Wireless in Local Loop (WLL):
What is WLL system?
o WLL is a system that connects subscribers to a public switched telephone network (PSTN) using radio signals as a substitute for copper transmission media for all or part of the connection between the subscriber and the switch.
o WLL is also known as Fixed Wireless Access (FWA), Radio in the loop (RITL), Fixed-Radio Access (FRA).
There are two alternatives to WLL:
> Narrowband WLL - offers replacement for existing telephone system
> Broadband WLL - provide high speed voice and data service
WLL configuration:
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| Figure 1: WLL configuration |
Architecture of WLL:
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| Figure 2: WLL architecture |
• The given architecture consists of three major components i.e WANU, WASU and SF
• Wireless Access network unit (WANU):
The WANU consists of various components which include
- Several base stations transceivers or radio ports (RP)
- a Radio port control unit
- an Access manager (AM)
- an HLR.
• It provides various functionalities like:
- Authentication
- Air interface privacy
- Over-the-air registration of subscriber units.
- Operations and Maintenance
- Routing
- Billing
- Switching functions
- Transcoding of voice and data.
Wireless access subscriber unit (WASU):
It provides an air interface UWLL towards the network and a traditional interface TWLL towards the subscriber.
• The power supply for it is provided locally.
• The interface includes
- protocol conversion and transcoding
- authentication functions
- signaling functions
• Switching Function (SF):
The switching function (SF) is associated with a switch that can be digital switch with or without Advanced Intelligent Network (AIN) capability, an ISDN switch or a Mobile Switching Centre (MSC).
The switching function (SF) is associated with a switch that can be digital switch with or without Advanced Intelligent Network (AIN) capability, an ISDN switch or a Mobile Switching Centre (MSC).
• The TWLL interface can be an RJ-11 or RJ-45 port.
• The UWLL interface can be AMPS, GSM, DECT and so one.
• The AWLL interface between the WANU and the SF can be ISDN-BRI or IS-634 or IS-653 or such variants.
Various issues are considered in WLL development which include:
- Spectrum: The implementation of WLL should be flexible to accommodate different flexible bands as well as non-continuous bands. More these bands are licensed by the government.
- Service quality: Customer expects that the quality of service should be better than the wireline counterpart. The quality requirements include link quality, reliability and fraud immunity.
- Network Planning: Unlike Mobile System, WLL assumes that user is stationary, not moving. Also, the network penetration should be greater than 90%. Therefore WLL should be installed based on parameters like Population Density etc.
- Economics: Major cost here is electronic equipment’s. In the current scenario, the cost of such electronic equipment is reducing periodically.
Advantages of WLL over the wired system to subscriber local loop support:
1. Time of installation: Much less than wired system.
2. Cost: the Overall total cost of wireless system components, installation, and maintenance is less than a wired system.
3. The scale of installation: Radio transceivers are installed only for those subscribers who need the service at the given time.
Wireless Local Loop Techniques are as follows:
o Multichannel Multipoint Distribution Service (MMDS)
o Local Multipoint Distribution Service (LMDS)
A) Multichannel Multipoint Distribution Service (MMDS)
o Used to support two-way services as well as an alternative for broadband services.
o In United States, the FCC(Federal communication commision) has allocated five frequency bands in the range of 2.15 - 2.68 GHz
o Used in Tv and Data signals
Advantages:
o Cost of subscriber and base stations is lowered because the equipment operating at low frequency is less expensive.
o MMDS signals can travel further without power losses.
o Less susceptible to rain absorption. MDDS signals do not get blocked by objects, which allow them to be sent fot longer distance.
Disadvantage:
o Low bandwidth as compared to LMDS
B) Local Multipoint Distribution Service (LMDS)
o Broadband wireless point to multipoint communication technology used to provide digital two-way voice, data, Internet and video services.
o Operates on microwave frequency across 26Ghz to 29Ghz bands.
o LMDS uses cellular like architecture.
o The propogation characteristics of signal limit the potential coverage area to a single cell site.
Advantages of LMDS:
o Easy and fast to deploy
o Realization of revenue is fast.
o Relatively less expensive if compared with cable alternatives.
o Easy and cost-effective network mainteinance, management, and operation
o Data rate is relatively high, in Mbps range.
o Scalable architecture with customer demands
Drawback of LMDS:
o Short range from the base station, as a result increase in number of base stations in order to service a specific area.
