More on the link layer Logical Link Control (LLC) Medium Access Control (MAC)

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More on the link layerLogical Link Control
(LLC)Medium Access Control (MAC)
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link layer functions

• services
• un-ACKed connectionless
• ACKed connectionless
• ACKed connection oriented
• framing
• encoding
• error control
• flow control (vs. congestion control?)
• MAC

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point-to-point vs. broadcast media

• point-to-point
• PPP for dial-up access
• point-to-point link between Ethernet switch and
  host
• broadcast (shared wire or medium)
• traditional Ethernet
• 802.11 wireless LAN

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data link protocols (logical)

• unrestricted simplex
• simplex stop-and-wait
• simplex for noisy channel
• discussion
• sliding window protocols

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sliding window protocols

• piggybacked ACKs
• less overhead (bandwidth, interrupts, buffering,
  ...)
• how to deal with timeouts?
• sequence numbers
• sending window
• receiving window

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1-frame sliding window
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1-bit sliding window
(seq, ACK, pkt) ACK last OK frame
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pipelining

• idea
• do not block transmitter during the roundtrip
  time
• increase the window size
• what happens with errors
• go back n
• selective repeat

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go back n

• if error
• receiver discards subsequent frames
• no ACKs for the discarded frames
• receiver window size 1
• transmitter times-out, resends unACKed frames
• inefficient if the error rate is high

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selective repeat

• if error
• receiver still stores the subsequent good frames
• transmitter retransmits the bad frame
• receiver window size gt 1
• efficient at higher error rate
• trade-off between bandwidth and buffer space

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sliding window schemes
go back n

• go back n (frames received out of sequence
  discarded)
• selective repeat (frames received out of
  sequence buffered)
• (after error, receiver may NAK frame 2
  to short-circuit sender timeout)

(a
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3-bit sequence numbers
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line utilization

• b channel capacity in bits per second
• f frame size in bits
• R round-trip propagation time
• frame transmission time (f / b) seconds
• line utilization f / (f bR)
• if f lt bR, then efficiency lt 1/2

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window size

• sender needs n buffers for window size n
• go back n
• sender needs enough buffers for
• timeout
• RTT of frame NACK
• selective repeat
• window size floor((maxseq1)/2)
• why?
• optimal window size, sequences

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(review this in detail)protocol specification by
FSM

1. state diagram key to state ovals (SRC), S in
   0,1, R in 0,1, C in 0,1,A,-
2. transition chart

(from Tanenbaum text)
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(review this in detail)Petri net for simplex
wait for ACK

• Tanenbaum 4/e, 233
• places, tokens, transitions, input/output arcs
• tokens not conserved
• no composite states
• sender, receiver, channel separated
• transitions may be viewed as a grammar

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multiple access protocols

• single shared broadcast channel
• multiple nodes can speak at once
• collisions lead to garbled data
• multiple access protocol (medium access control)
• distributed algorithm for sharing the channel
• algorithm determines which node can transmit

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types of MAC

• static bandwidth allocation channel partitioning
• TDM
• FDM
• CDMA (see handout)
• problems?
• deterministic sharing
• token-passing
• polling
• reservations, scheduling
• contention
• random access allow collisions, and then recover
• ALOHA, CSMA, more

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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)

• polling
• master node invites slave nodes to transmit in
  turn
• concerns
• polling overhead
• latency
• single point of failure (master)

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token rings

• (a) transmit token after frame is sent
• (b) transmit token while frame is being stripped

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ring topology

• self-healing ring (SHR)

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random access protocols

• when node has packet to send
• transmit at full channel data rate
• no a priori coordination among nodes
• gt 2 nodes transmit concurrently ? collision
• random access MAC protocol specification
• collision detection
• collision recovery
• examples
• ALOHA, slotted ALOHA
• CSMA, CSMA/CD, CSMA/CA

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key ideas of random access

• station model
• n independent stations (nodes, terminals)
• single channel
• all transmission/reception on shared channel
• nodes have equivalent ability
• node priority may vary dynamically
• time
• continuous
• slotted (discrete, timed intervals, master
  clock)

