Advanced Network Technologies Wireless 2
Dr. | Lecturer School of Computer Science
IEEE 802.11 Wireless LA Fi
IEEE 802.11 WiFi
IEEE 802.11 standard
Max data rate
802.11n (WiFi 4)
2.4, 5 Ghz
802.11ac (WiFi 5)
802.11ax (WiFi 6)
2020 (exp.)
2.4, 5 Ghz
35 – 560 Mbps
unused TV bands (54-790 MHz)
§ all use CSMA/CA for multiple access, and have base-station and ad- hoc network versions
Wireless and Mobile Networks: 7- 3
802.11 LAN architecture
hub, switch or router
v wireless host communicates with base station
§ base station = access point ( AP)
v Basic Service Set (BSS) (aka
“cell”) in infrastructure
mode contains:
§ wireless hosts
§ access point (AP): base station
§ ad hoc mode: hosts only
802.11: Channels, association
› 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
– AP admin chooses frequency for AP
– interference possible: channel can be same as that chosen by neighboring AP! › host: must associate with an AP
– scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address – selects AP to associate with
– may perform authentication
– will typically run DHCP to get IP address in AP’s subnet
802.11: passive/active scanning
AP1 1AP2 AP12 2
passive scanning:
(1) beacon frames sent from APs
(2) association Request frame sent: H1 to
selected AP
(3) association Response frame sent from
selected AP to H1
active scanning:
(1) Probe Request frame broadcast
(2) Probe Response frames sent
(3) Association Request frame sent:
H1 to selected AP
(4) Association Response frame sent
from selected AP to H1
spatial layout of nodes collisions
› collisions can occur: propagation delay means two nodes may not hear each other’s transmission
› collision: frame transmission time wasted
Wired Networks: CSMA/CD (collision detection)
– collisions detected within short time
– colliding transmissions aborted, reducing channel wastage
› collision detection:
– wired LANs: measure signal strengths, compare transmitted, received
– Can transmit and sense at the same time
– wireless LANs: received signal strength overwhelmed by local
transmission strength
– CSMA-CD cannot be used in wireless LAN
CSMA/CD (collision detection)
spatial layout of nodes
IEEE 802.11: multiple access
› 802.11: no collision detection!
– difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
– can not sense all collisions in any case: hidden terminal, fading – goal: avoid collisions: CSMA/C(ollision)A(voidance)
AB A’s signal
C’s signal strength
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS (Distributed inter-frame space) 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
802.11 receiver
– if frame received OK
return ACK after SIFS (Shorter inter-frame spacing)
Sender: if no ACK, increase random backoff interval, repeat 2
sender DIFS
Avoiding collisions (more)
idea: allowsenderto“reserve”channelratherthanrandomaccess 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!
Collision Avoidance: RTS-CTS exchange
reservation collision
Please think: How does A (B) know that RTS collide?
RTS(A) RTS(A)
802.11: advanced capabilities
Rate adaptation
› base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)
operating point
10-1 10-2 10-3 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
2. When BER becomes too high, switch to lower transmission rate but with lower BER
Exposed Terminal
Source: Wikipedia
Exposed Terminal
Ideal: S1->R1 and S2->R2 simultaneously
However: S2 can sense the carrier of S1 so that it keeps silence
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Cellular Internet Access Architecture and standards
Components of cellular network architecture
v covers geographical region
v base station (BS) analogous to 802.11 AP v mobile users attach to network through BS vair-interface: physical and link layer protocol between mobile and BS
v connects cells to wired tel. net. v manages call setup (more later!) v handles mobility (more later!)
Mobile Switching Center
Public telephone network
wired network
Mobile Switching Center
Cellular networks: the first hop
Two techniques for sharing mobile- to-BS radio spectrum
› combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots
› CDMA: code division multiple
time slots
frequency bands
2G (voice) network architecture
Base station system (BSS)
Public telephone network
Gateway MSC
Base transceiver station (BTS)
Base station controller (BSC) Mobile Switching Center (MSC)
Mobile subscribers
3G (voice+data) network architecture
radio network controller
Gateway MSC
Public telephone network
Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network
Public Internet
v voice network unchanged in core v data network operates in parallel
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN) General Packet Radio Service
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
1. Evolved packet core (EPC): simplified all-IP core network that unifies the cellular circuit-switched voice network and the packet switched cellular data network.
