Wireless Networks and 2G Sowjanya Talasila Kalpana Uppalapati Karthik Dhoopati Sudheer Adumulla

Agenda Introduction to Wireless Networks 2G Architecture SS7 and IS-95 protocols Connection Management Radio resource management MSC-MSC interfaces and protocols

Advantages of Wireless Reduced Cost Accuracy of data User mobility( Stay connected to the network) Flexible Return on investment and increased productivity

Limitations Energy constraints Variability in network performance

Wireless media Fading Interference

Types of wireless networks Centralized User terminals communicate with access points only Non-centralized Communication not limited between user terminals and access points

Comparision Services Coverage area Limitation Examples Cellular Handheld phones Continuous Limited bandwidth AMPS, IS-95 WLAN Extended LAN Local environments Limited range IEEE 802.11a,b,g GPS 3D position, velocity Anyplace on earth Cost GNSS, NAVSTAR

Wireless Network Mobile Switching Center (MSC) Mobile Data Set Base Station Controller (BSC) PSTN Mobile Voice Unit Packet Network Packet Inter-Working Function Base Transceiver System (BTS) Challenge is to keep connection and not loose any data during handoff operation

Components Base Transceiver System (BTS) Consists of one or more transceivers at a single location Base Station Controller (BSC) Provides allocation and management of radio resources Mobile Switching Center (MSC) Provides and controls mobile access to PSTN

Contd.. Public Switching Telephone Network (PSTN) Landline calls initiated from cell get connected via this network

Handoff The technique of handing over a call from one access point to another (as the user moves from one cell to another) without the call getting terminated is handoff

Contd..

Types of handoff Soft handoff Hard handoff

Contd..

Access Techniques Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Code Division Multiple Access (CDMA)

CDMA o r “Spread Spectrum” Contd.. FREQUENCY TIME User 3 User 2 User 1 TDMA FREQUENCY TIME FDMA 1 2 3 TIME FREQUENCY CODE CDMA o r “Spread Spectrum” User 3 User 2 User 1

Mobile Systems Single high powered transmitters High coverage Limited number of subscribers

Cellular Concept Number of low power transmitters Large number of subscribers Increased system capacity without adding more spectrum

Frequency reuse Frequency reuse refers to the use of same radio channels on the same carrier frequency to cover different areas which are seperated by a significant distance

Contd..

Different generations 1G – Analog, only voice communication 2G – Digital, only voice communication, SMS 2.5G – Digital, voice communication, simple web browsing 3G – Digital, data, voice communication , Multimedia

Interfaces and Protocols Bsc-msc interface and protocols. Msc-msc interface and protocols.

IS-95 Interfaces A Interface (BSC-MSC) .. This interface is between the BSC and the MSC. It supports both the control plane and user plane Abis Interface (BTS-BSC)—This is the interface between the BSC and BTS. This is internal interface and generally proprietary B Interface (MSC-VLR) This interface is defined by TIA IS-41

IS-95 interface (cont..) C Interface (MSC-HLR) This interface uses IS-41 messaging as well D Interface (HLR-VLR) – HLR-VLR signaling is based on IS-41 as well. It sits on top of SS7 E Interface (MSC-MSC)– Inter MSC signaling is defined in IS-41 L interface (MSC-IWF) This interface allows the ability for circuit switched data in second generation networks Um Interface (BS-MS) – This is the air interface between the mobile and the network

Multiple Access CDMA Benefits of CDMA: unique digital codes are used to differentiate subscribers codes are shared by both MS and BS all users share the same range of radio spectrum Benefits of CDMA: Capacity increases: 4 to 5 times (GSM) Improved call quality Simplified system planning Enhanced privacy Improved coverage characteristics Increased talk time for portables Bandwidth on demand

CDMA There are two CDMA common air interface standards: Cellular (824-894 MHz) - TIA/EIA/IS-95A PCS (1850-1990 MHz) - ANSI J-STD-008 They are very similar in their features, with exceptions of the frequency plan, mobile identities, and related message fields. IS-95A 45 MHz spacing for forward & reverse channel Permissible frequency assignments are on 30 kHz increments

Forward CDMA Channel of IS-95A From BSS to MS It carries traffic, a pilot signal, and overhead information. Pilot is a spread but unmodulated Pilot and overhead channels establish the system timing and station identity. Pilot channel is also used in the mobile-assisted handoff (MAHO) process as a signal strength reference.