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key ideas of random access

• carrier sense (or not)
• listen before speaking, and dont interrupt
• check if someone else is already sending data ...
• wait till the other node is done
• collision detection (or not)
• if someone else starts talking at the same time,
  stop
• if the data on the wire is garbled ...
• ... another node is transmitting, too, so stop
• randomness
• dont start talking again right away
• wait for a random time before trying again

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pure ALOHA

• in pure ALOHA, frames are transmitted at
  completely arbitrary times

temporal collisions destroy colliding frames
send when data arrives if collision, random
delay, resend
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ALOHA

• Vulnerable period for the shaded frame.

shaded frame vulnerability period modeled as two
frame times
intuition collisions of partially-overlapping
frames
slotted ALOHA during contention, frames
collide in some single slot/frame time
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slotted ALOHA

• assumptions
• all frames same size
• time divided into equal slots (time to transmit a
  frame)
• nodes start to transmit frames only at start of
  slots
• nodes are synchronized
• if two or more nodes transmit, all nodes detect
  collision

• operation
• when node obtains fresh frame, transmits in next
  slot
• no collision node can send new frame in next
  slot
• collision node retransmits frame in each
  subsequent slot with probability p until success

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pure vs. slotted ALOHA

• Throughput versus offered traffic for ALOHA
  systems.

max throughput ALOHA 1/(2e) 18 slotted
ALOHA 1/e 37
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CSMA carrier sensing multiple access

• 1-persistent CSMA
• if idle, send ... if collision, random delay,
  sense ...
• propagation delay ? collision
• two nodes waiting for idle ? collision
• idea behind Ethernet LAN protocol
• non-persistent CSMA
• if idle, send else, random delay
• p-persistent CSMA (slotted time)
• if idle, send with probability p if collision,
  random delay
• slotted transmission discipline

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persistent and non-persistent CSMA
non-persistent CSMA very good throughput under
high load 0.01-persistent CSMA best throughput
tolerance for delay can enhance throughput in
chaotic environments
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collision-free protocols

• basic bit-map protocol

- contention slots are constant overhead -
overhead means less as frames get large
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collision-free protocols
addresses
(AND of addresses)

• binary countdown protocol (log2n bits) dash
  indicates silence.

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limited-contention protocols

• Acquisition probability for a symmetric
  contention channel.

throughput of contention protocols under high load
1/e
- motivation for hybrid deterministic/probabilisti
c protocols - idea allow contention at low load,
use taking-turns at high load
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adaptive tree walk protocol

• tree for eight stations depth first search, LR

nodes
- at idle, each station ready to send data
signals 1 or not, according to a clever
plan - example only station H ready to send
slot1 1 (candidate sender is under 1),
s2 0 (not under 2), s3 0 (not under 6),
s4 0 (not G), s5 1 (H). - for binary tree,
sender is chosen in O(log2(n)) time
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WDM access protocol (one example out of 100s)

• Wavelength division multiple access.

- fixed data output control input - tunable
data input control output - on control channel
control slots on data channel, status slot -
classes of traffic CBR, VBR, datagram
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IEEE 802.3 Ethernet

• Metcalfe, Boggs, 1976 first LAN, the
  Ethernet
• TCP/IP/Ethernet a connectionless stack
• simple to use, reliable, cheap, scalable
• LAN collision domains (broadcast) (bus, hub)
• store-and-forward switches, point-to-point links
• 10 Mbps, 100 Mbps, gigabit, 10 gigabit
• becoming very rare for network paths not to
  traverse any Ethernet links

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original flavor 10 Mbps Ethernet

• common kinds of Ethernet cabling

only twisted pair and fiber are still being
deployed (except in specialized environments)
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10 Mbps (plain) Ethernet PHY
(a) and (b) (10base5, 10base2) seldom
deployed (c) 10baseT hub is a collision domain
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10 Mbps Ethernet PHY topologies
- broadcast medium - same logical topologies - no
loops (rings) allowed

