1. Medium Access Control Sublayer Chapter 4
Channel Allocation Problem
Multiple Access Protocols
Ethernet
Wireless LANs
Broadband Wireless
Bluetooth
RFID
Data Link Layer Switching
Physical
Link
Network
Transport
Application
MAC is in here
Gray units can be optionally omitted without causing later gaps
MAC Sublayer is responsible for deciding who sends next on a multi-access link
An important part of the link layer, especially for LANs

2. Channel Allocation Problem (1)
For fixed channel and traffic from N users
Divide up bandwidth using FTM, TDM, CDMA, etc.
This is a static allocation, e.g., FM radio
This static allocation performs poorly for bursty traffic
Allocation to a user will sometimes go unused
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3. Channel Allocation Problem (2)
Dynamic allocation gives the channel to a user when they need it. Potentially N times as efficient for N users. Schemes vary with assumptions:
Assumption
Implication
Independent traffic
Often not a good model, but permits analysis
Single channel
No external way to coordinate senders
Observable collisions
Needed for reliability; mechanisms vary
Continuous or slotted time
Slotting may improve performance
Carrier sense
Can improve performance if available
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4. Multiple Access Protocols
ALOHA »
CSMA (Carrier Sense Multiple Access) »
Collision-free protocols »
Limited-contention protocols »
Wireless LAN protocols »
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5. ALOHA (1)
In pure ALOHA, users transmit frames whenever they have data; users retry after a random time for collisions
Efficient and low-delay under low load `
Collision
Time
User A B C D E
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6. ALOHA (2)
Collisions happen when other users transmit during a vulnerable period that is twice the frame time
Synchronizing senders to slots can reduce collisions
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7. ALOHA (3) Slotted ALOHA is twice as efficient as pure ALOHA
Low load wastes slots, high loads causes collisions
Efficiency up to 1/e (37%) for random traffic models
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8. CSMA (1) CSMA improves on ALOHA by sensing the channel!
User doesn’t send if it senses someone else
Variations on what to do if the channel is busy:
1-persistent (greedy) sends as soon as idle
Nonpersistent waits a random time then tries again
p-persistent sends with probability p when idle
CSMA outperforms ALOHA, and being less persistent is better under high load
uses a refinement of p-persistent CSMA

9. CSMA (3) – Collision Detection
CSMA/CD improvement is to detect/abort collisions
Reduced contention times improve performance
Collision time is much shorter than frame time
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10. CSMA (4)
If two stations sense the channel to be idle and begin transmitting simultaneously, their signals will still collide.
An improvement is for the stations to quickly detect the collision and abruptly stop transmitting since they are irretrievably garbled anyway.
This strategy saves time and bandwidth. This protocol, is known as CSMA/CD(CSMA with Collision Detection).
In a hub, all stations are in the same Collision domain. They must use the CSMA/CD algorithm to schedule their transmissions.