Overhead Channels There are three types of overhead channel in the forward link: pilot, is required in every station sync paging Pilot channel pure short code with no additional cover or information content always code channel zero a demodulation reference for the mobile receivers and for handoff level measurements carries no information all stations use the same short code, distinguished by the phase

Overhead Channels Sync Channel Paging channel sync channel carries timing and system configuration information data rate is always 1200 bps Paging channel used to communicate with MSs when they are not assigned to a traffic channel successful accesses are normally followed by an assignment to a dedicated traffic channel paging channel may run at either 4800 or 9600 bps each BS must have at least one paging channel per sector, on at least one of the frequencies in use

Traffic Channel Traffic channels Timing assigned dynamically, in response to MS accesses, to specific MS always carries data in 20 ms frames carry variable rate traffic frames, either 1, 1/2, 1/4, or 1/8 of 9600 bps rate is independently variable in each 20 ms frame the 800 bps reverse link power control subchannel is carried on the traffic channel by puncturing 2 from every 24 symbols transmitted. Timing all base stations must be synchronized within a few microseconds

Overhead Messages System Parameter Message: configuration of the Paging Channel registration parameters parameters to aid pilot acquisition Access Parameter Message configuration of the Access Channel control parameters used to stabilize the Access Channel Neighbor List Message time offset of the pilot basic neighbor configuration CDMA Channel List Message CDMA frequency assignment that contain Paging Channels

Paging Channel Messages Page Message: contains pages to one or more mobile stations. Order Message: a broad class of messages used to control a particular MS. Channel Assignment Message: let BS to assign a MS to the traffic channel change Paging Channel Assignment

Access Channel Access Channel provides communications from MS to BS when MS is not using a Traffic Channel. All Access Channel use 4800 bps mode Access Channel Message: call origination response to pages orders registrations Control of Access Channel transmission is accomplished through the Access Parameter Message sent on the Paging Channel

Connection Management Mobile Origination Mobile Termination Call clearing procedure

Call origination

Call termination

Supplementary services Call waiting Three way calling Message notification on the paging system Message notification on the traffic channel

Call waiting

Three way calling

Call paging on paging/traffic channel

Radio Resource Management Purpose: Keeps a state of Radio Resources Allocates Physical Radio Resources Allocates Scrambling Codes (UL) Allocates Spreading Codes (DL) Knows Radio Network Configuration and State Data Informs PS for current resources state

Radio Resource Management Handles QoS Controls Cell Capacity and Interference in order to provide an optimal utilization of the wireless interface resources. Includes RRM Mechanisms: Power Control Handover Call Admission Control Load Control Packet Scheduling Resource Management

Handoff Based on Types Signal Quality (thresholds) Traffic Level (maximum cell capacity or maximum threshold) User Mobility Faster movements many handovers Types Softer (BSsection1  BS section2) Soft (BS1  BS2) Hard (lose connectivity, change frequency)

Contd..

