GC
2019-2020
Topic: Wireless and Mobile Networks
What you will learn
IEEE 802.11 Mobile IPv6
Centralized Architectures and CAPWAP Planning a wireless access network A glance at VANETs
1/67
GC
2019-2020
wireless hosts
laptop, tablet,
smartphone, or desktop may be stationary (non- mobile) or mobile
wireless does not always mean mobility
Elements of a wireless network
network infrastructure
2/67
GC
2019-2020
wireless link
used to connect a wireless host to a base station or to another wireless host
also used as backbone link
A MAC protocol coordinates link access Various link rates, transmission range
Elements of a wireless network (cont.)
network infrastructure Coverage area transmission
range
3/67
GC
2019-2020
base station
typically connected to a wired network
relay - responsible for sending and receiving packets to and from a wireless host associated with it
e.g., cell towers, 802.11 access points
Elements of a wireless network (cont.)
network infrastructure
4/67
GC
2019-2020
infrastructure mode
base station connects mobile hosts into a wired network handoff (or handover or roaming): mobile host changes base station
Elements of a wireless network
network infrastructure
handoff
5/67
GC
2019-2020
ad hoc mode
no base stations nodes can only transmit to other nodes within coverage area nodes organize themselves into a network: route among themselves
Elements of a wireless network
6/67
GC
2019-2020
Wireless network taxonomy
single hop multiple hops
infrastructure (e.g., APs)
no infrastructure
Wireless hosts connect to a base station, which connects to a larger wired network, e.g., the Internet (WiFi, WiMAX, cellular networks).
There is no base station. One of the nodes may coordinate the transmissions of the other nodes (e.g., Bluetooth networks).
There is a base station, which is wired to the larger network.
Wireless nodes may have to relay their communication through other wireless nodes to communicate via the base station.
Wireless Mesh Networks
No base station. Nodes may have to relay messages in order to reach a destination (MANET, VANET).
MANET: Mobile Ad hoc NETwork VANET: Vehicular Ad hoc NETwork
7/67
GC
2019-2020
Some popular wireless network standards
Indoor
10-30m
Outdoor
50-200m
Mid-range outdoor
200m – 4 Km
Long-range outdoor
5Km – 20 Km
.056 .384 1 4 5-11 54
2G: IS-95, CDMA, GSM 3G: UMTS/WCDMA, CDMA2000 802.15
802.11b 802.11a,g
600 802.11n
D a ta r a te ( M b p s )
Enhanced 3G: HSPA 4G: LTE 802.11a,g point-to-point
Key characteristics: coverage area and data rate
transmission range
802.11ac
1300
8/67
GC
2019-2020
The Wireless Channel
A radio channel between a transmitter unit u and a receiver unit v is established if and only if the power P
rof the radio signal received by the unit v is above a certain threshold β
β depends on the features of the wireless transceiver and on the communication data rate (the higher the data rate, the higher β )
The received power P
rdepends on the transmission power P
tand on the Path Loss PL(u,v), which models the radio signal degradation with distance
The Free Space propagation model assumes the ideal propagation condition that there is only one clear Line-Of-Sight (LOS) path between transmitter and receiver
The path loss PL(u,v) is proportional to d
2, with d being the distance between u and v
P r > β
P r = P t PL(u,v)
9/67
GC
2019-2020
Usually there is no clear line-of-sight path
Receiving power additionally influenced by other mechanisms (frequency dependent), such as:
Atmosphere (weather conditions)
Shadowing (the higher the frequency of a signal, the more it behaves like light) Reflection
Scattering
Diffraction (at edges)
Signal propagation
reflection scattering diffraction shadowing
caused by objects much larger than the wavelength of the signals
if the size of an obstacle is in the order of the wavelength or less
10/67
GC
2019-2020
Multi-path propagation
Signal can take many different paths between sender and receiver
The signal reaches a receiver directly and phase shifted (delay spread) Distorted signal depending on the phases of the different parts
Time dispersion: signal is dispersed over time
Inter Symbol Interference (ISI): it limits the bandwidth of a radio channel
If the receiver knows the delays of the different paths, it can compensate for the distorsion caused by the channel
The sender can first transmit a training sequence known by the receiver
The receiver then compares the received signal to the original training sequence and programs an equalizer used to compensate for the distortion
signal at sender
signal at receiver
LOS pulses Multipath pulses
11/67
GC
2019-2020
Effects of mobility
Due to mobility, channel characteristics change over time and location
If changes are too fast, the receiver cannot adapt fast enough the parameters of the equalizer and, thus, the error rate of transmission increases dramatically
Quick changes in the power received (short term fading)
Additional changes in distance to sender obstacles further away
Slow changes in the average power received (long term fading)
Typically, senders can compensate for long-term fading by increasing/decreasing sending power
The received power always stays within certain limits short term fading
long term fading
t Power
12/67
GC
2019-2020
Antenna Diversity
Due to multipath fading, the electromagnetic field distribution in two points that are few centimetres apart may be very different
Antenna diversity technique (also known space diversity) can be used to try to solve the problem
Access Point with two or more antennas
RX1
RX2
13/67
GC
2019-2020
Bit Error Rate-BER and Signal-to-Noise Ratio-SNR
larger SNR – easier to extract signal from noise SNR versus BER tradeoffs
given physical layer: increase power -> increase SNR-
>decrease BER
given SNR: choose physical layer that meets BER requirement, giving the highest throughput
• The SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)
10 20 30 40
QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)
SNR(dB)
B E R
10-1
10-2
10-3
10-5
10-6
10-7 10-4
14/67
GC
2019-2020
Motivation for a specialized MAC
Can we apply media access methods from fixed networks?