11. Collision-Free (1) – Bitmap
Collision-free protocols avoid collisions entirely
Senders must know when it is their turn to send
The basic bit-map protocol:
Sender set a bit in contention slot if they have data
Senders send in turn; everyone knows who has data
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12. Collision-Free (2) – Token Ring
Token sent round ring defines the sending order
Station with token may send a frame before passing
Idea can be used without ring too, e.g., token bus
Station
Direction of
transmission
Token
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13. Collision-Free (3) – Countdown
Binary countdown improves on the bitmap protocol
Stations send their address in contention slot (log N bits instead of N bits)
Medium ORs bits; stations give up when they send a “0” but see a “1”
Station that sees its full address is next to send
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14. Limited-Contention Protocols (1)
Idea is to divide stations into groups within which only a very small number are likely to want to send
Avoids wastage due to idle periods and collisions
Already too many contenders for a good chance of one winner
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15. Limited Contention (2) –Adaptive Tree Walk
Tree divides stations into groups (nodes) to poll
Depth first search under nodes with poll collisions
Start search at lower levels if >1 station expected
Level 0
Level 1
Level 2
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16. Ethernet Classic Ethernet » Switched/Fast Ethernet »
Gigabit/10 Gigabit Ethernet »
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17. Classic Ethernet (1) – Physical Layer
One shared coaxial cable to which all hosts attached
Up to 10 Mbps, with Manchester encoding
Hosts ran the classic Ethernet protocol for access
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18. Classic Ethernet (2) – MAC
MAC protocol is 1-persistent CSMA/CD (earlier)
Random delay (backoff) after collision is computed with BEB (Binary Exponential Backoff)
Frame format is still used with modern Ethernet.
Ethernet
(DIX)
IEEE 802.3
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19. Classic Ethernet (3) – MAC
Collisions can occur and take as long as 2 to detect
 is the time it takes to propagate over the Ethernet
Leads to minimum packet size for reliable detection
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20. Classic Ethernet (4) – Performance
Efficient for large frames, even with many senders
Degrades for small frames (and long LANs)
10 Mbps Ethernet,
64 byte min. frame
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21. Switched/Fast Ethernet (1)
Hubs wire all lines into a single CSMA/CD domain
Switches isolate each port to a separate domain
Much greater throughput for multiple ports
No need for CSMA/CD with full-duplex lines
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22. Switched/Fast Ethernet (2)
Switches can be wired to computers, hubs and switches
Hubs concentrate traffic from computers
More on how to switch frames the in 4.8
Switch
Twisted pair
Switch ports
Hub
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23. Switched/Fast Ethernet (3)
Fast Ethernet extended Ethernet from 10 to 100 Mbps
Twisted pair (with Cat 5) dominated the market
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24. Gigabit / 10 Gigabit Ethernet (1)
Switched Gigabit Ethernet is now the garden variety
With full-duplex lines between computers/switches
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25. Gigabit / 10 Gigabit Ethernet (1)
Gigabit Ethernet is commonly run over twisted pair
10 Gigabit Ethernet is being deployed where needed
40/100 Gigabit Ethernet is under development
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26. UCL Record
1.125 terabytes per second, a new optical communication system developed by UCL set new record for fastest data transfer rate for digital information.
At the quoted rate, entire HD series of the TV show Game of Thrones could be downloaded in less than one second.
They used noise reduction techniques normally found in wireless communications and applied them to optical transmission. I
Building on previous work, where the team transmitted optical signals over a world-record 5,890 km error-free, the new system employs a total of 15 data transmission channels, each carrying optical signal of different wavelengths.
Modulated using the 256QAM format normally employed in cable modems, the 15 signals were combined and then sent to a single optical receiver for detection.

27. Wireless LANs 802.11 architecture/protocol stack »
physical layer »
MAC »
frames »

28. Characteristics of selected wireless link standards
200
802.11n 54
802.11a,g
802.11a,g point-to-point
data
5-11
802.11b
(WiMAX) 4
3G cellular
enhanced
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
Data rate (Mbps) 1
802.15
.384
UMTS/WCDMA, CDMA2000 3G
.056
IS-95, CDMA, GSM 2G
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
5Km – 20 Km

29. Wireless network taxonomy
single hop
multiple hops
host connects to base station (WiFi, WiMAX, cellular)which connects to
larger Internet
host may have to relay through several wireless nodes to connect to larger
Internet: mesh net
infrastructure
(e.g., APs)
no base station, no connection to larger
Internet (Bluetooth, ad hoc nets)
no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET no
infrastructure
Access Point
Client
To Network

30. Wireless Link Characteristics
Differences from wired link ….
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”

31. Wireless & MAC
A station on a wireless LAN may not be able to transmit frames to or receive frames from all other stations because of the limited radio range of the stations.
In wired LANs, when one station sends a frame, all other stations receive it. The absence of this property in wireless LANs causes a variety of complications.
On a wireless network, the problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called the hidden terminal problem.
There is also the exposed terminal problem.
Nodes cannot detect collisions, i.e., sense while sending
Makes collisions expensive and to be avoided

32. Wireless LANs (2) – Hidden terminals
Hidden terminals are senders that cannot sense each other but nonetheless collide at intended receiver
Want to prevent; loss of efficiency
A and C are hidden terminals when sending to B

33. Wireless LANs (3) – Exposed terminals
Exposed terminals are senders who can sense each other but still transmit safely (to different receivers)
Desirably concurrency; improves performance
B  A and C  D are exposed terminals

34. Wireless LANs (4) – MACA
MACA protocol grants access for A to send to B:
A sends RTS to B [left]; B replies with CTS [right]
A can send with exposed but no hidden terminals
This protocol solves the issue of expensive collisions too, since the RTS/CTS are short packets – if there is a collision it will be brief, not as long as a data frame.
A sends RTS to B; C and E hear and defer for CTS
B replies with CTS; D and E hear and defer for data

