October 31, 2016
Categorised in: Data Communicaiton & Wireless Sensor Networks
A cellular network or mobile network is a wireless network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station.
In a cellular network, each cell uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed bandwidth within each cell.
Cellular Network Devices
In Advanced Mobile Phone System (AMPS), a cell phone subscription is identified using three numbers:
- Electronic Serial Number (ESN): unique 32 bit number programmed into the cell when it was manufactured.
- Mobile Identification Number (MIN): 10 digit phone number.
- System Identification Code (SID): Unique 5 bit code assigned by the FCC to each service provider.
What happens when you receive a call?
When you first power the phone, it listens for an SID on
the control channel.
Recall: control channel is special frequency that the phone and base station use to talk to each other about things like call setup and channel changing, etc.
If phone cannot find any control channels, then it is out of range and it lights up the “No Service” light.
When it receives in SID, the phone compares it to the SID programmed in the phone.
- If it matches, then phone is in the home system.
- If it does not match, the phone is roaming.
Phone transmits a registration request to the base, which forward this request to the MSC.
MSC uses this registration request to update a large database (called the location registry) which keeps track of the latest location of the cell phone.
This helps network find a phone when a call comes in for it.
Also it informs the MSC if the cell phone user is valid (legitimate paying customer).
MSC also learns of phone subscription features, like caller-id, etc., from the MSC.
Assume a call comes in for the phone.
The MSC tries to find the phone by looking up the database.
MSC uses a frequency in the cell in which the phone was last in, and transmits an “incoming call” message over the control channel with the phone’s ESN and MIN numbers.
This message also tells the phone which frequency to switch to communicate with the base and complete the conversation.
Call Reception; Call Handoff
The phone and base station tower switch to these
frequencies and the call is connected.
Now assume the phone user is moving around and moves to the edge of its serving cell.
Base station notes that the strength of the radio waves from this phone is diminishing.
Meanwhile, a nearby base station notes that the signal strength to this phone is increasing.
All base stations constant monitor the signal strength on all voice channels (all 416) in order to pinpoint users who may be moving into their coverage area.
When the signal gets weak enough at the first base station and strong enough at the second base station, the base stations send a signal to the MSC.
The MSC determines the new frequency in the new cell that user should switch to.
The new frequency is conveyed to the phone.
The phone switches to the new frequency (faultlessly) and the new base station tunes into this frequency and starts receiving signals from the phone.
This way the phone gets handed-over to the new base station.
When SID of the phone does not match the SID of the
nearest base station, the phone knows its roaming.
The MSC of the system that the phone is roaming in contacts the MSC of the phone’s home system.
The home MSC verifies the phone (valid, paying user, etc.) to the local MSC. The local MSC then keeps track of the phone as it moves thru the local system. Each time updating the database at the home system.
Cell to Cell Call
Let’s say there is a phone in a cell that wishes to talk to another phone in that cell.
Assume that both the phones are in a cell of their home system (thereby, they both have the same home system).
These two phones must talk to each other via the base station.
Future cell phones systems (perhaps 4G) may allow phones to connect directly with each other (peer-to-peer connection)
Ad hoc Wireless Networks
The principle behind ad hoc networking is multi-hop relaying in which messages are sent from the source to the destination by relaying through the intermediate hops (nodes).
In multi-hop wireless networks, communication between two end nodes is carried out through a number of intermediate nodes whose function is to relay information from one point to another. A static string topology is an example of such network:
0<-> 1<-> 2<-> 3<-> 4<-> 5<-> 6<-> 7
In the last few years, efforts have been focused on multi-hop “ad hoc” networks, in which relaying nodes are in general mobile, and communication needs are primarily between nodes within the same network.
An examples of such developments is the Bluetooth standard that is one of the first commercial realizations of ad hoc wireless networking developed by Bluetooth Special Interest Group (SIG):
–A piconet formed by a group of nodes establishes a single-hop (master node) point-to-point wireless link.
–A scatternet formed by multiple piconets (master nodes) can establish a multi-hop wireless network.
Though the IEEE 802.11 protocols have developed for the wireless networks, they don’t function well in multi-hop networks.
Realizing the necessity of open standards in this emerging area of computer communication, the mobile ad hoc networks (MANET) standards are being developed by the Internet Working Tasking Force (IETF) MANET working group.
Even though ad hoc wireless networks are expected to work in the absence of any fixed infrastructure, recent advances in wireless network architectures enable the mobile ad hoc nodes to function in the presence of infrastructure
Multi-hop cellular networks (MCNs), self-organizing packet radio ad hoc networks with overlay (SOPRANO), and mesh networks are examples of such types of networks.
Mesh networks serve as access networks that employ multi-hop wireless forwarding by non-mobile nodes to relay traffic to and from the wired Internet. In such an environment, hybrid technologies and/or hierarchical network organization can be used for ad hoc and infrastructure wireless links.
Cellular and Ad Hoc Wireless Networks
The following figure represents different wireless networks.
- Infrastructure: cellular wireless networks
- Ad hoc: wireless sensor networks
- Hybrid: mesh networks
Comparisons between Cellular and Ad Hoc Wireless Networks
Applications of Ad hoc Wireless Networks
–Ad hoc wireless networks is useful in establishing communication in a battle field.
Collaborative and Distributed Computing
–A group of people in a conference can share data in ad hoc networks.
–Streaming of multimedia objects among the participating nodes.
–Ad hoc wireless networks are useful in emergency operations such as search and rescue, and crowd control.
A Wireless Mesh Network is a mesh network that is built upon wireless communications and allows for continuous connections and reconfiguration around blocked paths by “hopping” from node to node until a connection can be established.