1. Chapter 10 – Wireless LANs
Local Area Networks
Chapter 10 – Wireless LANs

2. Wireless Communication
The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.
Companies jumped in, quickly developing incompatible wireless products in the 1990’s.
Industry decided to entrust standardization to the IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!!
Wireless communications compelling
Easy, low-cost deployment
Mobility & roaming: Access information anywhere
Supports personal devices
PDAs, laptops, data-cell-phones
Supports communicating devices
Cameras, location devices, wireless identification
Signal strength varies in space & time
Signal can be captured by snoopers
Spectrum is limited & usually regulated

3. Wireless Links Many end systems use wireless links:
Portable PCs within a wireless LAN
PDAs that connect to the Internet through wireless telephony infrastructure
Cameras, automobiles, etc.
Two standards for wireless networking:
IEEE b standard for wireless LANs (aka Wi-Fi)
Bluetooth standard that allows devices to communicate without being in line of sight
Wireless devices classified wrt power, range, and data rate
IEEE  high power, medium range, and high rate “access” technology
Bluetooth  low power, short range, low rate, “cable replacement” technology

4. IEEE 802.11 Wireless LAN Wireless LANs: mobile networking
IEEE standard:
MAC protocol
Unlicensed frequency spectrum: 2.4Ghz (802.11b) or 5-6 Ghz (802.11a)
Provides wireless Ethernet access at 11 Mbps or 54 Mbps (802.11a)
Basic Service Set (BSS) (a.k.a. “cell”) contains:
wireless hosts
access point (AP): base station
BSS’s combined to form distribution system (DS)

5. IEEE Wireless LAN

6. IEEE Wireless LAN

7. The Protocol Stack

8. The Protocol Stack

9. Wireless Standards Frequency, Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (FHSS) HR: High Rate
Orthogonal Frequency Division Multiplexing (OFDM, VOFDM, COFDM)

10. Wireless Physical Layer
Physical layer conforms to OSI (five options)
1997: infrared, FHSS, DHSS
1999: a OFDM and b HR-DSSS
2001: g OFDM
Infrared
Two capacities 1 Mbps or 2 Mbps.
Range is 10 to 20 meters and cannot penetrate walls.
Does not work outdoors.
FHSS (Frequency Hopping Spread Spectrum)
The main issue is multipath fading.
79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.
Same pseudo-random number generator used by all stations.
Dwell time: min. time on channel before hopping (400msec).

11. Wireless Physical Layer
Frequency Hopping Spread Spectrum

12. Wireless Physical Layer
DSSS (Direct Sequence Spread Spectrum)
Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA see Tanenbaum sec ).
Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.
1 or 2 Mbps.
802.11a OFDM (Orthogonal Frequency Divisional Multiplexing)
Compatible with European HiperLan2.
54Mbps in wider 5.5 GHz band  transmission range is limited.
Uses 52 FDM channels (48 for data; 4 for synchronization).
Encoding is complex ( PSM (Power saving mode) up to 18 Mbps and QAM above this capacity).
E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.
More difficulty penetrating walls.

13. Wireless Physical Layer
Direct Sequence Spread Spectrum

14. Wireless Physical Layer
802.11b HR-DSSS (High Rate Direct Sequence Spread Spectrum)
11a and 11b shows a split in the standards committee.
11b approved and hit the market before 11a.
Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec.
Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers.
Range is 7 times greater than 11a.
11b and 11a are incompatible!!
802.11g OFDM(Orthogonal Frequency Division Multiplexing)
An attempt to combine the best of both a and b.
Supports bandwidths up to 54 Mbps.
Uses 2.4 GHz frequency for greater range.
Is backward compatible with b.

15. Infrastructure NetworkB2 B1 A1
AP1
AP2
Distribution System
Server
Gateway to
the Internet
Portal
BSS A
BSS B
Permanent Access Points provide access to Internet

16. IEEE Wireless LAN

17. 802.11 Definitions Basic Service Set (BSS) Extended Service Set (ESS)
Group of stations that coordinate their access using a given instance of MAC
Located in a Basic Service Area (BSA)
Stations in BSS can communicate with each other
Distinct collocated BSS’s can coexist
Extended Service Set (ESS)
Multiple BSSs interconnected by Distribution System (DS)
Each BSS is like a cell and stations in BSS communicate with an Access Point (AP)
Portals attached to DS provide access to Internet

18. Ad Hoc Networks
Ad hoc network: IEEE stations can dynamically form network without AP
Formed “on the fly” when mobile devices are in proximity
Applications:
“Laptop” meeting in conference room, car
Interconnection of “personal” devices
Battlefield
IETF MANET (Mobile Ad hoc Networks) working group

19. Ad Hoc Networks

20. Hidden Terminal ProblemC A
Data Frame
A transmits data frame
C senses medium,
station A is hidden from C
Data Frame A B C
C transmits data frame & collides with A at B
(b)
New MAC: CSMA with Collision Avoidance

21. IEEE 802.11 MAC Protocol: CSMA/CA (collision avoidance)
CSMA: sender
if sense channel idle for Distributed Inter Frame Space (DIFS) sec.
then transmit entire frame (no collision detection)
if sense channel busy then binary backoff
CSMA receiver:
if received OK
return ACK after Short Inter Frame Spacing (SIFS)
(DIFS = SIFS + 2 × slot time)
Time slot= 20 micro s, SIFS=10 micro s, DIFS=50 micro s.

22. IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others
Other stations wait for a random backoff period after DIFS after current transmission
Avoids collisions
Collisions  uses exponentially increasing backoff period
Collisions detection is difficult:
Hidden terminal problem
Fading
NAV: Network Allocation Vector:
frame has transmission duration field
Others (hearing stations) defer access (to save power) for NAV time units

23. IEEE MAC Protocol

24. Hidden Terminal effect
Hidden terminals: A, C cannot hear each other
Obstacles, signal attenuation
Collisions at B
Goal: avoid collisions at B
CSMA/CA: CSMA with Collision Avoidance
Fading can also result in collisions

25. Collision Avoidance: RTS-CTS exchange
CSMA/CA: explicit channel reservation
sender: send short RTS: request to send
receiver: reply with short CTS: clear to send
CTS reserves channel for sender, notifying (possibly hidden) stations
Benefit: RTC-CTS avoids hidden station collisions

26. Collision Avoidance: RTS-CTS exchange
CA with RTS-CTS:
Collisions less likely, of shorter duration
End result similar to collision detection
IEEE allows:
CSMA
CSMA/CA: reservations
polling from AP

27. CSMA with Collision Avoidance
RTS
A requests to send B C
(a)
CTS A B C
B announces A ok to send
(b)
Data Frame
A sends B
C remains quiet
(c)

28. IEEE Wireless LAN
Stimulated by availability of unlicensed spectrum
U.S. Industrial, Scientific, Medical (ISM) bands
MHz, GHz, GHz
Targeted wireless 20 Mbps
MAC for high speed wireless LAN
Ad Hoc & Infrastructure networks
Variety of physical layers

29. Infrastructure NetworkB2 B1 A1
AP1
AP2
Distribution System
Server
Gateway to
the Internet
Portal
BSS A
BSS B

30. Distribution Services
Stations within BSS can communicate directly with each other
DS provides distribution services:
Transfer MAC SDUs between APs in ESS
Transfer MSDUs between portals & BSSs in ESS
Transfer MSDUs between stations in same BSS
Multicast, broadcast, or stations’s preference
ESS looks like single BSS to LLC layer

31. Infrastructure Services
Select AP and establish association with AP
Then can send/receive frames via AP & DS
Reassociation service to move from one AP to another AP
Dissociation service to terminate association
Authentication service to establish identity of other stations
Privacy service to keep contents secret

32. IEEE 802.11 MAC MAC sublayer responsibilities
Channel access
PDU addressing, formatting, error checking
Fragmentation & reassembly of MAC SDUs
MAC security service options
Authentication & privacy
MAC management services
Roaming within ESS
Power management

33. MAC Services Physical Distribution coordination function (DCF)
Contention Service: Best effort
Contention-Free Service: time-bounded transfer
MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs). MAC Service Data Unit (MSDU)
Physical
Distribution coordination function (DCF)
(CSMA-CA)
Point coordination
function
Contention-free service
Contention service
MAC
MSDUs

34. Distributed Coordination Function (DCF)
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access
Wait for
reattempt time
Time
Busy medium
DCF provides basic access service
Asynchronous best-effort data transfer
All stations contend for access to medium
CSMA-CA
Ready stations wait for completion of transmission
All stations must wait Interframe Space (IFS)

35. Priorities through Interframe Spacing
DIFS
PIFS
SIFS
Contention
window
Next frame
Defer access
Wait for
reattempt time
Time
Busy medium
High-Priority frames wait Short IFS (SIFS)
Typically to complete exchange in progress
ACKs, CTS, data frames of segmented MSDU, etc.
PCF IFS (PIFS) to initiate Contention-Free Periods
DCF IFS (DIFS) to transmit data & MPDUs

36. Contention & Backoff Behavior
If channel is still idle after DIFS period, ready station can transmit an initial MPDU
If channel becomes busy before DIFS, then station must schedule backoff time for reattempt
Backoff period is integer # of idle contention time slots
Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires
Station can contend when backoff timer expires
A station that completes a frame transmission is not allowed to transmit immediately
Must first perform a backoff procedure