35. 802.11 Architecture/Protocol Stack (2)
MAC is used across different physical layers
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36. physical layer
NICs are compatible with multiple physical layers
E.g., a/b/g
Name
Technique
Max. Bit Rate
802.11b
Spread spectrum, 2.4 GHz
11 Mbps
802.11g
OFDM, 2.4 GHz
54 Mbps
802.11a
OFDM, 5 GHz
802.11n
OFDM with MIMO, 2.4/5 GHz
600 Mbps
Remember, most LAN interfaces have a promiscuous mode, in which all frames are given to each computer, not just those addressed to it.
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37. physical layer
All of the transmission methods define multiple rates.
The idea is that different rates can be used depending on the current conditions.
If the wireless signal is weak, a low rate can be used. If the signal is clear, the highest rate can be used.
This adjustment is rate adaptation.
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38. 802.11 MAC (1) CSMA/CA inserts backoff slots to avoid collisions
MAC uses ACKs/retransmissions for wireless errors
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39. MAC (2)
Virtual channel sensing with the NAV and optional RTS/CTS (often not used) avoids hidden terminals
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40. 802.11 MAC (3) Different backoff slot times add quality of service
Short intervals give preferred access, e.g., control, VoIP
MAC has other mechanisms too, e.g., power save
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41. 802.11 Frames Frames vary depending on their type (Frame control)
Data frames have 3 addresses to pass via APs
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42. frame: addressing
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Address 4: used only in ad hoc mode
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 3: MAC address
of router interface to which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame

43. 802.11 frame: addressing Internet router H1 R1 AP
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
frame H1 R1
R1 MAC addr H1 MAC addr
dest. address
source address
802.3 frame

44. 802.11 frame: more frame seq # (for RDT) duration of reserved
transmission time (RTS/CTS)
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Type
From AP
Subtype To
More
frag
WEP
data
Power
mgt
Retry
Rsvd
Protocol
version 2 4 1
frame type
(RTS, CTS, ACK, data)

45. IEEE : multiple access
avoid collisions: 2+ nodes transmitting at same time
802.11: CSMA - sense before transmitting
don’t collide with ongoing transmission by other node
802.11: no collision detection!
difficult to receive (sense collisions) when transmitting due to weak received signals (fading)
can’t sense all collisions in any case: hidden terminal, fading
goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C
A’s signal
strength
space
C’s signal

46. IEEE 802.11 MAC Protocol: CSMA/CA
sender
1 if sense channel idle for DIFS then
transmit entire frame (no CD)
2 if sense channel busy then
start random backoff time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff interval, repeat 2
receiver
- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender
receiver
DIFS
data
SIFS
ACK

47. Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but they’re short)
BS broadcasts clear-to-send CTS in response to RTS
CTS heard by all nodes
sender transmits data frame
other stations defer transmissions
avoid data frame collisions completely
using small reservation packets!

48. Collision Avoidance: RTS-CTS exchangeB AP
RTS(A)
RTS(B)
reservation collision
RTS(A)
CTS(A)
DATA (A)
ACK(A)
defer
time

49. 802.11: mobility within same subnet
H1 remains in same IP subnet: IP address can remain same
switch: which AP is associated with H1?
self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1
router
hub or
switch
BBS 1
AP 1
AP 2 H1
BBS 2

50. 802.11: advanced capabilities
Rate Adaptation
base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
10-1
10-2
10-3
BER
10-4
10-5
10-6
10-7 10 20 30 40
SNR(dB)
1. SNR decreases, BER increase as node moves away from base station
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
2. When BER becomes too high, switch to lower transmission rate but with lower BER
operating point

51. Broadband Wireless 802.16 Architecture / Protocol Stack »
Physical Layer »
MAC »
Frames »

52. Google Project Loon
Many of us think of the Internet as a global community. But two- thirds of the world’s population does not yet have Internet access. Project Loon is a network of balloons traveling on the edge of space, designed to connect people in rural and remote areas, help fill coverage gaps, and bring people back online after disasters.

53. 802.16 Architecture/Protocol Stack (1)
Wireless clients connect to a wired basestation (like 3G)
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54. 802.16: WiMAX like 802.11 & cellular: base station model
point-to-point
like & cellular: base station model
transmissions to/from base station by hosts with omnidirectional antenna
base station-to-base station backhaul with point-to-point antenna
unlike :
range ~ 6 miles (“city rather than coffee shop”)
~14 Mbps
point-to-multipoint

55. 802.16 Architecture/Protocol Stack (2)
MAC is connection-oriented; IP is connectionless
Convergence sublayer maps between the two
Based on OFDM; base station gives mobiles bursts (subcarrier/time frame slots) for uplink and downlink

56. 802.16 MAC Connection-oriented with base station in control
Clients request the bandwidth they need
Different kinds of service can be requested:
Constant bit rate, e.g., uncompressed voice
Real-time variable bit rate, e.g., video, Web
Non-real-time variable bit rate, e.g., file download
Best-effort for everything else
Frames vary depending on their type
Connection ID instead of source/dest addresses
(a) A generic frame. (b) A bandwidth request frame
(b)
(a)