Example CSMA/CD
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium to detect a possible collision (original method in IEEE 802.3)
Problems in wireless networks
The transmission power in the area of the transmitting antenna is several magnitude higher than the receiving power
It is costly to build hardware that simultaneously transmits and listens in order to detect a collision
Collision happens at the receiver
Collision detection would not work well due to the hidden terminal problem
Since wireless networks do not use CD, once a station begins to transmit a frame, it transmit the frame entirely
15/67
GC
2019-2020
Hidden terminals: A and C cannot hear each other because of obstacles, signal attenuation
• A is transmitting to B, C cannot receive A
• C wants to send to B, C senses a “free” medium (Carrier Sense fails)
• collision at B, A cannot receive the collision (CD would fail)
• A is “hidden” for C (and vice versa)
Motivation - hidden and exposed terminals
B
A C
A
B C
Exposed terminals
B is sending to A, C wants to send to another terminal (for example, D) C has to wait, Carrier Sense signals the medium is in use
but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B
D
16/67
GC
2019-2020
IEEE 802.11n-2007 uses Multiple-In Multiple Out (MIMO) technology In order to mitigate RF impairments and enhance transmission rate, multiple antennas on both the sending side and the receiving side transmit/receive multiple spatial streams
RF band: 2.4 GHz, 5 GHz 40 MHz channel
Data rates from 54 Mbit/s to 600 Mbit/s (with 4x4 MIMO)
WiFi: IEEE 802.11 wireless LAN
Standards: 802.11 legacy, 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac, ...
Frequency ISM 2,4 GHz U-NII 5GHz ISM 2,4GHz ISM 2,4GHz
Data Rate 1, 2 6, 9, 12, 18, 24, 36,
48, 54
1, 2, 5.5, 11 1, 2, 5.5, 11, 6, 9, 12, 15, 24, 36, 48, 54
Physical layer FHSS, DSSS OFDM DSSS DSSS/OFDM
Modulation BPSK, QPSK BPSK, QPSK,
16/QAM, 64/QAM
BPSK, QPSK, CCK BPSK, QPSK, CCK, 16/QAM, 64/QAM
Compatibility NONE NONE 802.11 802.11, 802.11b
802.11 802.11a 802.11b 802.11g
17/67
GC
2019-2020
18/67
WiFi: IEEE 802.11 wireless LAN (cont.)
IEEE 802.11ac-2013 uses frequencies in the 5GHz band The 40 MHz of 802.11n is extended to 80 and 160 MHz
The first generation of 802.11ac products (Wave 1) supports data rates up to 1.3 Gbps, the second generation (Wave 2) supports data rates up to 3.5 Gbps.
Support of up to eight spatial stream is allowed, so that data rates up to 6.9 Gbps (with 8x8 MIMO) should be supported
Downlink multi-user MIMO (DL MU-MIMO) is defined: an access point (see
later) transmits simultaneously to multiple wireless stations (space division
multiplexing by transmit beamforming using multi-antennas)
GC
2019-2020
19/67
WiFi: IEEE 802.11 wireless LAN (cont.)