37. RTS
CTS
Data Frame
A requests to send B C A
A sends
C remains quiet
B announces A ok to send
(a)
(b)
(c)
ACK
(d)
B sends ACK

38. Carrier Sensing in 802.11 Physical Carrier Sensing
Analyze all detected frames
Monitor relative signal strength from other sources
Virtual Carrier Sensing at MAC sublayer
Source stations informs other stations of transmission time (in msec) for an MPDU
Carried in Duration field of RTS & CTS
Stations adjust Network Allocation Vector to indicate when channel will become idle
Channel busy if either sensing is busy

39. Transmission of MPDU without RTS/CTS
Data
DIFS
SIFS
Defer Access
Wait for Reattempt Time
ACK
NAV
Source
Destination
Other

40. Transmission of MPDU with RTS/CTS
Data
SIFS
Defer access
Ack
DIFS
NAV (RTS)
Source
Destination
Other
RTS
CTS
NAV (CTS)
NAV (Data)

41. Collisions, Losses & Errors
Collision Avoidance
When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots)
Station transmits frame when backoff timer expires
If collision occurs, recompute backoff over interval that is twice as long
Receiving stations of error-free frames send ACK
Sending station interprets non-arrival of ACK as loss
Executes backoff and then retransmits
Receiving stations use sequence numbers to identify duplicate frames

42. Point Coordination Function
PCF provides connection-oriented, contention-free service through polling
Point coordinator (PC) in AP performs PCF
Polling table up to implementor
CFP repetition interval
Determines frequency with which CFP occurs
Initiated by beacon frame transmitted by PC in AP
Contains CFP and CP
During CFP stations may only transmit to respond to a poll from PC or to send ACK

43. Contention-free repetition interval
PCF Frame Transfer
CF End
NAV
PIFS B
D1 + Poll
SIFS
U 1 + ACK
D2+Ack+Poll
U 2 + ACK
Contention-free repetition interval
Contention period
CF_Max_duration
Reset NAV
D1, D2 = frame sent by point coordinator
U1, U2 = frame sent by polled station
TBTT = target beacon transmission time
B = beacon frame
TBTT

44. DCF, PCF, and Frame Format

45. Distributed Coordination Function (DCF)
DCF is the access method used to support asynchronous data transfer on a best effort basis
All stations must support the DCF (DCF operates solely in the ad hoc network)
Operates solely or coexists with the PCF in an infrastructure network
DCF sits directly on top of the physical layer and supports contention services:
Each station with an MSDU queued for transmission must contend for access to the channel
Once the MSDU is transmitted, must recontend for access to the channel for all subsequent frames
Contention services promote fair access to the channel for all stations.
The DCF is carrier sense multiple access with collision avoidance (CSMA/CA).
CSMA/CD is not used because a station is unable to listen to the channel for collisions while transmitting
In IEEE , carrier sensing is performed at both the air interface, referred to as physical carrier sensing, and at the MAC sublayer, referred to as virtual carrier sensing
Physical carrier sensing detects the presence of other IEEE WLAN users by analyzing all detected packets, and also detects activity in the channel via relative signal strength from other sources
Virtual carrier sensing
Stations include MPDU duration in the header of request to send (RTS), clear to send (CTS), and data frames
An MPDU is a complete data unit that is passed from the MAC sublayer
to the physical layer
The MPDU contains header information, information, payload, and a 32-bit CRC
The duration field indicates the time (in microseconds) after the end of the present frame the channel will be utilized tocomplete the successful transmission of the data or management frame.
Stations in the BSS use the duration field to adjust their network allocation vector (NAV)
NAV indicates the amount of time that must elapse until the current transmission session is complete

46. Distributed Coordination Function (DCF)
DCF operates under the Contention Period (CP)
Three types of frames: management, control, and data
Management F: station association dis-association with AP
Control F: handshaking in CP, ACK data in CP, and end CFP
Basic DCF Access Method (no RTS-CTS):
When ST finds chaneel idle, it waits for DIFS and checks it again
If it is still idle, it transmits MPDU with medium busy time (including SIFS and ACK times)
Receiving st computes Checksum, if correct sends an ACK to source
All other STs in BSS hearing above messages adjust their NAV timers

47. Distributed Coordination Function (DCF)
RTS-CTS Data Mode
Priority Accsess: SIFS, PIFS (SIFS+1), and DIFS (SIFS+2)
In BSS, STs hearing RTS, CTS, F0, and ACK adjust their NAV
Sts: Basic mode, RTS/CTS mode if MPDU exceeds L, or always use RTS/CTS mode
Fairness: BEB starts with (1,8) and end at some maximum

48. Distributed Coordination Function (DCF)
MPDU (2300 bytes): collision lead to bandwidth loss
RTS is 20 bytes and CTS is 14 bytes
Fragmentation increases transmission reliability
Fragment MPDU, transmit Frag, receive ACK to completion
If no ACK, re-contend for medium and stat al last Frag.
In RTS-CTS mode, RTS-CTS used only in first frag.

