A new QoS Specification Is Needed For Reliable Wireless Multimedia Services

By Vijay K. Madisetti and Antonios D. Argyriou
iApplianceWeb
(12/06/02, 09:28:57 PM EDT)

Dr. Vijay K. Madisetti, Professor, Georgia Institute of Technology,
and CEO, Soft.Networks, LLC,
and Antonios D. Argyriou,
Yamacraw Graduate Student Researcher,
Yamacraw Project, Georgia Institute of Technology, Atlanta, Ga.

Future wireless all-IP networks, while offering the promise of exciting broadband applications,  are expected to consist of several, potentially incompatible, wireless access technologies that would be offered by a number of competing service providers.  Once past the access stage, the Internet is still expected to be the main traffic backbone. The diversity of access technologies, however, may affect the Quality of Service (QoS) . These QoS issues become even more important when the user may possibly want to switch from one access technology to the other that may either is priced differently or provides better service or it is available in a place where another one is not.

Additional issues arise when considering the widely differing types of services that the user may use. Streaming media, real-time communications, interactive communications, VoIP, and multimedia-commerce are representative of some of them. Each of these services imposes its distinct QoS requirements. These QoS demands from multimedia traffic are compounded in the case of a wireless network, where new problems arise due to the implied mobility of the users as well as due to the nature of the current IP protocols that support IP-based mobility, combined with a lossy and interrupt/outage-prone nature of the communications channel.

QoS requirements can be achieved by various techniques that span multiple layers of the network OSI hierarchy. However, we believe that transport layer is an oft overlooked  area with respect to the promise that it offers for improvement of mobility management, QoS, security, and throughput. Additionally a transport layer-based approach has to be able to interoperate with future QoS-aware backbone networks which will be primarily based on MPLS and DiffServ technologies.
Multimedia transmission over wireless channels

In particular, multimedia transmission over wireless links presents a great challenge for the internet engineering community today especially in the way reliable content can be delivered using the various methods of delivery: streaming, interactive (real-time), or simply non-real-time.

With streaming multimedia, constant transfer rates are of primary importance. This includes applications that are related to audio and video streams. So, brief disruptions in the transfer rate become noticeable by the user, resulting in jittering pictures or stuttering during sound playback. However, some initial delay is acceptable.

Another category is the interactive multimedia.  In this case the concern is with real-time data transfers and temporal fluctuations. So, while one may tolerate minor errors during content transfer, long delays and jitter are usually unacceptable from QoS considerations. Voice over IP and video telephony are representative of the applications that belong to this category. New exciting applications such as M-commerce would also fit this category. Non-real-time multimedia traffic such as Video on Demand (VoD), are concerned primarily about throughput and not with delay or jitter.

Wireless traffic problems

Each type of multimedia service has different QoS requirements and attempts to solve a differing set of problems.  These problems, specially applied to wireless networks, fall into seven broad categories.

Currently at the top of the list is the limited bandwidth of wireless networks. Not only will it always be limited when compared to wired connections, the wireless medium is subject to problems such as multi-path fading and noise susceptibility. This results in a second chronic problem, high packet loss rate and bit error rate.

Another major problem is the need for uninterrupted communication in the presence of roaming users implies handoff procedures between various wireless access points. However, this procedure comes with the price;  lost packets and interruption of an ongoing season.
 
The heterogenous nature of the 3G infrastructure (GPRS, UMTS, WLAN, Bluetooth) introduces a fourth problem in that each technology cannot be deployed everywhere and so wireless content delivery systems may have to be transparent to  various underlying  technologies. In addition there is the issue of inadequate performance of Transport Layer protocols (e.g., TCP/UDP). The old transport layer in IP networks based on TCP and UDP, that was designed for data networks, is not expected to perform satisfactorily in the wireless content delivery network scenario.

Due to the aforementioned problems, providing Quality of Service guarantees is not expected to be easy, and new and adaptable QoS service models are expected to be needed. Finally there is the issue of wireless Security.  Advanced authentication and encryption techniques that combine performance with low power requirements are needed to ensure widespread acceptance of wireless multimedia services.

Mobile/Wireless QoS Solutions

A number of approaches have been proposed to improve QoS at the network, transport and application layers, primarily related to QoS in the access networks. Of the many proposals most fall into several broad categories, dealing with the issue of QoS at the application, transport, network and data/physical link layers.