Public telephone network
Public Internet
Evolved Packet Core (all-IP)
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
2. LTE Radio Access Networks: uses a combination of orthogonal frequency-division multiplexing (OFDM) and time division multiplexing.
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
2. LTE Radio Access Networks: uses a combination of orthogonal frequency-division multiplexing (OFDM) and time division multiplexing.
frequency carriers
time slots (ms)
0 0.5 1 1.5 2 2.5
Mobility principles:
Addressing and routing to mobile users
What is mobility?
› spectrum of mobility, from the network perspective:
no mobility
mobile wireless user, using same access point
mobile user, disconnecting from network when moving.
high mobility
mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
Should Address always remain the same?
› Mobile phone: the phone number remains the same at all time when you travel
› How about IP Address?
Mobility: vocabulary
home network: permanent “home” of mobile
(e.g., 128.119.40/24)
permanent address:
address in home network, can always be
used to reach mobile e.g., 128.119.40.186
home agent: entity that will perform mobility functions on behalf of mobile
wide area network
Mobility: more vocabulary
permanent address: remains constant (e.g., 128.119.40.186)
Foreign (visited) network:
network in which mobile currently resides (e.g.,
care-of-address: address 79.129.13/24) in visited network.
(e.g., 79,129.13.2)
wide area network
correspondent: wants to communicate with mobile
foreign agent: entity in visited network that performs mobility functions on behalf of mobile.
How do you contact a mobile friend:
Consider friend frequently changing addresses, how do you find her?
› search all phone books?
› call her parents?
› expect her to let you know where he/she is?
I wonder where Alice moved to?
Mobility: approaches
› let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
– routing tables indicate where each mobile located – no changes to end-systems
› let end-systems handle it:
– indirect routing: communication from correspondent to mobile goes through home
agent, then forwarded to remote
– direct routing: correspondent gets foreign address of mobile, sends directly to mobile
Mobility: approaches
› let routing handle it: routers advertise permanent address of not
mobile-nodes-in-residence via usual routing table exchange.
– routing tables indicate where each mobile located – no changes to end-systems to millions of
› let end-systems handle it:
– indirect routing: communication from correspondent to mobile goes through home
agent, then forwarded to remote
– direct routing: correspondent gets foreign address of mobile, sends directly to mobile
Mobility: registration
home network
visited network
mobile contacts foreign agent on entering visited network
wide area network
foreign agent contacts home agent home: “this mobile is resident in my network”
end result:
› foreign agent knows about mobile
› home agent knows location of mobile
Mobility via indirect routing
home agent intercepts packets, forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent
visited network
home network
wide area network
correspondent addresses packets using home address of mobile
Indirect Routing: comments
› mobile uses two addresses:
– permanent address: used by correspondent (hence mobile location is transparent
to correspondent)
– care-of-address: used by home agent to forward datagrams to mobile
› triangle routing: correspondent-home-network-mobile
– inefficient when correspondent, mobile are in same network
Indirect routing: moving between networks
› suppose mobile user moves to another network
– registers with new foreign agent
– new foreign agent registers with home agent
– home agent update care-of-address for mobile
– packets continue to be forwarded to mobile (but with new care-of-address)
› changing foreign networks transparent: on going connections can be maintained!
Mobility via direct routing
correspondent forwards to foreign agent
foreign agent receives packets, forwards to mobile
visited network
home network
correspondent requests, receives foreign address of mobile
mobile replies directly to correspondent
Mobility via direct routing: comments
› overcome triangle routing problem
› non-transparent to correspondent: correspondent must get care-of-address
from home agent
– what if mobile changes visited network?
