VoIP / Multimedia over WiMAX (802.16)

Maneesh Bakshi


The paper discusses the implementation of Voice over IP (VoIP) and IP Multimedia Subsystem services (IMS) over the much sought after wireless standard WiMAX (802.16). The key issues addressed are : Introduction to the working of VoIP and multimedia transmission over wireless. The softswitching technique for compatibility with WiMAX. The challenges being faced that need to be overcome for successful deployment and their solutions. The pre-standard installations and developments happening around the world at this time. The various products offered by the vendors compliant to the technology, and the impact of these developments on the existing Telecom and Satellite/CableTV industry.

Keywords - WiMAX, VoIP, Multimedia, IMS, 802.16, SIP, H.323, IPTV, Softswitch

See also: TCP Optimizations over Wireless | QoS for WiMAX |
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Table of Contents

  1. Introduction to VoIP
  2. Introduction to IMS
  3. WiMAX - Quality of Service (QoS)

  4. Impact on the Telecom and Satellite/Cable TV Industry

  5. Recently Launched Products

  6. Pre WiMAX Happenings and Industry Forecasts

  7. Summary

  8. References

  9. List of Acronyms

1.0 Introduction to VoIP

Ever since its advent VoIP has opened new doors for telephony bringing forward immense possibilities. The basic reason for the popularity of VoIP is the cost which is very low as compared to the conventional telephony services. The concept of transmission of voice over data stream makes it possible to have VoIP transmitted and received using anything that uses IP - laptops, PC's, WiFi enabled handsets etc..

1.1 How does VoIP work?

VoIP uses Internet Protocol for transmission of voice as packets over IP networks. The process involves digitization of voice, the isolation of unwanted noise signals and then the compression of the voice signal using compression algorithms/codecs.After the compression the voice is packetized to send over an IP network, each packet needs a destination address and sequence number and data for error checking. The signaling protocols are added at this stage to achieve these requirements along with the other call management requirements. When a voice packet arrives at the destination, the sequence number enables the packets to be place in order and then the decompression algorithms are applied to recover the data from the packets. Here the synchronization and delay management needs to be taken care of to make sure that there is proper spacing. Jitter buffer is used to store the packets arriving out of order through different routes, to wait for the packets arriving late.

1.2 Signaling Protocols

Signaling in WiMAX

The signaling protocols H.323, SIP are used to setup the route for the transmission over the IP network, the Gateway protocols like the Media Gateway Control Protocol are used to establish control and status in the media and signaling gateways. Routing (UDP, TCP) and transport protocols (RTP) are used once the route is established for the transport of the data stream as shown in Figure 1

1.2.1 H.323
H.323 is the ITU-T standard for packet based multimedia communication, though originally developed for multimedia conferencing over LAN's it was later modified for VoIP as well. With versions coming out in 1996 and 1998, the standard has faced stiff competition from the other protocol SIP which was specifically designed for VoIP, but is more used because of its wide existence in the already installed networks. The standard is interoperable and has both point to point and multipoint capabilities. H.323 uses a number of other sub protocols for the various functions.

Also H.323 offers specifications for call control, channel setup, codecs for the transmission of Real time video and voice over the networks where the QoS and guaranteed services are not available. For the transport RTP is used for real time audio and video streaming.

1.2.2 Session Initiation Protocol (SIP)
SIP is the IETF standard for VoIP signaling. It is based on the existing protocols like SMTP and HTTP, and uses a text based syntax that is comparable to HTTP uses in web addresses. A web address is comparable to a telephone number in a SIP network, also the PSTN phone numbers are also compatible in a SIP network ensuring interfacing with PSTN systems. SIP also provides a mobility function to the users. SIP also supports multiple media sessions during a single call hence users can - share a game, use instant message (IM), and talk at the same time. SIP works with most protocols like RTP, Session Description Protocol (SDP), Session Announcement Protocol (SAP). A lot of other protocols are also needed when it comes to the transport and signaling with the PSTN networks - RSVP, LDAP, RADIUS. SIP works on a client server architecture, where the clients are referred to as User Agents (UA). UA interact with the server mainly through a PC with a telephony agent or IP phone. The servers are of four types :

The SIP messages used for communication between the client-server are INVITE, ACK, OPTIONS, REGISTER, CANCEL, BYE

1.3 Softswitch

The new innovation in switching - Softswitch is far less expensive both in terms of purchase and maintenance as compared to the conventional switches used in PSTN networks. Interoperability is primarily the biggest cutting edge advantage that the service provider gets out of using a Softswitch. Softswitch is an enhancement over the existing gatekeeper technology which supported H.323, since H.323 was only restricted to LAN's the activities and capabilities of the gatekeeper were restricted to a few gateways which were managed by a single gatekeeper. As the networks became larger, there was a need for more efficient and smart solutions for managing all these services, which were answered by the softswitch. The softswitch coordinates the call control, signaling, and the other features that enables making a call across networks possible.It provides all the essential call control and service logic functions, coordinates routing of signaling messages between networks, it also provides all the administrative functions like billing statistics and providing other value added services to the users. The various components of the Softswitch are described and shown in Figure 2: Components of Softswitch