IEEE 802.11ax (Wi-Fi 6) should be ratified in Q1 2020 High transmission efficiency in dense deployment scenarios
802.11ax radios will transmit and receive on either the 2.4 GHz or 5 GHz frequency bands, and, in the future, also in the 6 GHz band
802.11ax radios will be able to communicate with 802.11a/b/g/n/ac radios 802.11ax offers theoretical speeds of up to 10Gbps
Spatial Reuse techniques are defined to reduce the influence of exposed terminal problem
Some techniques are defined to reduce power consumption
Multi-user capabilities in downlink and in uplink (the AP coordinates the uplink transmissions)
DL MU-MIMO, UL MU-MIMO
OFDMA: the channel is divided into sub ‐ channels, called Resource Units (RUs), for transmissions to/from multiple stations
The number of subcarriers per RU
GC
2019-2020
IEEE 802.11 network - Infrastructure mode
Distribution System Portal 802.x LAN
Access Point
802.11 LAN BSS
2802.11 LAN
BSS
1Access
Point STA
1STA
2STA
3ESS
Wireless Station (STA) Access Point (AP) Basic Service Set (BSS)
group of STAs and a central AP using the same radio frequency
BSS Identifier (BSSID): MAC address of the AP
Distribution System
interconnection network to form a logical network (ESS: Extended Service Set) based on several BSS
Each ESS is identified by a Service Set IDentifier (SSID) consisting of up to 32 alphanumeric (case sensitive) characters The SSID is included in beacons sent by the AP (see later)
The architecture of the distribution system is not specified in IEEE 802.11 Portal
bridge to other (wired) networks
20/67
GC
2019-2020
IEEE 802.11 network - Ad-hoc mode
Direct communication within a limited range
Station (STA):
terminal with access mechanisms to the wireless medium
Independent Basic Service Set (IBSS):
group of stations using the same radio frequency for an IBSS, the SSID is chosen by the station that starts the network
802.11 LAN IBSS
2802.11 LAN
IBSS
1STA
1STA
4STA
5STA
2STA
321/67
GC
2019-2020
802.11b/g – Channels and EIRP
22/67
GC
2019-2020
802.11b/g – Channels and EIRP (cont.)
Maximum EIRP (Equivalent Isotropic Radiated Power) for IEEE802.11b 100 mW (20 dBm)
IEEE802.11g 50 mW (17 dBm)
When installing an access point, a network administrator assigns an SSID (displayed when you view “available wireless networks” on your notebook) and a channel number
Actually, an AP can support multiple SSID Sets of non-overlapping channels (in Europe)
• 1 - 6 - 11
• 2 - 7 - 12
• 3 - 8 - 13
Maximum transmission rate of 33 Mbps by installing three 802.11b APs at the same physical location (Co-location)
23/67
GC
2019-2020
Joining a BSS with AP
A STA willing to join a BSS must get in contact with the AP
Passive scanning
• The station scans the channels for a Beacon frame that is periodically broadcast (normally, every 100ms) by every AP
– Beacons include the Timestamp (for synchronization of stations), the SSID, the transmission parameters (e.g., the channel number, the supported data rates), the Beacon Interval (useful for power management, based on alternating between sleep and wake states), the Traffic Indicator Map (a list of stations whose frames have been buffered at the AP), the forms of
authentication and encryption (see later).
– Actually, the beacons are not always periodic because a beacon is also deferred if the medium is busy (see later)
Active scanning (the station tries to find an AP)
• Directed probe: The client sends a probe request with a specific destination SSID; only APs with a matching SSID will reply with a probe response
• Broadcast probe: The client sends a null SSID in the probe request; all APs receiving the probe-request will respond with a probe-response for each SSID they support
– Useful for service discovery
24/67
GC
2019-2020
Joining a BSS with AP (cont.)
An STA selects one of the APs for association
It may be required to authenticate itself
• Pre-shared Key (PSK) authentication
• Server-based authentication. The IEEE 802.11i standard (WiFi Protected Access 2- WPA2 for WiFi Alliance) secures WLANs
– It provides for much stronger security than Wired Equivalent Privacy (WEP) – It supports several forms of encryption schemes, including AES
– It provides for an extensible set of authentication mechanisms, and a key distribution mechanism
– IEEE 802.1x based authentication + EAP (Extensible Authentication Protocol) – Typically, username and password are employed
– The RADIUS server and protocol are de facto standard components for 802.11i – EAP messages are encapsulated using EAPoL (EAP over LAN) when sent over
the 802.11 wireless link
AP: access point
AS:
Authentication server (RADIUS) wired
network STA:
client station
802.1x (EAPoL) Radius (over UDP/IP) 25/67
GC
2019-2020
AP: access point
AS:
Authentication server wired
network STA:
client station
1 Discovery of security capabilities
STA and AS mutually authenticate, together generate Master Key (MK). AP serves as “pass through”
2
3 STA derives 3 Pairwise Master Key (PMK)
AS derives same PMK, sends to AP 4 STA, AP use PMK to derive
Temporal Key (TK) used for message encryption, integrity
Joining a BSS with AP (cont.)
802.11i: four phases of operation
WPA3 designed to overcome inherent weaknesses of WPA2
Instead of PSK, Simultaneous Authentication of Equals (SAE) is used as an initial key exchange protocol
Opportunistic Wireless Encryption (OWE) for improved security on public
networks, like hotels and airports. OWE will provide automatic encryption
with no user intervention required
GC
2019-2020
Joining a BSS with AP (cont.)