Wireless Personal Area Networks
Bluetooth
Bluetooth wireless technology is a short-range radio technology, which is developed for Personal Area Network (PAN). Bluetooth is a standard developed by a group of electronics manufacturers that allows any sort of electronic equipment -- from computers and cell phones to keyboards and headphones -- to make its own connections, without wires, cables or any direct action from a user.
Topology:
Before learning about the topology let us understand some basic terms:
Master & Slave: When two bluetooth devices want to connect, the one requesting the connection is known as master and the other is known as slave. The master always controls the link created between the two devices. A master can setup 7-8 active connections.
PARK Mode: When a master instructs a slave to 'sleep' for the specified amount of time by putting them in low-power mode, it is known as PARK Mode.
Now let us understand the different topology present in Bluetooth.
There are two types of topology
o Piconet
o Scatternet
A) Piconet:
o The Piconet is a small ad hoc network of devices (normally 8 stations) as shown in Figure 3.
o Devices in a Piconet hop together according to master's clock value and its 48 bit device ID
o Slave to Slave transmission is not possible. If they want to communicate then either they can form a separate piconet where one of them is master or use a higher layer protocol.
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| Figure 3: Piconet |
It has the following features:
o One is called Master and the others are called Slaves
o All slave stations synchronizes their clocks with the master
o Possible communication - One-to-one or one-to-many
o There may be one station in parked state
o Each piconet has a unique hopping pattern/ID
o Each master can connect to 7 simultaneous or 200+ inactive (parked) slaves per piconet
B) Scatternet:
o A collection of overlapping piconets is called scatternet.
o Devices participating in two or more piconets are known as bridge devices and participate in each piconet in time-sharing manner.
o A bridge node that participates in several piconets can be either:
> Slave in all piconets. In this case, when leaving the old piconet, the slave has to inform the master for the duration of its absence.
> Master in one piconet and slave in all others. In this case, all traffic in the old piconet is suspended until the master returns to the piconet. The suspension of traffic is achieved by putting the piconets slaves into low power HOLD mode.
> Slave in all piconets. In this case, when leaving the old piconet, the slave has to inform the master for the duration of its absence.
> Master in one piconet and slave in all others. In this case, all traffic in the old piconet is suspended until the master returns to the piconet. The suspension of traffic is achieved by putting the piconets slaves into low power HOLD mode.
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| Figure 4: Scatternet |
By making one slave as master of another Piconet, Scatternet is formed by combining several Piconets as shown in Figure 4.
Key features of the scatternet topology are mentioned below:
• Linking of multiple co-located piconets through the sharing of common master or slave devices.
• A device can a master as well as a slave.
• High capacity system
Bluetooth Protocol Stack
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| Figure 5: Bluetooth protocol stack |
A) Radio Layer:
• The Radio layer provides electrical specification in order to send and receive bitstreams over the channel.
• It makes use of 2.4 GHz license free band.
• Time division duplex & Frequency hopping is used for transmission with hopping rate of 1600 hops/s.
• 79 hop carriers equally spaced with 1 MHz.
• GFSK is used for modulation.
B) Baseband layer:
• The Baseband layer enables operations of Bluetooth link medium.
• It controls the following:
o Device Addressing
o Channel control by using paging and inquiry methods
o Power saving
o Flow control & synchronization among devices.
C) Link Manager Protocol (LMP):
• The Link management layer runs the Link Manager Protocol (LMP).
• The functions are covered by LMP:
o Authentication, encryption and pairing
o Capability negotiation
o Synchronization
o Power control
o QoS negotiation
o State and transmission mode change
o Link Supervision
D) Logical Link Control and Adapation Protocol (L2CAP):
• Logical Link Control and Adaptation Protocol (L2CAP) supports higher level protocol multiplexing, packet segmentation and reassembly, and the conveying of quality of service information.
• L2CAP provides:
o Connection-oriented and connectionless data services to upper layer protocols with protocol multiplexing capability.
o Group abstractions.