At the application layer, there are currently several approaches at the application layer that  provide  mobility management for media data over wireless links, most of which are based on either SIP (Session Initiation Protocol) and/or  Mobile IP standards.

While SIP is not a QoS management protocol, there are ongoing efforts that are positioning SIP to be used as a protocol that is responsible for mobility management at the application layer, with an additional objective to improve QoS for real-time and non-real-time multimedia applications. SIP relies on the usage of SIP's registration mechanism for providing terminal and personal mobility.

Every user has a URI, which could be an e-mail address. When the user moves to a new place, it registers its new position with the SIP registrar server, so that the SIP registrar server knows the user's current position. This kind of registration can provide user-level mobility. In order to achieve terminal mobility a SIP mobility implementation could poll the operating system (OS) in order to find out if handoff took place so that the SIP will register with the new SIP registrar after handoff.

SIP based mobility, thus, offers attractive benefits when used in mobile multimedia applications. However, there are some inherent problems with this approach that make the adoption of this scheme difficult. For example it cannot handle mid-call subnet changes, since it is an application layer solution. This is where it requires the support of a lower level mobility protocol, e.g., Mobile IP. One other important issue is that of inter-operability with Mobile IP. Home Agent and Foreign Agent registrations in mobile IP serve the same purpose with the SIP REGISTER messages and their joint deployment becomes problematic.

Mobile IP and Network Layer QoS

A number of network layer approaches exist that intend to improve mobility management at the IP layer, and thus the QoS indirectly. The Network Layer is the primary OSI layer for enforcing QoS policies in computer networks. This is because the core network components (routers), that play the major role in network performance, operate at the network layer (IP).

Mobile IP is the most well-known mobility management solution. However, handoff delay, and overheads of Mobile IP's triangular routing, triangular registration, and IP encapsulation are major issues that present a bottleneck for mobile IP to become a wide spread acceptable solution for real-time interactive multimedia communications over the wired or wireless IP network.

Various mobile IP modifications  such as HAWAII, Cellular IP, and various Domain based approaches are focusing on improving mobility management especially in the case of frequent handoffs. However they do not explicitly address QoS-related problems.

An interesting approach attempts to improve the QoS by providing a more scalable reservation system for wireless applications through localized RSVP messages. In this approach RSVP is used in such a way that when a mobile node moves to a new point of attachment new PATH and RESV messages are send only locally, between the mobile node and an anchor point, creating thus a hierarchy in the QoS reservation system. This approach has the potential of improving significantly resource reservation time.

QoS in the Transport Layer

Transport layer is a neglected area concerning QoS related research. However, there are a few notable exceptions, but, as in the application and network layers solutions are not without their problems. But there are answers to these drawbacks in the form of enhancements to the Stream Control Transport layer protocol.

For example, a new experimental transport protocol, the Application-Oriented Transport Protocol (AOTP)that lies above IP and provides transport services with new functionality added specifically, to trade off reliability, throughput and/or jitter, in order to support applications of the upper layer with the required QoS. AOTP is claimed to provide adjustable partially reliable service, priority-based error recovery strategies and dynamic playback management. This approach however represents a "multimedia-aware" protocol that is targeting wire-line networks without any kind of provisioning for wireless systems.

And while older transport layer protocols, such as TCP and UDP, do not appear to be able to fully meet the stringent QoS requirements for interactive multimedia services, especially in the wireless, mobile environment, there is a new IETF transport level protocols, called Stream Control Transport Protocol (SCTP), that was designed to transfer reliably SS7 signaling messages over IP-based networks. However it soon proved to be not only an "application specific" protocol but it could also overthrow TCP.

SCTP introduces the idea of multi-homing, where a host has multiple interfaces and IP addresses by which it is reachable. An association between two endpoints can exist between any of these addresses. If one of the paths that correspond to one address fails then an alternative can be used without interrupting the connection between the endpoints. The two endpoints can monitor the status of the paths by sending a special kind of SCTP message called the Heartbeat. Primary goal of the above protocol property was error resilience.

Additionally SCTP provides the ability to maintain multiple streams of messages inside a single association. This makes possible to maintain a sequence of messages only per stream basis (partial in-sequence delivery) thus reducing unnecessary head-of-line blocking between streams of messages that are independent.