• no path has gt 4 repeaters
• - network diameter lt 2500 m

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intermission
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Ethernet MAC sublayer protocol

• - no two nodes are farther apart than A and B
• t is the diameter of the network,
• the one-way propagation time between the
  farthest nodes

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Ethernet CSMA/CD (Collision Detection)
contention slot time 2t max(signal round-trip
time)
(10 Mbps Ethernet slot 51.2 microsec 512
100-nanosec-wide bits)
contention period series of slot-length
collisions/jamming frames half-duplex cannot
receive while listening for own transmission
collision

• sense if idle, send if collision, abort,
  random delay

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performance of Ethernet
efficiency of 10 Mbps Ethernet, 512-bit slot times
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switched Ethernet vs. hub
half-duplex
collision domain
full-duplex point-to-point links
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100 Mbps (fast) Ethernet cabling
T4 4 each, cat 3 unshielded twisted pairs,
3 pairs simplex forward, 1 pair simplex
reverse (dynamic) ternary encoding
(trits, not bits) TX 2 each cat 5 unshielded
twisted pairs (opposing simplex) FX 2 multimode
fiber (opposing simplex), point-to-point links
only
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gigabit Ethernet cabling
100 m and 25 m copper segments used, e.g., in
data center or POP of ISP
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Ethernet MAC sublayer framing
(a) DIX (Digital, Intel, Xerox) (b) IEEE 802.3
preamble for receiver clock sync dest addr 0
unicast, 1 multicast, all 1s broadcast type
network protocol to call at dest, OR length
data bytes (type embedded in data) pad frame
length (without preamble) gt 64 bytes 512 bits
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Why MAC is a sublayer

• Ethernet (one of the MAC protocols) interfaces
  directly to network layer (IP)
• the Ethernet MAC offers best-effort,
  no-guarantee, datagram service
• this is great for TCP/IP, nothing else is needed
• but, other network layer protocols expect link
  layer error control and flow control services
• IEEE 802.2 (LLC) supports these services, built
  on various MAC sublayers (e.g., Ethernet)

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IEEE 802.2 logical link control
- Ethernet MAC sends best effort datagrams - LLC
supports flow-control error-control
PHY
PHY

• (a) network layer sees the same LLC, regardless
  of type of MAC
• (b) LLC encapsulates network layer packet,
• MAC encapsulates LLC frame before passing
  to PHY

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wireless LANs

• IEEE 802.11 (WiFi)
• Distributed Coordination Function (DCF )
• CSMA-CA
• Point Control Function (PCF)
• centrally controlled by basestation (access
  point)
• short-range RF (rooms, battlefields)
• ad hoc and basestation flavors
• many PHY layer options
• 802.11, 802.11a, 802.11b, 802.11g, 802.11n
  (pre-std)

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The 802.11 Protocol Stack
11 Mbps
54 Mbps
54 Mbps
1-2 Mbps
1997
1999
2001
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CSMA fails off the wire
why CSMA falls short in packet radio networks
and mobile ad hoc networks

• C wants to send to B
• channel sounds clear to C
• C cannot hear A
• the hidden station problem

C wants to send to D channel sounds busy to C D
cannot hear B the exposed station problem
(not unhidden station problem)
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wireless LAN protocols (CSMA-CA)

• MACA(W), multiple access collision avoidance
• A sends RTS to B, B sends CTS to A
• all potential interrupters hear
  B's CTS, wait for frame
• (Wireless sense first, ACK every frame,
  sophisticated backoff)

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802.11 MAC sublayer protocol

• The use of virtual channel sensing using CSMA/CA.

- C hears As RTS D hears Bs CTS (NAV
transmitter quiet time) - notice how politely C
and D each set aside time for A and B
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802.11 MAC sublayer protocol

• A fragment burst.