Only after the association is completed, a station can transmit and receive data frames
IEEE 802.11f standardizes the exchange of information between APs to support roaming (handoff)
AP 2 informs AP1 of the re-association with AP2
AP1 forwards buffered packets to AP2 and de-registers H1 AP 1 AP 2
H1
BBS 2 BBS 1
1 2 3
1 AP 2
AP 1
H1
BBS 2 BBS 1
1 2
2
3 4
Passive scanning Active scanning (broadcast probe) 2: Association Request frame
3: Association Response frame Association process
STA → AP: Association Request frame AP → STA: Association Response frame
27/67
GC
2019-2020
IEEE 802.11 MAC Protocol
Performs the following functions:
Control Medium Access
• Virtual resource reservation is possible MAC PDU (frame) format
Error control
Data segmentation and reassembly
Three frame types:
1. Control: positive ACK, handshaking for accessing the channel (RTS, CTS)
2. Data Transfer: information to be transmitted over the channel 3. Management: synchronization, authentication, roaming,
power management, …
28/67
GC
2019-2020
Data transfer services
Three access methods:
A mandatory basic method for based on a version of CSMA/CA An optional method avoiding the hidden terminal problem A contention-free method for time-bounded service (for real-time traffic)
The first two methods are summarized as Distributed Coordination Function (DCF)
Asynchronous Data Service
The third method (not really implemented) is called Point Coordination Function (PCF)
PCF is based on the polling of the stations and controlled by the AP (Point Coordinator)
The Point Coordinator splits the time in superframes
• Each superframe consists of a Contention-Free (CFP) period and a Contention Period (CP)
29/67
only asynchronous data service is offered
GC
2019-2020
Time Slot
The system is synchronous Time is divided into slots
A slot is the system time unit and its duration depends on the implementation of the physical layer. For example:
802.11g: 20 μs 802.11a: 9 μs
Stations are synchronized with the AP in the infrastructure mode and among each other in the ad hoc mode
Synchronization is maintained through beacon frames (remember that beacons include the timestamp)
30/67
GC
2019-2020
Inter-Frame Spaces (IFSs)
The waiting time before medium access is controlled through different inter-frame spaces (priorities of medium access)
SIFS (Short IFS)
• the highest priority level, for ACK, CTS, polling response PIFS (PCF IFS) = SIFS + 1 slot time
• medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS) = SIFS + 2 slot times
• the lowest priority, for the asynchronous data service
t medium busy SIFS
PIFS DIFS DIFS
next frame contention
direct access if medium is free ≥ DIFS
31/67
GC
2019-2020
CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance
A station (wireless host or an AP) ready to send starts sensing the medium.
Carrier Sense is based on the Clear Channel Assessment signal (CCA) if the medium is idle for at least the duration of DIFS, the station can access the medium at once
This allows for short access delay under light load
if the medium is busy or the medium is not idle for at least DIFS, the station has to wait until the medium is idle for DIFS again, then the station must additionally wait a random backoff time (multiple of slot-time) chosen within a Contention Window (CW) Collision Avoidance
The station counts down and transmits the packet when the backoff counter reaches zero. However, if another station occupies the medium during the back-off time of the station, the back-off timer stops. The countdown will continue after the medium is idle for DIFS again.
Basic DCF using CSMA/CA
t medium busy
DIFS DIFS
next frame contention window (randomized back-off mechanism)
slot time direct access if
medium is free ≥ DIFS
32/67
GC
2019-2020
Basic DCF
t busy
bo
estation
1station
2station
3station
4station
5packet arrival at MAC DIFS
bo
ebo
ebo
ebusy
elapsed backoff time bo
rresidual backoff time busy medium not idle
(frame, ack etc.)
bo
rbo
rDIFS
bo
ebo
ebo
ebo
rDIFS
busy
busy DIFS
bo
ebusy
bo
ebo
ebo
rbo
rA collision of an unicast frame triggers a retransmission with a new random selection of the backoff time (see next slides)
A simpler version (no ACKs)
33/67
GC
2019-2020
IEEE 802.11 unicast data transfer
An additional feature is provided for unicast data transfer
The receiver of the data transmission answers with an
acknowledgement (ACK) at once (after waiting for SIFS) if the packet was received correctly (CRC)
If no ACK is returned, the sender automatically retransmits the frame
t SIFS
DIFS
data ACK
waiting time other
stations receiver
sender data
DIFS
contention
34/67
GC
2019-2020
Exponential backoff
For a retransmission a new random backoff time is chosen Retransmissions are not privileged
The system tries to adapt to the current number of stations trying to send
The number of slots is a random variable uniformly distributed in [0,CW-1]
The contention window starts with a size of, e.g., CW min = 7
Each time a collision occurs, the contention window doubles up to a maximum, CW max .