49. Point Coordination Function (PCF on top of DCF)
PCF (optional) operates under the Contention-Free Period (CFP)
Medium access contr. by Point Coordinator PC (AP/BSS, polling)
Polled Sts can transmit (No CSMA)
CFP Repetition Interval (Manag duration): (1) PCF, and (2) DCF

50. Point Coordination Function (PCF on top of DCF)
Light traffic: shorter CFP if previous DCF traffic is not complete
PC: PIFS, Beacon, (CF-poll/data/Data+CF-poll), CF-end.
CF-aware st:
Gets CF-poll,
Responds: CF-ACK, Data+CF-ACK,
Then PC responds by Data+CF-ACK+CF-poll

51. Point Coordination Function (PCF on top of DCF)
When ST receives a poll from IP:
Transmit a F to another ST in the BSS
When Dest receives F, a DCF-ACK is returned to source
PC waits for PIFS after ACK before continuation

52. Frame Types Management frames Control frames Data frames
Station association & disassociation with AP
Timing & synchronization
Authentication & deauthentication
Control frames
Handshaking
ACKs during data transfer
Data frames
Data transfer

53. Frame Structure MAC Header: 30 bytes Frame Body: 0-2312 bytes6 6 6 2 6
0-2312 4
Frame
Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence
control
Address 4
Frame
body
CRC
MAC Header: 30 bytes
Frame Body: bytes
CRC: CCITT-32 4 bytes CRC over MAC header & frame body

54. Frame Control (1) Protocol version = 0
Address 2
Frame
Control
Duration/ ID 1 3
Sequence
control 4
body
CRC
Protocol
version
Type
Subtype To DS
From
More
frag
Retry
Pwr
mgt
data
WEP
Rsvd 6
0-2312
MAC header (bytes)
Protocol version = 0
Type: Management (00), Control (01), Data (10)
Subtype within frame type
Type=00, subtype=association; Type=01, subtype=ACK
MoreFrag=1 if another fragment of MSDU to follow

55. Frame Control (2)
Address 2
Frame
Control
Duration/ ID 1 3
Sequence
control 4
body
CRC
Protocol
version
Type
Subtype To DS
From
More
frag
Retry
Pwr
mgt
data
WEP
Rsvd 6
0-2312 To DS
From
Address 1 2 3 4
Destination
address
Source
BSSID
N/A
Receiver
Transmitter
Meaning
Data frame from station to station within a BSS
Data frame exiting the DS
Data frame destined for the DS
WDS frame being distributed from AP to AP
To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS

56. Frame Control (3) Retry=1 if mgmt/control frame is a retransmission
Address 2
Frame
Control
Duration/ ID 1 3
Sequence
control 4
body
CRC
Protocol
version
Type
Subtype To DS
From
More
frag
Retry
Pwr
mgt
data
WEP
Rsvd 6
0-2312
MAC header (bytes)
Retry=1 if mgmt/control frame is a retransmission
Power Management used to put station in/out of sleep mode
More Data =1 to tell station in power-save mode more data buffered for it at AP
WEP=1 if frame body encrypted

57. Physical Layers 802.11 designed to Physical layer LLC Physical layer
convergence
procedure
Physical medium
dependent
MAC
PLCP
preamble
LLC PDU
MAC SDU
header
CRC
PLCP PDU
designed to
Support LLC
Operate over many physical layers

58. IEEE 802.11 Physical Layer Options
Frequency Band
Bit Rate
Modulation Scheme
802.11
2.4 GHz
1-2 Mbps
Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum
802.11b
11 Mbps
Complementary Code Keying & QPSK
802.11g
54 Mbps
Orthogonal Frequency Division Multiplexing
& CCK for backward compatibility with b
802.11a
5-6 GHz

59. 802.11 - MAC management Synchronization Power management
try to find a LAN, try to stay within a LAN
timer etc.
Power management
sleep-mode without missing a message
periodic sleep, frame buffering, traffic measurements
Association/Reassociation
integration into a LAN
roaming, i.e. change networks by changing access points
scanning, i.e. active search for a network
MIB - Management Information Base
managing, read, write

60. Synchronization using a Beacon (infrastructure)
beacon interval B B B B
access
point
busy
busy
busy
busy
medium t B
value of the timestamp
beacon frame

61. Synchronization using a Beacon (ad-hoc)
beacon interval B1 B1
station1 B2 B2
station2
busy
busy
busy
busy
medium t B
value of the timestamp
beacon frame
random delay

62. Power management Idea: switch the transceiver off if not needed
States of a station: sleep and awake
Timing Synchronization Function (TSF)
stations wake up at the same time
Infrastructure
Traffic Indication Map (TIM)
list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)
list of broadcast/multicast receivers transmitted by AP
Ad-hoc
Ad-hoc Traffic Indication Map (ATIM)
announcement of receivers by stations buffering frames
more complicated - no central AP
collision of ATIMs possible (scalability?)