• A sends a burst to B C and D wait for it
• after each data or control frame, a system of
  delay intervals

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802.11 delay timing

• how can PCF and DCF protocols coexist?
• end of frame or ACK starts series of timers
• Short, PCF, DCF, Expanded InterFrame Spacing
• Short for burst fragment, receiver ACK, or CTS
• PCF for central control (beacons, polling)
• DCF for contention (RTS)
• Extended for bad frame reporting (NAK)

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802.11 MAC sublayer protocol
PCF and DCF coexist in a single collision domain

• Interframe spacing in 802.11.

fragment
CTS
FRAME
dead
ACK
central ctl
RTS
NAK
ACK
the starting guns go off at different times for
different frame types
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RF signaling

• RF signal strength at node's own antenna
• 2-antenna implementation?
• shared channel
• RF signal strength from distant antennae
• how to detect?
• interference
• fading
• multipath

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IEEE 802.11

• access point infrastructure largely 802.11
• 100 Million 802.11 chipsets per annum (out of
  date statistic)
• strong application development efforts
• IEEE 802.11 spec CSMA-CA
• RTS/CTS channel reservation, ACK
• explicit ACK
• CSMA sender will not hear interference, fading,
  multipath
• contention short RTS frames, collisions waste
  less
• if senders CTS times out, it knows the RTS
  failed
• random backoff (countdown proceeds while channel
  is idle)

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802.11 distribution services

• association (connect to access point cell)
• beacons in the jungle there can only be one
• next, DHCP discovery
• disassociation
• reassociation (cell-to-cell handover)
• distribution (how to route frames)
• integration (802.11 ? external network format)

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802.11 intracell services

• authentication (challenge frame, key, encryption)
• if invoked, a pre-condition for association
• deauthentication
• privacy (data encryption)
• data delivery (best effort)

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802.11 data frame

• type data, ctl, mgt subtype RTS, CTS, probe
  (scanning for new AP)
• to DS/from DS activation of address 3, 4 for
  distribution system APs
• MF more frags retry more (frames)
• duration, sequence

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broadband wireless comparison ...

• 802.11 (WiFi)
• mobile Ethernet LAN
• centralized infrastructure (APs and cell
  architecture)
• or, distributed architecture
• ad hoc
• mobile ad hoc (MANET)
• best effort delivery
• short range, half-duplex
• power concerns
• limited budget (commodotized)

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... broadband wireless comparison

• 802.16 (WiMax)
• wireless local loop for buildings
• metro area coverage, full duplex
• many users aggregated per endpoint
• connection oriented
• FEC (Hamming codes), security
• base station control (centralized control)
• fixed, directional antennas

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802.16 Protocol Stack
modulation schemes vary with range to end-points
(what is wrong with this picture?)
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The 802.16 Physical Layer

• The 802.16 transmission environment.

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The 802.16 Physical Layer

• shown TDD (time division duplexing) PHY frames
• not shown FDD (frequency division duplexing)
• millimeter RF waves are not omnidirectional

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The 802.16 Frame Structure

• (a) A generic frame. (b) A bandwidth request
  frame.

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802.16 MAC sublayer protocol

• service classes
• constant bit rate service (regular slots)
• real-time variable bit rate service (regular
  polling)
• non-real-time variable bit rate service (frequent
  polling)
• best effort service (contend for request slots)
• aggregator switch at subscriber building may
  negotiate with base station for all users, and
  arbitrate received bandwidth between users

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personal area networks (PANs)

• Bluetooth (Ericsson, IBM, Intel, Nokia, Toshiba)
• cable replacement
• specifies complete networking stack
• TDM 10 m 2.4 Ghz FHSS 79 1-MHz channels
• IEEE 802.15
• only PHY LL
• interferes with 802.11 (2.4 GHz)
• master/slave piconets
• slaves can bridge piconets to form scatternets

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remarks

• networks run on various link layer technologies
• point-to-point links vs. shared media
• wide varieties within each class
• link layer performs key services
• encoding, framing, and error detection
• optionally error correction and flow control
• shared media introduce interesting challenges
• decentralized control over resource sharing
• partitioned channel, taking turns, and random
  access
• Ethernet as a wildly popular example
• next switches and bridges

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summary/glossary