For i=1, CW 1 = CW min
For i>1, CW i = (2 CW
i-1+ 1), with i>1 being the number of consecutive attempts for transmitting the MPDU
For any i, CW i ≤ CW max
35/67
GC
2019-2020
DCF with RTS/CTS handshaking
t SIFS
DIFS
data ACK
defer access other
stations receiver
sender data
DIFS
contention RTS
SIFS CTS SIFS
NAV (RTS) NAV (CTS)
After waiting for DIFS (plus a random backoff time if the medium was busy or was not idle for at least DIFS) the sender can issue a Request To send (RTS)
The RTS packet includes the receiver of the data transmission to come and the duration of the whole data transmission (in the duration field)
Every station receiving this RTS has to set its Net Allocation Vector (NAV) in accordance with the duration field
The NAV then specifies the earliest time instant at which the station can try to
access the medium again 36/67
GC
2019-2020
DCF with RTS/CTS handshaking (cont.)
After waiting for SIFS, the receiver of the data transmission answers with a Clear To Send (CTS)
Based on duration field, every station receiving this CTS has to adjust its NAV Finally, the sender can send the DATA after SIFS
After SIFS, the receiver of the DATA acknowledges if the transfer was correct
RTS/CTS is a virtual reservation scheme (virtual carrier sensing) Designed to solve the hidden terminal problem
Collisions can only occur at the beginning while the RTS is sent Using RTS/CTS can result in a non-negligible overhead
An RTS threshold can determine when to use the additional mechanism (basically at larger data frame size) and when to disable it (short data frames) Area covered by
RTS/CTS
RTS/CTS works well, as all stations are within the transmission range of the AP
A is “hidden” for C C is “hidden” for A
RTS
A
CTSB C
D
37/67
GC
2019-2020
802.11 frame
frame
control duration address 1
address 2
address 4 address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seq control
Type From
Subtype To AP AP
More
frag More WEP
data Power
Retry mgt Rsvd
Protocol version
2 2 4 1 1 1 1 1 1 1 1
duration of reserved
transmission time (RTS/CTS)
used to filter duplicates
This bit is set to 1 if the frame is a retransmission
Set to 1, this bit indicates that the station goes into power-save mode
38/67
GC
2019-2020
The address 1 identifies the physical receiver(s) of the frame The address 2 represents the physical transmitter of the frame
The address 3 and the address 4 are necessary for the logical assignment of frames ( logical sender, BSS identifier, logical receiver)
If the address 4 is not needed, the field is omitted
802.11 frame: addressing (cont.)
DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address. It is the MAC address of the receiving AP within the DS TA: Transmitter Address. It is the MAC address of the transmitting AP within the DS
scenario to AP from
AP
address 1 address 2 address 3 address 4
ad-hoc network 0 0 DA SA BSSID -
infrastructure network, from AP
0 1 DA BSSID SA -
infrastructure network, to AP
1 0 BSSID SA DA -
infrastructure network, within DS
1 1 RA TA DA SA
39/67
GC
2019-2020
802.11 frame: addressing (cont.)
Internet router
H1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr
dest. address source address
802.3 frame
logical and physical sender
physical receiver logical receiver
40/67
GC
2019-2020
41/67
Access point operation: root mode
The root mode is the default mode
GC
2019-2020
42/67
Access point operation: bridge mode
An AP operating in bridge mode essentially becomes a wireless bridge
APs in bridges mode are typically used to link together two or
more wired segments in different buildings.
GC
2019-2020
43/67
Access point operation: repeater mode
The AP in repeater mode connects to clients (wireless hosts) as an access point, while it connects to a root mode AP as a client
There must be at least a 50% overlap of the cells involved
This configuration is recommended only if absolutely necessary because of the reduced throughput.
GC
2019-2020
Mobility in the same IP subnet
The mobile node H1 moves from BSS1 to BSS2.
Remaining in the same IP subnet, H1 keeps its IP address and all of its ongoing TCP connections
switch: which AP is associated with H1?
Backward-Learning solves the problem: the switch will see a frame from H1 and “ remember ” which switch port can be used to reach H1
• AP2 could send a broadcast Ethernet frame with H1’s source address to the switch just after the new association (some 802.11f
specifications handle this issue) 44/67
H1
BSS1 BSS2
L2 switch IP Router
AP1 AP2
GC
2019-2020
Moving within the Internet: MIPv6
Mobile IPv6 allows devices to be reachable and maintain ongoing TCP connections (e.g., FTP) while moving within the Internet topology
Subnet B Internet IPv6
Home Agent Correspondent node
IPv6 router Mobile node
Subnet A
45/67
GC
2019-2020
Mobile Node (MN): a mobile node is a node that changes its location within the Internet topology
Correspondent Node (CN): any node that communicates with the mobile node
Home address: a stable address that belongs to the mobile node and is used by correspondent nodes to reach mobile nodes
based on the 64-bit prefix assigned to the home link combined with the mobile node’s interface identifier
used also to allow a mobile node to be reachable by having a stable entry in the DNS
Home link: a link to which the home address prefix is assigned
Home Agent (HA): a router located on the home link that acts as a proxy for the mobile node while away from the home link.