• L2CAP provides three different types of logical channels, these are:
o Connectionless - broadcast
o Connection-oriented - data transfer with QoS flow specification.
o Signaling - exchange signaling messages between L2CAP entities.
E) Host Controller Interface (HCI):
• The Host Controller Interface (HCI) provides a command interface to the Baseband Link Controller and Link Manager.
• Access to hardware status and control registers.
• A uniform method of accessing the Bluetooth baseband capabilities.
• The HCI exists across 3 sections, The Host, Transport Layer, and Host Controller. Each section has a different role in HCI system.
• HCI defines the set of functions of a Bluetooth module that are accessible to the host and its application.
• HCI can be seen as a boundary.
F) RFCOMM:
• The RFCOMM protocol runs a serial line RS-232 control and data signal emulation protocol
• Provides a serial line interface to all the applications, hence also called as cable replacement protocol.
• Based on the ETSI std TS 07.10.
• Supports multiple serial ports over a single physical channel.
G) Service Discovery Protocol (SDP):
• The Service Discovery Protocol (SDP) provides a means for applications to discover, which services are provided by or available through a Bluetooth device.
• New service is discovered as follows:
o Client sends request to search for an interested service.
o Server responds to the client with list of available services that match to the client’s criteria.
o Client uses list to retrieve additional attribute for the service of interest.
H) Telephony Control Protocol Specification (TCS):
• Define call control signaling for establishment of voice, data calls between devices
Bluetooth Security
A) Bluetooth security features
o Authentication and Encryption feature is supported by Bluetooth.
o Authentication: It verifies the user on the other side of the link.
o Encryption: It ensures confidentiality of data.
i. Pairing:
When two devices communicate for the first time, a secret key is generated which is shared and stored in both the devices. In future, there is no pairing procedure of the same device.
ii. Security modes of a device:
There are three security modes to a device.
- Non-secure: Security procedure is not initiated by the device.
- Service level enforced security: Before channel establishment at the L2CAP level, the device does not initiate security procedures.
- Link level enforced security: Security procedure is initiated before link set up at LMP is completed.
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| Figure 5: Security in Bluetooth |
2. Bluetooth security levels
Authentication: After determining the type of service, authentication is performed. It cannot be performed when ACL link is established, performed only when connection request to service is submitted. Bidirectional: client authenticates server and vice versa.
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| Figure 6: Authentication Procedure in Bluetooth |
Authentication procedure: -
1. The connection request is sent to L2CAP.
2. L2CAP requests access from the security manager.
3. The security manager enquires the service database.
4. The security manager enquires the device database.
5. If necessary, the security manager enforces the authentication and encryption procedure.
6. The security manager grants access, and L2CAP continues to set up the connection.
1. The connection request is sent to L2CAP.
2. L2CAP requests access from the security manager.
3. The security manager enquires the service database.
4. The security manager enquires the device database.
5. If necessary, the security manager enforces the authentication and encryption procedure.
6. The security manager grants access, and L2CAP continues to set up the connection.
Authorization:
There are two kinds of device trust levels:
- Trusted device: It has fixed relationship (paired) and unrestricted access to all services.
- Untrusted device: This device has been previously authenticated, a link key is stored, but the device is not marked as trusted in device database.
- An unknown device is also an untrusted device. No security information is available for this device.
Services that require authorization and authentication — Trusted devices are granted automatic access. Rest devices need a manual authorization.
Services that require authentication only — As stated, authorization is not necessary
A default security level is defined to serve the needs of legacy applications and will be used unless other settings related to a service are found in a security database.
Services that require authentication only — As stated, authorization is not necessary
A default security level is defined to serve the needs of legacy applications and will be used unless other settings related to a service are found in a security database.
IEEE 802.11 standards:
o Specifies physical layer and MAC layer standards, operating at 2.4GHz ISM band with 1 or 2 Mbps data rate.
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| Figure 7: IEEE Comparison chart |
WiMax vs LTE:
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| Figure 8: LTE vs WiMax |
5 Broadcast Communication Networks Version 2 CSE IIT, Kharagpur
4 comments:
good explanation. keep it up from Quora Gaffer
Thanks, Ajay!
Good,keep it up
Thanks Ameer!
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