63. Power saving with wake-up patterns (infrastructure)
TIM interval
DTIM interval D B T T d D B
access
point
busy
busy
busy
busy
medium p d
station t T
TIM D
DTIM
awake d
data transmission
to/from the station B
broadcast/multicast p
PS poll

64. Power saving with wake-up patterns (ad-hoc)
ATIM
window
beacon interval B1 A D B1
station1 B2 B2 a d
station2 t B A
transmit ATIM D
beacon frame
random delay
transmit data a d
awake
acknowledge ATIM
acknowledge data

65. 802.11 - Roaming No or bad connection? Then perform: Scanning
scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer
Reassociation Request
station sends a request to one or several AP(s)
Reassociation Response
success: AP has answered, station can now participate
failure: continue scanning
AP accepts Reassociation Request
signal the new station to the distribution system
the distribution system updates its data base (i.e., location information)
typically, the distribution system now informs the old AP so it can release resources

66. WLAN: IEEE 802.11b What’s new? Frequency Security Cost Availability
Define a new PHY layer. All the MAC schemes, management procedures are the same
User data rate max. approx. 6 Mbit/s
Frequency
On certain frequencies in the free 2.4 GHz ISM-band
Security
Limited, WEP insecure, SSID
Cost
100€ adapter, 250€ base station, dropping
Availability
Many products, many vendors
Special Advantages/Disadvantages
Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system
Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only

67. Bluetooth Most compelling application addressed by Bluetooth:
A convenient, untethered means to interconnect electronic devices
Examples: portable phones, PDAs, laptops, desktops, digital cameras, fax machines, printers, keyboard, mouse, etc.
Line-of-sight infrared technology has been used for such communications
Using RF wireless communication, Bluetooth does not require LoS
It can support multipoint as well as point-to-point communication
Bluetooth architecture:
Mobile devices need short-range transceivers
Transceivers operate in 2.5 Ghz unlicensed frequency band
Provide data rates of up to 721 kbps + 3 voice channels (64 kbps)
Operating range is 10 to 100 meters
Each device is identified by a 12-bit address

68. Bluetooth (Cont’d) Frequency hopping Error recovery:
Transceiver minimizes the effect of interference from other signals
Hops to a new frequency after transmitting or receiving a packet
Error recovery:
Transceiver forward error correction (FEC)
Automatic Repeat reQuest (ARQ) for retransmission
Bluetooth protocol suite includes:
Baseband protocol
Enables physical RF wireless connection between devices
A connection of 2-7 Bluetooth devices forms a small networkpiconet
Link manager protocol
Handshaking between two devices to establish connection
L2CAP protocol
During a connection, adapts upper layer protocols for transmission over the baseband

69. Bluetooth - Physical Upwards!
79 channels, each 1MHz, using FSK, with 1 bit per
symbol = 1Mbps
Much of the 1Mbps is taken up with protocol overheads – caused
by frequency hopping ( ms needed to stabilise radio after
the hop!)
Leaves about 366 bits for actual data – of which 126 bits are headers
– leaving 240 bits for data per slot!

70. Bluetooth Frequency Hopping

71. Point to Point Data Link Control
One sender, one receiver, one link: easier than broadcast link:
No Media Access Control
No need for explicit MAC addressing
Examples:
Dialup link phone line  56 Kbps modem connections
SONET/SDH link
X.25 connection
ISDN line
Popular point-to-point DLC protocols:
PPP (point-to-point protocol)
HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

72. PPP Design Requirements [RFC 1547]
Packet framing:
Encapsulation of network-layer datagram in data link frame
Carry network layer data of any network layer protocol (not just IP)
Ability to demultiplex upwards
Bit transparency:
Must carry any bit pattern in the data field with no constraints
Error detection (no correction)
PPP receiver must be able to detect bit errors
Connection liveness:
Detect, signal link failure to network layer
Network layer address negotiation:
Endpoint can learn/configure each other’s network address

73. Error recovery, flow control, data re-ordering
PPP Non-Requirements
No error correction/recovery
No flow control
PPP receiver is expected to receive frames at full physical layer speed  higher layer could drop packets or throttle sender
Out of order delivery OK
No need to support multipoint links (e.g., polling)
Other link layer protocols can support multipoint links
E.g., HDLC
Error recovery, flow control, data re-ordering
all relegated to higher layers!|