The home agent redirects packets addressed to a mobile node’s home address to its current location (care-of address) using IP-in-IP tunneling Foreign link: any link (other than the home link) visited by a mobile node
MIPv6 terminology
46/67
GC
2019-2020
MIPv6 terminology (cont.)
Care-of Address (CoA): an address that is assigned to the mobile node when located in a foreign link
formed based on stateless or stateful (DHCP) autoconfiguration Binding: the association of the mobile node’s home address with a care-of address for a certain period of time
The binding is refreshed (if the refresh timer expires) or updated when the mobile node gets a new care-of address
Binding cache: a cache containing a number of bindings for one or more mobile nodes
A binding cache is maintained by both the home agent and the correspondent node
Binding Update List: a list maintained by the mobile node containing all bindings that were sent to the mobile node’s home agent and
correspondent nodes
maintained in order for the mobile node to know when a binding needs to be refreshed
47/67
GC
2019-2020
Overview of MIPv6
While at home, the MN uses its permanent IP address (home address) When the mobile node moves from its home link to a foreign link, it first forms a CoA based on the prefix of the foreign link announced through the router advertisements (stateless autoconfiguration is assumed) Following address configuration, the mobile node informs its HA of such movement by sending a Binding Update (BU) message
Subnet B Internet IPv6
Subnet A Home Agent
Correspondent node
IPv6 router Binding Update
48/67
GC
2019-2020
Overview of MIPv6 (cont.)
The home agent validates the message and, if the binding update is accepted, it creates an entry for the binding information in its binding cache (or updates an already existing entry)
Then, it acknowledges the binding update sent by the MN
Subnet B Internet IPv6
Subnet A Home Agent
Correspondent node
IPv6 router Binding Ack
49/67
GC
2019-2020
Overview of MIPv6 (cont.)
The home agent sends a proxy neighbor advertisement (actually, more than once) addressed to the all-nodes multicast address on the link (FF02::1)
The advertisement includes the mobile node’s home address in the target address field and the home agent’s link-layer address
• Hence, the home agent ensures that any IP packet addressed to the mobile node is forwarded to the home agent’s link-layer address
The Home Agent flag is set in a router advertisement to indicate that the router sending this router advertisement is also functioning as a Mobile IPv6 home agent on this link
The home agent also defends the mobile node’s addresses (remember the DAD procedure related to IPv6)
Upon receiving a packet addressed to the mobile node’s home address, the home agent tunnels it to the mobile node’s care-of address, which is included in the mobile node’s binding cache entry (IPv6-in-IPv6)
50/67
GC
2019-2020
Overview of MIPv6 (cont.)
The tunnel entry point is the home agent (source address in the outer header), and the tunnel exit point is the mobile node’s care-of address The tunnel is bidirectional
The tunnel to the mobile node can be secured using IPsec
Subnet B Internet IPv6
Subnet A Home Agent
Correspondent node
IPv6 router tunnel
51/67
GC
2019-2020
Overview of MIPv6 (cont.)
Routing packets through the home agent adds delays and uses more network bandwidth than direct communication
Routing optimization is about routing packets between a mobile node and a correspondent node, using the shortest possible path
The mobile node informs the correspondent node of its current location The correspondent node maintains a binding cache similar to the one maintained by the home agent.
Subnet B Internet IPv6
Subnet A Home Agent
Correspondent node
binding update and binding Ack
IPv6 packets
52/67
GC
2019-2020
Fat APs also have enhanced capabilities such as Access Control Lists (ACLs), QoS-related functions, such as enforcing IEEE 802.1p priority or DSCP (DiffServ)
Each AP is independently managed
The downside of such APs is complexity, so that they have uses only in small network installations
Autonomous Architecture
There is no backhauling of wireless traffic from the FAT AP to another device
Fat APs can provide VLAN tagging, based on the SSID that the client uses to associate with the AP (Multiple SSIDs), and “router-like”
functions, such as DHCP server capabilities
The Access Points (Fat APs) completely implement and terminate the 802.11 functions
53/67
GC
2019-2020
Centralized Architecture
CAPWAPProtocol between WLAN Controller and AP for Configuration and Control
WLAN Controllers
LightweightAccess Points (APs)
An important motivation is the location of APs
Aiming at providing optimum radio connectivity for end stations, APs are typically mounted in areas which are hard-to-reach
Network managers prefer to install APs just once and not have to perform complex maintenance on them
54/67
GC
2019-2020
Centralized Architecture (cont.)
