57 bits
26 bits
S indicates data or control
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Many Types of Logical Channels

• Control channels
• Broadcast control channel (BCCH)
• From base station, announces cell identifier,
  synchronization
• Common control channels (CCCH)
• Paging channel (PCH) Base transceiver station
  (BTS) pages a mobile host (MS)
• Random access channel (RACH) MSs for initial
  access, using slotted Aloha
• Access grant channel (AGCH) BTS informs an MS
  its allocation
• Dedicated control channels
• Standalone dedicated control channel (SDCCH)
  signaling and short message between MS and an MS
• Traffic channels (TCH)

• Example call setup from an MS

BTS
MS
SDCCH message exchange
Communication using TCH
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GPRS GSM Data Services

• Using GSM, an MS can use a (logical) traffic
  channel to send data
• data rate standardized at 9.6 kbps
• General Packet Radio Service (GPRS)
• allocate multiple slots from the same frame by
  reserving different number of slots and using
  different coding scheme, an MS achieves different
  rate (kbps)
• simplified signaling process still uses a random
  channel to request frequency and time slot

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GPRS Signaling
PRACH Pkt. Random Access Channel PAGCH Pkt.
Access Grant Channel PTCH Pkt. Traffic
Channel USF uplink state flag
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UMTS Enhancements of GSM

• UMTS (Universal Mobile Telecommunications System)
  
• Use CDMA for channel partitioning
• less fragmented channels
• additional requirement allocate different amount
  of bw to mobile stations
• W-CDMA
• chipping rate 5 MHz, 3.840 Mchip/s

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Orthognal Variable Spreading Factor (OSVF)

• By assigning a code with a low spreading factor,
  a node receives higher bw.

1,1,1,1,1,1,1,1
...
1,1,1,1
1,1,1,1,-1,-1,-1,-1
1,1
1,1,-1,-1,1,1,-1,-1
...
1,1,-1,-1
X,X
1,1,-1,-1,-1,-1,1,1
1
X
1,-1,1,-1,1,-1,1,-1
X,-X
...
1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,-1
SFn
SF2n
1,-1,-1,1,1,-1,-1,1
...
1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
SF1
SF2
SF4
SF8
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Outline
Outline

• Admin. and recap
• Example MAC protocols
• GSM
• Channel partitioning (time, freq., code) and
  slotted Aloha
• Ethernet

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Ethernet

• Dominant LAN technology
• First widely used LAN technology
• Kept up with speed race 10 Mbps, 100 Mbps, 1
  Gbps, 10 Gbps

Metcalfes Ethernet sketch
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Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble 8 bytes
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011 (why the preamble?)
• Source and dest. addresses 6 bytes
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC CRC-32 checked at receiver, if error is
  detected, the frame is simply dropped

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The Basic MAC Mechanisms of Ethernet
get a packet from upper layer K 0 n
0 // K control wait time n no. of
collisions repeat wait for K 512 bit-time
while (network busy) wait wait for 96
bit-time after detecting no signal transmit
and detect collision if detect collision
stop and transmit a 48-bit jam signal
n m min(n, 10), where n is the
number of collisions choose K randomly
from 0, 1, 2, , 2m-1. if n repeat else give up
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Ethernets Exponential Backoff

• Goal adapt retransmission attempts to estimated
  current load
• compared with CSMA, 1/2m can be considered as p
• not a static p---adjusted using exponential
  backoff
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

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Ethernet From Bit to Electrical Signal

• Use Manchester encoding
• One voltage change per bit
• for a 1, a voltage change from 1 to 0
• for a 0, a voltage change from 0 to 1
• Example

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Ethernet Technologies 10Base2

• 10 10Mbps 2 under 200 meters max cable length
• Thin coaxial cable in a bus topology

Issues of such connectivity?
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10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Hub to which nodes are connected by twisted pair,
  thus star topology
• there is a bus inside the hub boost signal from
  one port to all other ports

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Interconnecting with hubs

• Multiple hubs interconnect to form a larger
  Ethernet network
• extends max distance between nodes more ports

Issue individual segment collision domains
become one large collision domain
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Ethernet Bridges

• Link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• segments become separate collision domains

LAN (IP network)
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Bridge Forwarding
Key issue How do determine to which LAN segment
to forward frame?
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Ethernet Bridge Self Learning

• A bridge has a bridge table
• Entry in bridge table
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• Bridges learn which hosts can be reached through
  which interfaces
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender/location pair in bridge table

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Filtering/Forwarding

• When bridge receives a frame
• index bridge table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
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Ethernet Bridge Example

• Suppose C sends frame to D and D replies back
  with frame to C.