74. PPP Data Frame Flag: delimiter (framing)
Address: does nothing (only one option)
Control: does nothing; in the future possible multiple control fields
PPP sender can allow sender to skip address and control bytes
Protocol: upper layer protocol to which frame delivered
Examples: PPP-LCP, IP, IPCP, etc
RFC 1700 and RFC 3232 define 16-bit protocol codes for PPP

75. PPP Data Frame (Cont’d)
Info:
Variable length upper layer data being carried
Default maximum is 1500 bytes
Can be changed when the link is initially configured
Check:
Uses cyclic redundancy check (CRC) for error detection
Two or 4 bytes CRC

76. Byte Stuffing
“Data transparency” requirement: data field must be allowed to include flag pattern < >
Q: is received < > data or flag?
Sender:
Adds (“stuffs”) an escape byte < > before each < > data byte
Receiver:
Discards the escape byte and continues data reception
Single  flag byte
If two < > bytes in a row  discard the first escape byte and continue data reception

77. Byte Stuffing flag byte pattern in data to send flag byte pattern plus
stuffed byte in transmitted data

78. PPP Link and Network Control Protocols
Before exchanging network-layer data, data link peers must:
Configure PPP link (max. frame length, authentication)
Learn/configure network
layer information
For IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address
PPP link always begins and ends in the dead state

79. PPP Link Control Protocol (LCP)
Link establishment state:
Entered on an event that indicates presence of a physical layer, which is ready to be used: carrier detection, user intervention
One end of the link uses configure-request frame to indicate its configuration options
PPP frame with protocol filed set equal to LCP
Information field contains the specific configuration request
Options:
Maximum frame size for the link
Specification of authentication protocol to be used (if any)
Option to skip the address and control fields in PPP frames
The other side responds with configure-ack, configure-nak, or configure-reject frame
Network layer configuration begins after link is established:
Options negotiation done and authentication performed (if any)
Network layer specific control packets are exchanged with each other

80. PPP Network Control Protocol (IPCP)
If IP is running over PPP, IP control protocol (IPCP) is used
IPCP is carried within a PPP frame
Protocol field will have IPCP  indicated by 0x8021
IPCP allows two IP modules to exchange or configure IP addresses
IPCP also allows two IP modules to negotiate whether or not IP datagrams will be sent in compressed form
Similar network control protocols for other network protocols:
Examples: DECnet, AppleTalk, etc.
Link goes in open state after network configuration
PPP can start exchanging network layer datagrams
To check the link status, use echo-request and echo-reply LCP frames
Terminating state
One side sends LCP terminate-request and other responds with LCP terminate-ack frame
Link goes to the dead state again

81. Asynchronous Transfer Mode (ATM)
Two types of networks have existed side by side:
Telephone networks  carry real-time voice
Data networks  carry non real-time datagrams
ATM standards were developed in mid-1980’s
Goal: design a network technology that will be appropriate for both types of traffic
Standard developed by ATM Forum and ITU for broadband digital services networks
ATM technology:
A full suite of communication protocols form application to physical layer
Calls for packet switching within virtual circuits  virtual channels
Deployed in both telephone networks and Internet backbones
High performance ATM switches can deliver terabits per second!
Still could not replace TCP/IP based networks at desktop level

82. Characteristics of ATM
ATM service models:
Constant bit rate (CBR)
Variable bit rate (VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
ATM uses fixed-length packets  cells
Header: 5 bytes and payload: 48 bytes
Fixed length cell and simple header facilitate high speed switching
ATM VCs  virtual channels
Header includes virtual channel identifier (VCI) field
VCI is used by switches to forward the cells
Connection-oriented service
Cells always arrive in-order
ATM does not provide acks as other connection-oriented protocols do
Effectively, a VC is full duplex
Channel capacity and other properties may be different in two directions
Date rates:
155 Mbps, 622 Mbps, and higher

83. Characteristics of ATM (Cont’d)
No link-by-link retransmissions
If an ATM switch detects error in a header, it tries to correct it
Simply drops the cell if error cannot be correctedno retransmission request
Congestion control
Only for ABR service class
Network provides feedback to sender to regulate its rate
ATM protocol stack consists of three layers:
ATM physical layer
ATM layer
ATM adaptation layer (AAL)
Analogous to transport layer in TCP/IP stack
Multiple types of AALs

84. Cell Header Formats In both cases, cells consist of: Header fields
5 byte header and 48 byte payloads
Headers are slightly different for two interfaces (GFC field is unused any way)
Header fields
VPI is a small integer that selects a particular virtual path
VCI selects a particular VC from within the chosen virtual path