Another important feature is the distinction between the delivery mechanism and reliable datagram transfer. This provides a more flexible usage of the protocol so that is adapted to the specific needs of the application using it. It is, for example, possible for a scenario where one application requires partial ordering of the delivered datagrams, while another could be satisfied with reliable transfer that does not imply any kind of sequencing.

An SCTP implementation, fully compatible with the RFC, appears to be able to perform better than TCP even in the case of a wireless system. Even though SCTP does not incorporate any novel features particularly suited for wireless systems, it is a powerful new transport technology that has a number of advantages over TCP in this regard.

Multi-homing can greatly improve performance: even a standard SCTP stack operating in a mobile computer with more than one network interface card can significantly increase data transfer reliability. Moreover SCTP supports IPv6 and it can it can operate at the same time by using IPv4-IPv6 addresses. This feature is of importance since IPv6 is expected to soon replace the older IP version.

We have proposed to the IETF an enhancement to the SCTP base protocol, called Voice over Mobile IP (VoMo), that makes it practical in the wireless environment. VoMo is actually a collection of technologies which include: an intelligent address distribution (IAD) mechanism; a neighborhood based mobility (NBM) mechanism and a QoS-aware Transport Layer that has a number of capabilities as relates to multimedia over mobile wireless connections including  path quality monitoring, network status estimation, efficient stream & physical path management, rate control and content adaptation mechanisms, and load-Balancing and bandwidth aggregation mechanisms. More details on operation of the QoS features may be found at Yamacraw Project Website .


Six Steps In VoMo Multimedia QoS Management

The VoMo platform consists of two distinct phases that are related to multimedia mobility management. One is the initialization process phase which is performed at each subnet even when no mobile nodes are inside a cell. The other is the connection process phase  which handles all the necessary steps that a mobile node (MN) has to go through in order to connect to the wireless network.

In the first phase, the initialization of the network, all contiguous cells to the currently active cell, will be included in a “neighborhood_list,” ensuring that any handoff will be smoother based on the movement of the mobile node (MN).

The second phase, the connection process, involves six distinct steps. First, the Mobile Node enters a subnet and sends a registration request. Second, a central server (e.g., DHCP) provides a list_of_addresses based on the neighborhood_list. Third, the MN uses as its primary Care-of-Address one of the list_of_addresses that corresponds to its current point of attachment.

When the MN moves to another point of attachment (or a Base Station, BS) two alternative scenarios may arise: When the MN movement is inside the subnet, it MUST switch to another IP listed in its list_of_addresses according to movement detection information, or, when MN is moving to a new subnet, it sends a registration request to the other subnet requesting new list_of_addressees

Depending on the decision made in step four, the MN decides when it has changed its BS via information on movement detection obtained from the L2 or L3 protocol layers. Upon completion of its decision, the MN,  having moved to a new subnet discards the old list_of_addresses and starts using a new_primary_address from a newly allocated list_of_addresses.

QoS Features Supporting Multimedia Services

In the VoMo platform, multimedia data sets are distinguished at the transport layer according to the classes these data sets are assigned to by the application layer, creating an opportunity to provide Quality of Service (QoS). The classes of data sets are assumed to match those proposed by the Universal Mobile Telephone System (UMTS): e.g., conversational class, streaming class, interactive class, and background class. Adopting the application classes as the above is not restrictive for any kind of application layer protocol and may offer the right abstraction for manipulating data at the modified SCTP layer.

The QoS features intelligently route data to outgoing appropriate interfaces based on network status information, available network resources, application layer requirements, and application bit-rate (BER) demands.

Due to wireless and mobility issues, network bandwidth and noise characteristics of paths between various communications endpoints are expected to vary over time.  VoMo offers the ability to adapt the quality of the content being transmitted based on the channel or communications capabilities available to the service.

The VoMo platform implements load balancing at the transport layer. While the foundations rely on the multi-homing technology of SCTP, several new features are provided. The user can use a new Stream Ordering (SO) service where he can specify an ordering in the stream data delivery. This can be useful in the case where the user wants to transfer a single file and wants to split it into streams for transmission across various available interfaces. VoMo(tm) handles this case by using the SO service to split data into streams, and to tailor the transmission by sending the streams out according to the available bandwidth at each interface.