1.4 Challenges Faced

1.4.1 Voice Quality
Presently, the telephone companies are losing so much of their business to the VoIP counterparts, but still the speculation exists on the kind of voice quality that VoIP service providers would give to customers. The techniques used to measure the voice quality of a VoIP call are the Mean Opinion Score (MOS) and Perceptual Speech Quality Measurement (PSQM)

The other factors affecting the voice quality are :

1.4.2 Security

Securing the voice communication is also a big challenge for VoIP over WiMAX as care has to be taken that it cannot be eavesdropped or intercepted. The Double encryption process - X.509 for Authentication and 152-bit AES, 3DES or 56-bit DES for data flow ensure the transmission is secure and eavesdropping is very difficult on the traffic.

x.509 56 bit DES

When a Subscriber Station (SS) needs to associate with a Base Station (BS), it sends an authorization request along with authentication information in a X.509 certificate (Figure 3). The BS after verifying the certificate responds by sending the authorization message which has the authorization key encrypted with subscriber's public key, to enable the subscriber to register with the network. An IP address is given to the SS by the DHCP. The DHCP server also provides the address of the TFTP server, from where the SS gets the vendor specific configuration information file. After this, the BS accepts the subscriber. The data stream is further encrypted using 56-bit DES, 3DES or 152-bit AES (Figure 4). This prevents the possibility of eavesdropping on the data and theft of service as the links between the BS and SS are encrypted. Also WiMAX has built in virtual LAN (VLAN) support which provides protection for data transmitted by multiple users on the same BS.

1.4.3 E911/CALEA Support

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2.0 IP Media Subsystem Services - IMS

Today WiMAX is the only standard capable of providing the Quadruple play Technologies - Data, Voice, Video, Mobility (802.16e) using a single network. Collectively these services are referred to as IP Media Subsystem Services - IMS (Figure 5)

Quadruple Play - WiMAX


Uses the simple principle of converting a TV program to IP. This packetized program is streamed to the viewer, who can easily view it using a normal TV along with a Set Top Box (STB) as shown in Figure 6.


2.2 Wimax IPTV - How Does it work?

IPTV service providers have secured agreements with national broadcasters and programmers to offer all the channels currently being offered by the Cable/Satellite TV companies. The process involves receiving the programs directly from the broadcasters after encoding it into MPEG-2 format at a constant bit rate. The MPEG-2 stream is encapsulated into UDP/IP and is then sent as individual multicast streams to satellite uplink. The IP QoS is also applied by the IP streaming platform. These IP multicast streams are then uplinked to the IPTV service provider's satellite in a Digital Video Broadcasting (DVB) Format. The service provider has all the required setup for the converting the DVB format back into the IP, who delivers the video streams an IP or encapsulated as ATM providing superior viewing quality to the customer. There is also an option of providing on site encoding services for streaming of off-air programming.


2.3 Services Offered

3.0 WiMAX QoS

WiMAX provides Quality of Service at both physical and MAC layer using the following techniques:

4.0 Impact on the Telecom and Satellite/Cable TV Industry

The Quadruple Play as we call it, will definitely have a catastrophic effect on the existing Telecom and Satellite/Cable TV Industry. The reasons being

The one solution to all the Data, Voice, Video, Mobility services is definitely a big plus point for the consumer. WiMAX leaves behind all the competing technologies such as Fiber to the Home (FTTH), Broadband over Power Lines (BPL), xDSL, High-Speed Downlink Packet Access or 3.5G (EvDoHSPDA), Enhanced Data rates for GSM Evolution (EDGE) by miles, as none of these technologies provide all the services the way WiMAX can. Therefore WiMAX emerges as the best available solution for these services

5.0 Products Available

All the pro dcts can be found at http://www.wimaxforum.org/kshowcase/view

WiMAX Products

Base Stations

Subscriber station
Mobile Phones Products are also being offered by Adaptix, Axxcelera Broadband Wireless, Proxim Wireless, Orthogon Systems and Alvarion.

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6.0 Pre WiMAX Happenings and Industry Forecasts

7.0 Summary

In this paper, we discussed the transmission of VoIP and IMS over WiMAX, the changes required in the existing protocols. The working of a Softswitch which makes VoIP over WiMAX possible. The possible challenges encountered and their solutions. Working of Wimax IPTV. The effects of the advent of quadruple play offered by WiMAX on the existing telecom and Cable TV industry. Lastly we looked into the products being launched compliant to the standard and some pre standard implementations happening around the world.

8.0 References

9.0 List of Acronyms

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Last Modified on : April 24, 2006
Note: This paper is available online at http://cse.wustl.edu/~jain/cse574-06/wimax_voip.htm