57. 802.16: WiMAX: downlink, uplink scheduling
transmission frame
down-link subframe: base station to node
uplink subframe: node to base station
pream.
DL-
MAP
UL- DL
burst 1
SS #1
burst 2
burst n
Initial
maint.
request
conn.
downlink subframe
SS #2
SS #k
uplink subframe …
base station tells nodes who will get to receive (DL map)
and who will get to send (UL map), and when
WiMAX standard provide mechanism for scheduling, but not scheduling algorithm

58. Bluetooth Bluetooth Architecture » Bluetooth Applications / Protocol »
Bluetooth Radio / Link Layers »
Bluetooth Frames »
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59. 802.15: personal area network
less than 10 m diameter
replacement for cables (mouse, keyboard, headphones)
ad hoc: no infrastructure
master/slaves:
slaves request permission to send (to master)
master grants requests
802.15: evolved from Bluetooth specification
GHz radio band
up to 721 kbps P S P
radius of
coverage M S P S P M S
Master device
Slave device
Parked device (inactive) P

60. Bluetooth Architecture
Piconet master is connected to slave wireless devices
Slaves may be asleep (parked) to save power
Two piconets can be bridged into a scatternet
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61. Bluetooth Applications / Protocol Stack
Profiles give the set of protocols for a given application
25 profiles, including headset, intercom, streaming audio, remote control, personal area network, …
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62. Bluetooth Radio / Link Layers
Radio layer
Uses adaptive frequency hopping in 2.4 GHz band
Link layer
TDM with timeslots for master and slaves
Synchronous CO for periodic slots in each direction
Asynchronous CL for packet-switched data
Links undergo pairing (user confirms passkey/PIN) to authorize them before use (much better than etc)
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63. Bluetooth Frames
Time is slotted; enhanced data rates send faster but for the same time; addresses are only 3 bits for 8 devices
(b)
(a)
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64. RFID Gen 2 Architecture » Gen 2 Physical Layer »
Gen 2 Tag Identification Layer »
Gen 2 Frames »
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65. Gen 2 Architecture
Reader signal powers tags; tags reply with backscatter
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66. Gen 2 Physical Layer Reader uses duration of on period to send 0/1
Tag backscatters reader signal in pulses to send 0/1
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67. Gen 2 Tag Identification Layer
Reader sends query and sets slot structure Tags reply (RN16) in a random slot; may collide Reader asks one tag for its identifier (ACK) Process continues until no tags are left

68. Gen 2 Frames Reader frames vary depending on type (Command)
Query shown below, has parameters and error detection
Tag responses are simply data
Reader sets timing and knows the expected format
Query message

69. Data Link Layer Switching
Uses of Bridges »
Learning Bridges »
Spanning Tree »
Repeaters, hubs, bridges, .., routers, gateways »
Virtual LANs »

70. Uses of Bridges Common setup is a building with centralized wiring
Bridges (switches) are placed in or near wiring closets

71. Learning Bridges (1) A bridge operates as a switched LAN (not a hub)
Computers, bridges, and hubs connect to its ports

72. Learning Bridges (2)
Backward learning algorithm picks the output port:
Associates source address on frame with input port
Frame with destination address sent to learned port
Unlearned destinations are sent to all other ports
Needs no configuration
Forget unused addresses to allow changes
Bandwidth efficient for two-way traffic

73. Learning Bridges (3) Bridges extend the Link layer:
Use but don’t remove Ethernet header/addresses
Do not inspect Network header

74. Spanning Tree (1) – Problem
Bridge topologies with loops and only backward learning will cause frames to circulate for ever
Need spanning tree support to solve problem
Subset of forwarding ports for data is use to avoid loops
Selected with the spanning tree distributed algo by Perlman

75. Spanning Tree (3) – Example
After the algorithm runs:
B1 is the root, two dashed links are turned off
B4 uses link to B2 (lower than B3 also at distance 1)
B5 uses B3 (distance 1 versus B4 at distance 2)

76. Repeaters, Hubs, Bridges, Switches, Routers, & Gateways
Devices are named according to the layer they process
A bridge or LAN switch operates in the Link layer

77. Virtual LANs (1)
VLANs (Virtual LANs) splits one physical LAN into multiple logical LANs to ease management tasks
Ports are “colored” according to their VLAN

78. Virtual LANs (2) – IEEE 802.1Q
Bridges need to be aware of VLANs to support them
In 802.1Q, frames are tagged with their “color”
Legacy switches with no tags are supported

79. Virtual LANs (3) – IEEE 802.1Q
802.1Q frames carry a color tag (VLAN identifier)
Length/Type value is 0x8100 for VLAN protocol

80. End
Chapter 4