The VoMo load-balancing mechanism operates in close cooperation with the QoS features. The QoS module makes decisions that satisfy the application requirements and the Load-Balancing mechanism is invoked when the use of more than one interface is needed.

Benefits of VoMo

A multimedia wireless infrastructure based on this framework will have a number of important benefits, including reduced handoff delay; simplification of the handoff infrastructure; more efficient QoS management and load balancing at the transport layer; the ability to implement content class-driven path selection; and improved error resilience.

Performance degradation due to handoff effects will be reduced and lost packets due to cell handoff are expected to be eliminated. By establishing a multi-homed connection near the cell boundaries, the mobile device, when experiencing handoff, will gradually switch to another interface which corresponds to the new cell. For instance, let us suppose that packets are lost during handoff while they are being transmitted from the old cell. In this case, when the mobile device enters the new cell and realizes it did not receive an acknowledgement, it will retransmit the missing data.

For the sake of completeness we present simulation results that were performed by using the NS-2 network simulator. Part of VoMo functionality has been implemented as a pluggable module to an NS-2 network simulator and compared to the base SCTP configuration. In this specific setup ,  two hosts communicate through 2 links of 500Kbps each and delay of 100ms, using reliable transfer for both SCTP and VoMo.

To simulate performance in environments with high loss links, which is typically the case in wireless links, we added a packet loss rate equal to 2%. Despite this, however, the total amount of used bandwidth is far more increased in the case of VoMo. Moreover in the case where the two links have far different loss conditions, VoMo selects the a path with a very small packet loss rate so that it can provide an outgoing flow with more stable rate. This is particularly important for delay-sensitive applications.

Using VoMo For MPEG 4/MPEG 7 Wireless Delivery

MPEG-4 video coding and MPEG-7 content retrieval technology are expected to be the driving forces that will deliver multimedia content over the next generation mobile devices. These applications are well suited for advanced transport services offered by the VoMo platform.

The MPEG-4 standard is actually a set of tools that are available to the developer, and can be used according to required coding requirements, decoder complexity, and data format, to name a few options.  MPEG-4 is characterized by profiles, which are sets of tools that provide a specific functionality. For example the Simple Profile is uses the H.263 video conferencing standard. This profile is tailored for low bit-rate applications which would usually include handheld wireless devices.  Other complex MPEG-4 profiles exist which include synchronized interactive environments with a number of arbitrary-shaped video objects, and associated 2-D and 3-D vector graphics.

A typical MPEG-4 streaming video platform performs rate control offering VBR and CBR services. However, this approach is network agnostic and is based on efficient implementations of MPEG-4 coding standards and the underlying media protocols RTP/RTCP/RTSP. 

The MPEG-4 Simple Profile produces a base layer (BL) and several enhancement layers (EL) which can enhance the base layer description quality. Since the BL is the most important description it should receive the best protection, preferably using forward error correction (FEC).

A particular advantage of VoMo in this instance is that it allows the handling of each layer according to its importance. The base layer can be assigned to a VoMo stream that corresponds to a highly reliable path.

The MPEG-4 Core Visual Profile adds support for the encoding of objects that are arbitrarily-shaped. If this profile were used, MPEG-4 encoding is performed at an object by object basis.

Important objects may be encoded preferentially, and VoMo would consider these MPEG-4 objects as individually different transport entities.  This implies that the delivery of each one of them may be handled separately by a suitable mapping procedure between these objects and transport layer streams. Interactive object manipulation only affects those streams that are carrying these objects, while the rest of the bit stream stays unaffected.

Additional Sources Of Information

1. M. Handley et al.,"SIP: Session Initiation Protocol", RFC 2543, March 1999.
2. C. Perkins, "IP Mobility Support", RFC 2002, October 1996.
3. K. Kim et al. "Domain Based Approach for QoS Provisioning over Mobile IP", in Proc. Of ICON 2001, Bangkok, Thailand.
4. Ashutosh Dutta, et al "Application Layer Mobility Management Scheme for Wireless Internet," 3Gwireless 2001, San Francisco, pp. 7, May 2001.
5. V. Tsaoussidis, S. Wei , "QoS Management at the Transport Layer", ITCC 2000, Las Vegas, Nevada.
6. R. R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. J. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000.
7. http://www.umts-forum.org/
8. http://www.isi.edu/nsnam/ns/
9. http://mpeg.telecomitalialab.com/

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