Accommodating mobility with direct routing
› anchor foreign agent: FA in first visited network
› data always routed first to anchor FA
› when mobile moves: new FA arranges to have data forwarded from old FA (chaining)
wide area network
anchor foreign agent
foreign net visited at session start
correspondent
correspondent agent
new foreign agent
new foreign network
› RFC 3344
› has many features we have seen:
– home agents, foreign agents, foreign-agent registration, care-of-addresses › three components to standard:
– indirect routing of datagrams – agent discovery
– registration with home agent
Mobile IP: indirect routing
packet sent by home agent to foreign agent: a packet within a packet
foreign-agent-to-mobile packet
dest: 128.119.40.186
dest: 79.129.13.2
Permanent address: 128.119.40.186
packet sent by correspondent
Care-of address: 79.129.13.2
dest: 128.119.40.186
dest: 128.119.40.186
Mobile IP: agent discovery
› agent advertisement: foreign/home agents advertise service by broadcasting ICMP (Internet Control Message Protocol) messages (typefield = 9)
router address
sequence #
RBHFMGV bits
registration lifetime
mobility agent advertisement extension
0 or more care-of- addresses
H,F bits: home and/or foreign agent
R bit: registration required
standard ICMP fields
Mobile IP: registration example
home agent HA: 128.119.40.7
visited network: 79.129.13/24 foreign agent
COA: 79.129.13.2
ICMP agent adv.
79.129.13.2
mobile agent
MA: 128.119.40.186
registration req.
registration req.
COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format ….
COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 ….
registration reply
registration reply
HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format ….
HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 ….
Mobility in Cellular Networks
Components of cellular network architecture
correspondent
wired public telephone network
different cellular networks, operated by different providers
Handling mobility in cellular networks
› home network: network of cellular provider you subscribe to (e.g., Vodafone)
– home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)
› visited network: network in which mobile currently resides
– visitor location register (VLR): database with entry for each user currently in network – could be home network
GSM: indirect routing to mobile
home MSC consults HLR, gets roaming number of mobile in visited network
home network
correspondent
home Mobile Switching Center
call routed
to home network
Public switched telephone network
Mobile Switching Center
mobile user
visited network
home MSC sets up 2nd leg of call to MSC in visited network
MSC in visited network completes call through base station to mobile
GSM: handoff with common MSC
Mobile Switching Center
old new routing routing
› handoff goal: route call via new base station (without interruption)
› reasons for handoff:
– stronger signal to/from new BSS (continuing connectivity, less battery drain)
– load balance: free up channel in current BSS
› handoff initiated by old BSS
GSM: handoff with common MSC
Mobile Switching
1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs
2. MSC sets up path (allocates resources) to new BSS
3. new BSS allocates radio channel for use by mobile
4. new BSS signals MSC, old BSS: ready
5. old BSS tells mobile: perform handoff to new BSS
6. mobile, new BSS signal to activate new channel
7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call
8 MSC-old BSS resources released
Handoff algorithm: a brief overview
Signal Strength of Two Base Stations: when to handoff?
Handoff algorithm: a brief overview
› Naive way: Compare the RSSs (Received Signal Strength) of two B at
› Pnew> Pold
Ping-pong effect
Handoff back and forth.
Smarter ways
› RSS: initiate handoff to BS new if
› Pnew> Pold
› RSS with threshold(PT): choose BS new if
› Pnew> Pold and Pold< PT
› RSS with hysteresis(PH): choose BS new if › Pnew> Pold+PH
› RSS with threshold(PT) and hysteresis(PH): choose BS new if › Pnew> Pold+PH and Pold< PT
› Even better: Add a Dwell Timer to the above algorithms: start timer when above condition is met; initiate handoff if condition persists when timer expires
Wireless, mobility: impact on higher layer protocols
› logically, impact should be minimal ...
- best effort service model remains unchanged
- TCP and UDP can (and do) run over wireless, mobile
› ... but performance-wise:
- packet loss/delay due to bit-errors (discarded packets, delays for link-layer
retransmissions), and handoff
- TCP interprets loss as congestion, will decrease congestion window un-necessarily
- delay impairments for real-time traffic