WIRELESS SYSTEM
Mobile phone. Cellular phone. Smart phone. 3G/4G System.
Market growth rate of wireless phone technology.
http://www.jana.com/blog/mobile-subscriptions-to-exceed-global-population/
• Ubiquitous presence of cell-phones
• Wireless applications still small but growing
◊ WLAN rapidly growing
▪ Bluetooth, …
◊ Wireless WAN is rapidly capturing more market
▪ Wireless broadband, LBS (Location-based services)
Location-based services: Locate friends, nearest ATM machine,
track package, mobile advertising, …
BUT …
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Lack of standardization in wireless devices
Lack of standardization of interfaces
Spectrum shortage
Lack of killer application
Lukewarm performance of WLAN
• Lack of universal coverage
• Expensive service rate
HOWEVER …
• Internet is just about ready to accept ‘wireless’ in all its
innovative designs (if only if we can imagine tomorrow).
For mobile users:
◊ personalized data-retrieval
◊ online trading
◊ 3D TV viewing
• Thinner low power smartphones with longer battery lives are
coming up.
• Built-in next generation interfaces is expected.
• TCP/IP friendly Link-layer protocol is coming up.
Overall evolution of Information technology.
• Wired → Wireless Desktop → Mobile
Addition of at least one more degree of freedom
• Data → Multimedia
• Individual stand-alone PC → networked computing
• Moving beyond phones and PC → sensor based systems
Mobile phones in various phases.
◊ 1st Generation: Analog voice + Cordless phones + packet radio
◊ 2nd Generation: Digital voice + seemless roaming + Integrated
paging + Wireless LANs, MANs and WANs
◊ 3rd Generation: Internet access + video calls + mobile TV
◊ 4th Generation: WiMax mobile broadband access + VoIP +
100 Mb/s Internet access speeds from fast mobile devices
+ 1 Gb/s access for low speed mobile devices (pedestrians)
+ scalable bandwidth 5-20 MHz, sometimes up to 40 MHz
+ soft handover across heterogeneous networks
+ high QoS for next generation multimedia support
The phenomenal growth rate of this industry is due to:
Multiaccess technology.
Share the available radio spectrum among multiple users
simultaneously. Bandwidth sharing.
AMPS (Advanced Mobile Phone Systems) + D-AMPs
Mobile phones and devices are scattered over a geographic
region. Each region is divided into cells. Each cell supports a set
of radio frequencies, which neighboring cells do not support.
Reuse of same frequency for geographically separated cells.
However that depends on the strength of the signal within
propagation range.
Channel sharing: (a) Frequency Division Multiplexing, (b) Code
Division Multiple Access, and (c) TDMA
FDM: Entire frequency spectrum is partitioned into several
frequency bands. A channel is allocated a freqency band
permanently.
CDMA: Each channel uses a unique code for transmitting. All
channels use the same frequency spectrum all the time. Signal
generation in this case is complex and requires special hardware.
CDMA is used for 3G transmission/reception. For ad-hoc wireless
systems, one cannot expect these hardware support. Hence, no
CDMA.
TDMA: This and its variations are used in many ad-hoc designs.
Each cell has a base station that listens to and transmit at these
frequencies. Frequency reuse is the issue here.
A mobile telephone 𝑀𝑇𝑖 is logically in a cell 𝑖 under the control of
a base-station 𝑏𝑖 .
If 𝑀𝑇𝑖 moves away from its current position, its received signal at
the current base would reduce, and at some other base 𝑏𝑗 it
would grow stronger. This means eventually 𝑏𝑖 → 𝑏𝑗 (handover).
Handover takes 300ms.
If base handover takes place, 𝑀𝑇𝑖 would be assigned a different
frequency channel. Two types of handover.
Hard handover: 𝑏𝑖 drops its client 𝑀𝑇𝑖 abruptly before new
channel is acquired. If new channel cannot be found, the user
has to wait.
Soft handover: User would not notice 𝑏𝑖 → 𝑏𝑗 . It would be
smooth with no disruption of the current call.
General protocol for mobile migration.
◊ All base-stations are connected to a MTSO (Mobile Telephone
Switching Office). In turn, these are connected to MTSO, next
level up.
http://www.tech-faq.com/mtso-mobile-telephone-switching-office.html
◊ There are 832 duplex channels, each consisting a pair of
simplex channels of 30 KHz wide, from 824-849 MHz, and from
869-894 MHz. AMPS uses FDM to separate the channels.
◊ These channels are clustered in 4 categories
▪ Control (base to mobile)
▪ Paging (base ot mobile)
▪ Access (bidirectional) for call setup, channel assignment
▪ Data (bidirectional) for voice, fax, data
802.11 MAC Protocol
Different from Ethernet or 802.3 (CSMA/CD) protocol. With
CSMA/CD a station waits till the ether is perceived silent. This
cannot be done in this case.
http://en.wikibooks.org/wiki/Communication_Networks/Error_Control,_Flow_Control,_MAC
Fig A shows Hidden Terminal Problem. To C, everything is quite
as A is hidden from it. If C transmits now, it would collide at B
with A’s transmission. Can’t be done.
Fig B shows an Exposed station problem. If C wants to send to D
but thinks that it will collide at D. Therefore, it waits wasting time
and opportunity.
Multiple Access Collision Avoidance (MACA)
◊ Use of additional signaling packets.
◊ Sender sends a RTS (Request To Send).
◊ If Receiver agrees to receive, it would transmit CTS (Clear to
Send).
◊ If Sender receives CTS, only then it starts transmitting.
◊ A typical call dynamics.
Outgoing:
▪ Cellphone 𝑀𝑇𝑖 types in the number 𝑝𝑗 , and sends them
to the nearest 𝑏𝑘 on the access channel.
▪ If collision, it tries again.
▪ 𝑏𝑘 after getting it sends it to its MTSO or its partner.
▪ MTSO checks if the caller is its client. If client then
◦ it looks for an idle channel to place the call
◦ if found, the call is placed in it for the receiver.
Incoming:
▪ If phone is idle, it listens to the paging channel.
▪ When a call is placed to a mobile phone (from a landline or
mobile), a packet is sent to callee’s home MTSO to locate
her.
▪ When found, a packet is sent to callee’s current cell’s
base-station to send a broadcast on the paging channel
of the form ‘Is that you?’
▪ When the callee identifies, the base then informs the callee
that a call awaits on channel x. The ringing tone starts.
Routing imperatives for mobile hosts.
◊ Every host has a permanent home location and a permanent
home address.
◊ The network domain is partitioned into cells where one or more
foreign agents reside. Also, each cell would have a home
agent.
◊ Each foreign agent keeps track of mobile hosts visiting its cell.
◊ A mobile unit entering into a new cell must register with its
foreign agent.
Registration process with a foreign agent
◊ A foreign agent periodically broadcasts its presence and its
address. A wandering host may wait for it, or it might broadcast a
packet saying: “Any foreign agent around?”
◊ Mobile host registers with the foreign agent giving its home
address, machine address, and some security info.
◊ Foreign agent contacts the mobile host’s home agent, and
informs that one of its host is registered with it. Home agent
receives foreign agent’s address. The security info serves to
inform the home agent that the referred mobile host is authentic.
◊ When the mobile host gets authenticated by its home agent,
the forward agent inserts its information in its routing table.
◊ If the host leaves the current area, it gets deregistered.
Packet routing for mobile host is as shown by the following
diagram.
http://www.cisco.com/en/US/docs/ios/solutions_docs/mobile_ip/mobil_ip.html