2G Access Technologies Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

FDMA FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth Used mainly for analog transmission

TDMA A narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into time slots Voice data takes up significantly less transmission space TDMA has three times the capacity of an analog system Operates in either 800MHz(IS-54) or 1900MHz(IS-136) frequency bands

CDMA After digitizing data, spreads it out over the entire available bandwidth Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum At the receiver, that same unique code is used to recover the signal

Benefits of CDMA Capacity increases: 4 to 5 times Improved call quality Simplified system planning Enhanced privacy Improved coverage characteristics Increased talk time for portables

Signaling System #7(SS7) Signaling in Telecommunications Network Channel Associated Signaling (CAS) Common Channel Signaling (CCS) Signaling System Number (SS7) is a form of Common Channel Signaling. SS7 is a communications protocol that provides signaling and control for various network services and capabilities

Switch A B Voice Trunks Signaling Link Channel Associated Signaling: Uses In-Band Signaling Signaling is transmitted in the same frequency band as used by voice. Voice path is established when the call setup is complete, using the same path that the call setup signals used Common Channel Signaling: Uses Out-of-Band Signaling Employs dedicated path for signaling Voice trunks are used only when a connection is established ,not before Faster call setup Switch A B Voice Trunks Signaling Link

Functions of SS7 Roaming Subscriber Identification Call forwarding Basic call setup, management and tear down Wireless services Roaming Subscriber Identification Local Number Portability (LNP) Toll-free services (800) and premium (0907) Advanced call features Call forwarding Number display Conference/party calls

Properties of SS7 Interconnected network elements exchange messages using standard protocol Uses 56 or 64 kbps bi-directional channels called signalling links All signalling is out-of-band on dedicated channels rather than in-band Three network nodes called signalling points Service Switching Point Service Transfer Point Service Control Point Each point has a unique number code used in messages

SSP/STP/SCP Used to originate, terminate and tandem calls Can request routing information from the SCP STP Packet switching hub Removes need for all Signalling Points to be interconnected May act as a firewall between network operators SCP Routing databases SSP’s and SCP’s are also called end points

SS7 Protocol Stack

Message Transfer Part (MTP)

Message Transfer Part (MTP) Divided into three parts MTP Level 1 Provides an interface to the actual physical channel over which communication takes place CCITT recommends 64Kbps transmission whereas ANSI recommends 56 Kbps MTP Level 2 Ensures accurate end-to-end transmission Implements flow control, sequence validation and error checking MTP Level 3 Provides message routing Detects and reroutes on link failure or congestion

ISDN User Part (ISUP)

ISDN User Part (ISUP) Defines set-up, management and release of trunk circuits for voice/data traffic Calls that originate and terminate at the same SSP do not use ISUP Messages are sent from a switch, to the switch where the next circuit connection is required Call circuits are identified using circuit identification code (CIC)

Telephone User Part (TUP)

Telephone User Part (TUP) Handles analogue circuits only TUP is used to handle call set-up, management, release for the analogue network Used in countries with less mature networks such as China, South America As digital networks become the norm ISUP is replacing TUP

Signalling Connection Control Part (SCCP)

Signalling Connection Control Part (SCCP) Provides connection oriented / connectionless services to the network Provides Global Title Translation (GTT) A GTT may be 800 numbers, Mobile Subscriber ID’s, etc SCCP translates the GTT into the actual destination SSP point code and subsystem number (SSN) SSN are codes for applications rather than standard end-points SCCP is used as the transport layer by TCAP

Transaction Capabilities Applications Part (TCAP)

Transaction Capabilities Applications Part (TCAP) Handles exchange of non-circuit related data between applications on the SS7 network using SCCP messages For example when a mobile subscriber connects to the network, TCAP carries the Mobile Application Part (MAP) messages between the mobile switches and their supporting databases used for identifying and authentication the device and subscriber SCP’s use TCAP to query routing tables in the SCP’s

Transaction Capabilities Applications Part (TCAP) TCAP has enabled the development of Intelligent Network (IN) services by supporting connectionless non-circuit related data exchange between signalling points TCAP messages consist of Transaction Portion Basic packet identifier data (type, ID’s, etc) Component Portion May be thought of as “function calls” such as invoking, returning, rejecting, etc

References Few websites found through Google:- www.intellinet-tech.com/Solutions/wirelessnetworks.htm www.informit.com/articles/article.asp?p=26330 Few Slides from Dr.Dantu’s 5520 course

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