APs are connected to a WLAN controller (WLC) through a secure tunnel The tunnel should ensure low delay for packets
The Control and Provisioning of Wireless Access Points (CAPWAP) is the protocol used in order to communicate
CAPWAP is responsible for discovery and selection of an WLC by the AP CAPWAP uses UDP ports 5246 (control channel) and 5247 (data channel) APs backhaul wireless
802.11 frames to the WLC encapsulated within CAPWAP data packets CAPWAP control packets are encrypted
CAPWAP data packets encryption is possible, but this may result in severe
throughput degradation 55/67
GC
2019-2020
Split MAC architecture: the implementation of the MAC functions is divided between the AP and the WLC
APs are lightweight in the sense that they handle only a part of MAC functionalities
Vendors differ in the type of MAC splitting between the AP and the WLC Typically, APs
•
handle real-time MAC functions, such as beacon generation, probe response, control frame processing (RTS, CTS) and so on
•
leave all the non−real−time MAC functionalities (authentication, association, ...) to be processed by the WLC
APs provide the wireless encryption, while using the WLC for the actual key exchange
Centralized Architecture (cont.)
56/67
GC
2019-2020
The WLC
manages the firmware and configurations of the controlled APs
performs Radio Resource Management based on configuration and monitoring of the controlled APs
•
Through CAPWAP control messages, the APs send statistics (number of transmit retries, number of erroneous frames, …) to the WLC
•
For example, if two APs controlled by a WLC are interfering with each other, the WLC can send a signal to one of the APs to reduce its strength
Handles QoS enforcement and ACL-based filtering
Manages layer-2 and layer-3 mobility (the WLC acts as Mobile IP Home Agent) The extent of the several functions varies according to the vendor
implementation
Centralized Architecture (cont.)
...WLC location..
On a per-building basis On a per-campus basis ?
57/67
GC
2019-2020
Planning a wireless access network
58/67
GC
2019-2020
59/67
Looking beyond IEEE802.11ax
WLAN future evolution
802.11 engineers are already working on a new standard, currently called EHT (Extreme High Throughput), which would push WiFi into the 6 GHz band
EHT is expected to support 16 streams, twice as many as 11ax IEEE 802.11ay will be the follow-up of IEEE 802.11ad
Key application: transmission of audio/visual data
Wi-Fi APs running EHT and 802.11ay will be capable of generating more than 10Gb/s of traffic
GC
2019-2020
Implications for backhaul cabling
To support the multi-gigabit backhaul, future installations can be expected to utilize APs with two or more 10GBASE-T (aggregated) uplinks
Both ISO/IEC and ANSI/TIA cabling standards recommend that a minimum of two category 6A cables be installed to each SO (Service Outlet) that will support an AP.
Each category 6A cable will provide Class Ea cabling channels that support 10 Gb/s of data bandwidth to 100 meters
backhaul cabling
At least two cat6A cablesThe category of installed cabling must be considered to provide sufficient bandwidth for current and future applications
For distances up to 30 meters, designers might also consider category 8 cabling. Each cat 8 cable will support Class II channels up to 30 meters, providing 40 Gb/s of bandwidth
Multimode optical fibers, OM3 or higher, should be also considered, in
particular, for outdoor locations where distances are greater than 100m
GC
2019-2020
Cabling standards for AP deployment
The ISO/IEC 11801-6 and the Cenelec EN 50173-6 propose what it considered to be an optimal pattern for locating wireless access points The design is based on an array of tight-fitting hexagonal cells
The coverage area of each cell is limited to a 12-meter radius Recommendation: terminating the cables for each cell at an outlet located as close to the centre of the cell as possible
12 m
61/67
GC
2019-2020
Cabling standards for AP deployment (cont.)
TIA TSB-162-A (Telecommunications Cabling Guidelines for Wireless Access Points) suggests a square grid of cabling areas, each about 18 meters wide
TC
62/67
GC
2019-2020
TIA-4966 recommends that AP density within large open indoor spaces be based on expected occupancy (see table)
When developing a coverage plan for facilities with multiple partitioned areas, density should be based on square footage
One access point per 230 square meters for a typical office building
One access point per 150 square meters in case of facilities that may be less
RF friendly 63/67
Cabling standards for AP deployment (cont.)
GC
2019-2020
Planning the cabling architecture
Careful planning based on the RF environment, interference levels and sources, future capacity needs
It is recommended that an RF survey be conducted to optimize the AP location(s) within a given cell
Sample floor plan
64/67
GC
2019-2020
RF environment
In environments where capacity and other concerns are minimal, simply assessing RF propagation may be sufficient
Some programs allow network planners to input the site’s layout, conduct AP modelling, and compare RF simulations and surveys
RF planning diagram with one access point deployed
(grey area denotes limited or no coverage) 65/67
GC
2019-2020
RF environment (cont.)