85. Cell Header Formats (Cont’d)
VPI and VCI
At UNI, 8 bit VPI means that host may have up to 256 virtual paths, each containing 65,536 VCs (16 bits)
Actually slightly less as some VCs are used for control functions
PTI field defines the type of payload
E.g., 000 means user data cell with no congestion and cell type 0 while 010 means user data cell that experienced congestion
A cell sent by the user as 000 may arrive as 010
Types are user supplied but congestion info is network supplied
CLP is set by a host to differentiate between high and low priority traffic
In case of congestion, switch will first drop cells with CLP 1 before dropping cells with CLP 0
HEC byte provides error control over the header
All single bit and 90% of multibit errors can be corrected
A 48 byte payload follows header
Not all 48 bytes available for payload as some of the AAL protocols put their headers and trailers inside the payload

86. Connection Setup ATM supports two types of VCs
Permanent VCs: present at all times like leased lines
Switched VCs: have to be setup for each session
Connection setup is not part of ATM layer
Described by ITU protocol Q.2931, which is part of control plane
Connection setup is a two-step process
First, a VC is acquired for signaling
To establish such a circuit, cells containing a request are sent to virtual path 0, VC 5
If first step is successful, a new VC is opened on which connection setup request and replies are transmitted

87. Messages for Connection Setup in ATM
Four messages are used for establishment
Host sends a SETUP message on a special VC
Network responds with CALL PROCEEDING at each hop
When SETUP arrives at destination it responds with CONNECT that propagates back towards originator
Each switch returns a CONNECT ACK to originator
Two messages are used for release of a VC
Host wishing to release sends a request
Intermediate switches respond as request propagates

88. Connection Setup (Cont’d)
Multicast connection setup
A multicast channel has one sender and multiple receivers
Constructed by first setting up connection to one destination
ADD PARTY messages are sent to add more receivers to the VC previously returned
ATM addresses
Setup messages include destination address
ATM addresses come in three forms
Type 1: 20 bytes long OSI addresses
First byte indicates which of three formats
Bytes 2 and 3 specify country; byte 4 gives format for the rest of address that contains 3-byte authority, 2-byte domain, 2-byte area, and 6-byte add.
Type 2: bytes 2 and 3 designate an international organization and rest is same as in type 1
Type 3: 15 digit decimal ISDN telephone number

89. ATM Adaptation Layer
ATM layer does not provide error or flow control to applications
Only 53 byte cells are output
Not directly useable for applications
ATM Adaptation Layer (AAL) was defined to bridge this gap
AAL protocols:
Four protocols to handle four classes of service
AAL1 – AAL4
Requirements for classes C and D were so similar that AAL3 and AAL4 are combined into AAL ¾
AAL1 for CBR and AAL2 for VBR
AAL5 proposed by computer industry in contrast to telecommunication industry that proposed AAL1 – AAL3/4  for IP datagrams

90. Structure of the AAL AAL has two parts:
ATM physical layer
ATM layer
Convergence sublayer (service specific part)
Covergence sublayer (common part)
Segmentation reassembly sublayer
AAL has two parts:
Convergence sublayer
Interfaces with application for framing and error detection
Two parts: service-specific part and common part
Segmentation And Reassembly (SAR) sublayer
Adds headers and trailers to data units given by convergence layer to form cell payloads

91. Convergence and SAR Layer Operations
Convergence sublayer adds its header/trailer to the message
Message is broken into byte units, which are passed to SAR
SAR adds its own header/trailer and passes each piece to ATM layer
Some AAL protocols have null header/trailer
Above figure represents the most general case

92. IP over ATM ATM is widely used as Internet backbone
Permanent VCs between each pair of entry/exit point
Permanent VCs avoid having to establish dynamic VCs for transiting cells
Fro n entry points, n(n-1) permanent VCs are needed
Routers have 2 addresses:
An IP address
An ATM (LAN) address
ATM network needs to transit datagram to the exit router
Uses permanent VC
Uses AAL5

93. Practice Problem # 1
Q: Consider a CSMA/CD network running at 1 Gbps over a 1 km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size? A:
For a 1 km cable, the one-way propagation time is 5 msec or 2t = 10 msec. Shortest frame should take more than this time to transmit to allow the sender to identify any collisions in the worst case.
At 1Gbps, the number of bits that should be transmitted during 10 msec = 10,000 bits = 1250 bytes.
Thus, the frame should not be shorter than 1250 bytes.

94. Practice Problem # 2
Q: A 4-Mbps token ring has a token holding timer value of 10 msec. What is the longest frame that can be sent on this ring? A:
At 4 Mbps, a station can transmit 40,000 bits or 5000 bytes in 10 msec.
This is an upper bound on frame length.
From this amount, some overhead bytes must be subtracted, giving a slightly lower limit for the data portion.