• Bridge receives frame from C to D
• notes in bridge table that C is on interface 1
• because D is not in table, bridge sends frame
  into interfaces 2 and 3
• frame received by D

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Bridge Learning Example
C 1

• D generates frame for C, sends
• Bridge receives frame
• notes in bridge table that D is on interface 2
• bridge knows C is on interface 1, so selectively
  forwards frame to interface 1

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Bridges Spanning Tree

• For increased reliability, desirable to have
  redundant, alternative paths from source to dest
• With multiple paths, cycles result - bridges may
  multiply and forward frame forever
• Solution organize bridges in a spanning tree by
  disabling subset of interfaces

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Bridges vs. Routers

• both store-and-forward devices
• routers network layer devices (examine network
  layer headers)
• bridges are link layer devices
• routers maintain routing tables, implement
  routing algorithms
• bridges maintain bridge tables, implement
  filtering, learning and spanning tree algorithms

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Routers vs. Bridges

• Bridges and -
• Bridge operation is simpler
• Bridge tables are self learning
• - All traffic confined to spanning tree, even
  when alternative bandwidth is available
• - Bridges do not offer protection from broadcast
  storms (flooding of packets)

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Routers vs. Bridges

• Routers and -
• arbitrary topologies can be supported
• provide protection against broadcast storms
• - require IP address configuration (not plug and
  play)
• - require higher packet processing
• bridges do well in small (few hundred hosts)
  while routers used in large networks (thousands
  of hosts)

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Gbit Ethernet and Ethernet Switches

• Gbit Ethernet typically use Ethernet switches
• Essentially a multi-interface bridge
• layer 2 (frame) forwarding, filtering using LAN
  addresses
• Switching A-to-A and B-to-B simultaneously, no
  collisions
• cut-through switching frame forwarded from input
  to output port without awaiting for assembly of
  entire frame

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Not an atypical LAN (IP network)
Dedicated
Shared
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Summary Comparison
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Outline

• Admin. and recap
• Example MAC protocols
• GSM
• Channel partitioning and slotted Aloha
• Ethernet
• Random MAC protocol (CSMA/CD Exponential
  backoff)
• Wireless LAN

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802.11 Traffic Services and Access Methods

• Two types of traffic services
• Asynchronous Data Service (mandatory)
• exchange of data packets based on best-effort
• implemented by random access
• Time-Bounded Service (optional)
• Two types of coordination function (aka MAC)
• DCF (Distributed Coordination Function)
• PCF (Point Coordination Function)
• access point polls

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IEEE 802.11 Wireless LAN

• Basic Service Set (BSS) (a.k.a. cell) contains
• wireless station (WS)
• access point (AP) base station
• BSSs combined to form distribution system (DS)
• Two operation modes
• infrastructure mode
• everything through AP
• peer-to-peer mode
• called ad hoc network

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Random Access Carrier Sense in 802.11
B
A
C

• The hidden-terminal problem
• A is sending to B, but C cannot receive from A
• Friis Law (power decay proportional to distance
  squared)
• Therefore C sends to B, without detecting the
  transmission from A to B
• In summary, A is hidden for C

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The Exposed Terminal Problem
B
A
C
D

• B is sending to A, C intends to send to D
• C senses an in-use medium, thus C waits
• But A is outside the radio range of C, therefore
  waiting is not necessary
• In summary, C is exposed to B
• Implication false carrier sense

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Summary of Problems of Wireless MAC

• How to achieve carrier sense?
• in Ethernet, we use carrier sense to avoid and
  detect potential collision
• for wireless networks, the hidden-terminal, and
  the exposed-terminal problems make carrier sense
  (i.e., listen before talk) neither sufficient nor
  necessary
• not detected transmission at the sender does not
  imply no current transmission to the receiver
• detected transmission at the sender does not
  imply transmission will cause collision
• How to integrate random access (DCF) and taking
  turns (PCF)?