Signal weakness could be due to a number of variables, including the presence of RF-blocking materials such as cabinets or equipment, or structural impediments such as concrete walls
Adding additional APs significantly improves coverage
RF planning diagram using three APs
66/67
GC
2019-2020
Along with ensuring an adequate RF signal, the aggregate throughput should be considered
The space has to be divided into cells
Planning for higher capacity
Floor plan configured for a square-grid deployment (TIA TSB-162-A)
67/67
GC
2019-2020
Planning for higher capacity (cont.)
AP positions and density can be adjusted to suit the occupancy
RF planning diagram for high density AP deployment
68/67
GC
2019-2020
AP location and cabling
In addition to the RF environment and capacity planning, there are a number of other factors to consider in cabling and locating wireless access points (accessibility, electric power requirements, aesthetics, ..)
FD/HC
Structured cabling diagram
69/67
GC
2019-2020
Ad Hoc Wireless Networks
Collection of mobile hosts capable to form a temporary network without the support of any fixed infrastructure (infrastructureless, self-
organizing, self-configuring)
Network operations, such as routing and resource management, are performed in a distributed and cooperative manner
Due to limited radio transmission range, multi-hop routing is usually used
Each node can act as a host and as a router: a packet is forwarded from one node to another until it reaches the destination
70/67
GC
2019-2020
Applications of Ad Hoc Wireless Networks
Some interesting scenarios:
Military applications Emergency operations Vehicular communications Underwater communications
Acoustic communications
71/67
GC
2019-2020
Ad Hoc Wireless Networks (cont.)
Due to mobility associated with the nodes, network topology may experience continuous changes
Different power levels among different nodes introduce asymmetric links
Resources are typically constrained (bandwidth, battery power, etc.)
A B A B
72/67
GC
2019-2020
Effectiveness of RTS/CTS in Ad Hoc Networks
The effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within the transmission range of receivers (so that they can receive CTS packet successfully)
Some node out of the transmission range of the receiver may still interfere with the receiver
Nodes within the interference range (R i ) of a receiver are called hidden nodes
tx rx R
txd
R
ihidden node
r
hidden node
Area covered by RTS/CTS
Interference area not covered by RTS/CTS
73/67
GC
2019-2020
Radio ranges
Three radio ranges related to a wireless radio:
Transmission Range (R tx ): it represents the range within which a packet is successfully received if there is no interference from other radios. The transmission range is mainly determined by transmission power and radio propagation properties (i.e., attenuation).
Carrier Sensing Range (R cs ): it is the range within which a transmitter triggers carrier sense detection. This range is mainly determined by the antenna sensitivity and by the transmission power.
Interference Range (R i ): it defines the interference area A i = π R i 2 around a receiver. All nodes located in this area are hidden nodes of the
receiver. When the receiver is receiving a packet, if a hidden node starts a transmission, a collision will happen at the receiver.
Transmission range and Carrier sensing range are fixed and affected by the properties of the wireless radios
The interference range is not fixed, but related to the transmitter-receiver distance and can go far beyond the transmission range
74/67
GC
2019-2020
75/72
Vehicular Ad Hoc Networks (VANETs) VANET
Ad hoc network in which nodes are vehicles
IEEE 802.11p
• IEEE 802.11 amendment for the support of vehicular communications
GC
2019-2020
76/72
Routing problem in VANETs
SOURCE
DESTINATION The high speed of vehicles
causes frequent topology changes
It would be very difficult to keep routing tables updated
The destination is no longer reachable
Traffic on the wireless channel
would increase because of the exchange of control messages
GC
2019-2020
77/72 To borrow a technique thought for Intelligent Broadcasting
Routeless Routing
The high speed of vehicles causes frequent topology changes
Traffic on the wireless channel
would increase because of the exchange of control messages
Routing problem in VANETs
It would be very difficult to keep routing tables updated
GC
2019-2020
78/72 SOURCE
1 2
3
Contention-Based Forwarding (CBF)
SOURCE
d1 d2 d3
WT(d)
0
d TX range WT1
MaxWT
WT2 WT3
TX range
GC
2019-2020
79/72
RETRANSMISSION
Unicast transmissions with CBF
1. Only one node is the recipient of packets
2. All nodes that receive a given packet and are in the direction “source->
destination” (or “last forwarder-> destination”) calculate the waiting time and participate in the contention process
3. The node whose timer first expires forwards the packet .
4. Each node that overhears the transmission discards the packet (implicit ACK)
RETRANSMISSION
RETRANSMISSION RETRANSMISSION RETRANSMISSION
DESTINATION
DESTINATION