95. Practice Problem # 3
Q: At a transmission rate of 5 Mbps and a propagation speed of 200 m/msec, to how many meters of cable is the 1-bit delay in a token ring interface equivalent? A:
At 5 Mbps, a bit time is 200 nsec.
In 200 ns, the signal travels 40 m.
Thus, insertion of one new station adds as much delay as insertion of 40 meters of cable.

96. Practice Problem # 4
Q: A very heavily loaded 1-km long, 10 Mbps token ring has a propagation speed of 200 m/msec. There are 50 stations uniformly spaced along the ring. Data frames are 256 bits, including 32 bits of overhead. Acknowledgements are piggybacked onto the data frames are are thus included as spare bits within the data frames and are effectively free. The token is 8 bits. Is the effective data rate of this ring higher or lower than the effective data rate of 10 mbps CSDM/CD network? A:
Measured from the time of token capture, it takes 25.6 msec to transmit a packet.
Additionally, a token must be transmitted, taking 0.8 msec
Token must propagate 20 meters taking 0.1 msec.
Thus we have sent 224 bits in 26.5 msec, which results in an effective data rate of 8.5 Mbps. This is more than the effective bandwidth for the Ethernet (4.7 Mbps(why?)) under the same parameters.

97. Practice Problem # 5
Q: Ethernet frame must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64 byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?
A: The maximum wire length in Fast Ethernet is 1/10 as long as in the regular Ethernet.

98. Practice Problem # 6
Q: A large FDDI ring has 100 stations and a token rotation time of 40 msec. The token holding time is 10 msec. What is the maximum achievable efficiency of the ring? A:
With a rotation time of 40 msec and 100 stations, the time for the token to move between stations is 40/100=0.4 msec.
A station may transmit for 10 msec, followed by a 0.4 msec gap while the token moves to the next station.
The best case efficiency is then 10/10.4=96%.

100. IEEE Wireless LAN

101. Power management Idea: switch the transceiver off if not needed
States of a station: sleep and awake
Timing Synchronization Function (TSF)
stations wake up at the same time
Infrastructure
Traffic Indication Map (TIM)
list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)
list of broadcast/multicast receivers transmitted by AP
Ad-hoc
Ad-hoc Traffic Indication Map (ATIM)
announcement of receivers by stations buffering frames
more complicated - no central AP
collision of ATIMs possible (scalability?)

102. Power saving with wake-up patterns (infrastructure)
TIM interval
DTIM interval D B T T d D B
access
point
busy
busy
busy
busy
medium p d
station t T
TIM D
DTIM
awake d
data transmission
to/from the station B
broadcast/multicast p
PS poll

103. Power saving with wake-up patterns (ad-hoc)
ATIM
window
beacon interval B1 A D B1
station1 B2 B2 a d
station2 t B A
transmit ATIM D
beacon frame
random delay
transmit data a d
awake
acknowledge ATIM
acknowledge data

104. 802.11 - Roaming No or bad connection? Then perform: Scanning
scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer
Reassociation Request
station sends a request to one or several AP(s)
Reassociation Response
success: AP has answered, station can now participate
failure: continue scanning
AP accepts Reassociation Request
signal the new station to the distribution system
the distribution system updates its data base (i.e., location information)
typically, the distribution system now informs the old AP so it can release resources

105. WLAN: IEEE 802.11b Frequency Security Cost Availability
On certain frequencies in the free 2.4 GHz ISM-band
Security
Limited, WEP insecure, SSID
Cost
100€ adapter, 250€ base station, dropping
Availability
Many products, many vendors
Special Advantages/Disadvantages
Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system
Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only
What’s new?
Define a new PHY layer. All the MAC schemes, management procedures are the same
User data rate max. approx. 6 Mbit/s

106. Channel selection (non-overlapping)
Europe (ETSI)
channel 1
channel 7
channel 13
2400
2412
2442
2472
2483.5
22 MHz
[MHz]
US (FCC)/Canada (IC)
channel 1
channel 6
channel 11
2400
2412
2437
2462
2483.5
22 MHz
[MHz]

107. WLAN: IEEE 802.11a Frequency Connection set-up time Security
US 5 GHz: free , , GHz ISM-band
Connection set-up time
Connectionless/always on
Security
Limited, WEP insecure, SSID
Availability
Some products, some vendors
Quality of Service
Typ. best effort, no guarantees (same as all products)
Special Advantages/Disadvantages
Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band
Disadvantage: stronger shading due to higher frequency, no QoS

108. Operating channels for 802.11a / US U-NII36 40 44 48 52 56 60 64
channel
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350
[MHz]
16.6 MHz
center frequency =
*channel number [MHz]
149
153
157
161
channel
5725
5745
5765
5785
5805
5825
[MHz]
16.6 MHz

109. IEEE Wireless LAN