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Basic Solution Using RTS/CTS to Address the
Carrier Sense Problem

• Short signaling packets---virtual carrier sense
• RTS (request to send) and CTS (clear to send)
• to avoid collision at the receiver, any station
  who hears a CTS should not transmit
• frames need to contain sender address, receiver
  address, transmission duration

B
A
C
E
F
D
Example A sends to B
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Basic Solution Using Inter Frame Spacing to
Prioritize Access

• Different inter frame spacing (IFS) if the
  required IFS of a type of message is short, the
  type of message has higher priority
• SIFS (Short Inter Frame Spacing)
• highest priority, for ACK, CTS, polling response
• PIFS (Point Coordination Function Spacing)
• medium priority, for time-bounded service using
  PCF
• DIFS (Distributed Coordination Function Spacing)
• lowest priority, for asynchronous data service

DIFS
PIFS
SIFS
medium busy
next frame
contention
t
Access point access if medium is free ? DIFS
random direct access if medium is free ? DIFS
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Basic Control Flow of RTS/CTS

• Sender sends RTS with NAV (Network allocation
  Vector, i.e. reservation parameter that
  determines amount of time the data packet needs
  the medium) after waiting for DIFS
• Receiver acknowledges via CTS after SIFS (if
  ready to receive)
• CTS reserves channel for sender, notifying
  possibly hidden stations
• any station hearing CTS should be silent for NAV
• Sender can now send data at once

DIFS
data
RTS
sender
SIFS
SIFS
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
new contention
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802.11 RTS/CTS ACK

• 802.11 adds ACK in the signaling to improve
  reliability
• implication to avoid conflict with ACK, any
  station hearing RTS should not send for NAV
• thus a station should not send for NAV if it
  hears either RTS and CTS
• Note RTS/CTS is optional in 802.11, and thus may
  not be always turned on---some network interface
  cards turn it on only when the length of a frame
  exceeds a given threshold

DIFS
data
RTS
sender
SIFS
SIFS
SIFS
ACK
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
new contention
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802.11 PCF for Polling
SIFS
PIFS
D
D
point coordinator
SIFS
U
polled wireless stations
NAV
NAV
contention free period
t
medium busy
contention period
D downstream poll, or data from point
coordinator U data from polled wireless station
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802.11 - Frame Format

• Before the MAC header are
• an 80-bit preamble of alternating 0 and 1 for
  clock sync.
• a physical layer header (PLCP) which is always
  transmitted at 1 Mbps, including signaling fields
  such as sending rate
• Duration ID NAV
• The four addresses are used to encode various
  addresses
• e.g., Addr 1 is always the recipient address
  (i.e., the immediate recipiet of the frame), Addr
  2 is always the transmitter addr
• CRC check sum

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802.11 Frame Control Field
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Outline

• Admin. and recap
• Example MAC protocols
• GSM
• Channel partitioning and slotted Aloha
• Ethernet
• Random MAC protocol (CSMA/CD Exponential
  backoff)
• Wireless LAN
• Random MAC protocol (CSMA/CA RTS/CTS) Polling
• Bluetooth

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Bluetooth Design Objective

• Design objective a cable replacement technology
  to connect a small number of devices
• 1 Mb/s
• range 10 meters
• single chip radio baseband (means digital part)
• low power
• low price point (target price 5)
• Traffic Services
• SCO Synchronous connected link (fixed periodical
  traffic)
• ACL Asynchronous connectionless link

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Bluetooth

• Nodes in Bluetooth form piconet one master and
  upto 7 slaves
• Each radio can function as a master or a slave
• SCO a slave reserves with the master a slot for
  a synchronous connected link
• ACL The master polls slaves for asynchronous
  connectionless traffic

A piconet
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Bluetooth Links
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Coexistence of Bluetooth and 802.11

• Bluetooth shares the same freq. range as of
  802.11
• There are can be multiple piconets in close
  range, causing inteference (how about multiple
  802.11?)
• Question how to share among piconets and with
  802.11?

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Bluetooth Frequency Hopping

• Divide spectrum into 79 frequencies
• Master conducts pseudorandom frequency hopping
• The slaves follow the pseudorandom jumping
  sequence of the master

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Bluetooth Frequency Hopping
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MAC Summary

• In practical protocols, various MAC techniques
  are often combined to achieve objectives
• GSM
• Channel partitioning and slotted Aloha
• Ethernet
• Random MAC protocol (CSMA/CD Exponential
  backoff)
• Wireless LAN
• Random MAC protocol (CSMA/CA RTS/CTS) Polling
• Bluetooth
• Time partitioning, polling, and random hopping
• For physical layer, please see the optional
  slides linked on the schedule page

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Backup
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Comparisons of Different Ethernet Standards