Downloads |
White Paper
Videoconferencing, e-learning, software distribution - these are all applications that can benefit from multicast, the bandwidth-conserving technology that allows a host to send packets to a subset of all hosts as a group transmission. Third-generation (3G) multicast applications in mobile networks face different challenges than those in wireline networks. Cellular networks are point-to-point and multicast does not scale well as a point-to-point service. The major challenge is to find an alternative approach to the high cost of multicasting to a wide area with multiple cells. Cisco Systems® sees tremendous promise in multicast applications and fully supports different approaches to multicast services. These approaches to multicast for mobile differ among Code Division Multiple Access (CDMA) networks and General Packet Radio Service (GPRS) networks. Proprietary and standards-based efforts, such as Qualcomm's MediaFLO and the Digital Video Broadcast Handheld (DVBH) broadcast technology, promoted by such mobile innovators as Nokia and Texas Instruments, are pioneering multicast for early mobile adopters. Cisco® native IP and Cisco IP/Multiprotocol Label Switching (IP/MPLS) backbone technologies and Cisco technologies for the radio access network (RAN) support the evolving approaches to multicast for mobile networks, which must include carrier-class security, quality of service (QoS), provisioning, high availability, and management features.
This paper presents the different emerging multicast standards and protocols, their capabilities, and state of current adoption by mobile operators with CDMA and GPRS Networks. Cisco supports multicast services over an IP or common IP/MPLS backbone and RAN built on Cisco products and solutions.
Summary
Multicast is a well-established bandwidth-conserving technology that reduces traffic by allowing a host to send packets to a subset of all hosts as a group transmission instead of having to send packets to every single user. IP multicast delivers application source traffic to multiple receivers without burdening the source or the receivers while using a minimum of network bandwidth, eliminating redundancy, and making easily scalable and economical distributed applications possible (Figure 1). Multicast packets are replicated in the network at the point where paths diverge by Cisco routers enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, resulting in the most efficient delivery of data to thousands and even millions of business or consumer users. Multicast application types include one-to-many (such as video, TV, radio, stock news), few-to-few (small video or audio conferences), few-to-many (publishing), many-to-many (stock trading, gaming), and many-to-few (subscriptions).
Figure 1
Multicast Advantages
The Gartner Group estimates that multicast will be used by 80 percent of all Global 2000 companies for live video and video-on-demand services by 2006. Even low-bandwidth applications can benefit from using multicast when there are thousands of receivers. High-bandwidth applications, such as MPEG video, may require a large portion of the available network bandwidth for a single stream. In these applications, IP multicast is the only way to send to more than one receiver simultaneously.
Multicast for mobile networks is a work-in-progress. Standards-based multicast has not yet been implemented in CDMA networks. Nevertheless, early approaches to multicast for both types of cellular data environments are very promising. Major engineering resources have been devoted to multicast by mobile operators and cellular vendors, as a result of the growing sophistication and requirements of users as they sample 3G services.
Challenge
In mobile networks, the most expensive and scarce resource is radio spectrum. To provide multicast or broadcast services to a meaningful proportion of a cell coverage area may require significant amounts of power dedicated to the multicast or broadcast transmission. This is one of the challenges in delivering rich media streaming with Third-Generation Partner Project (3GPP) Broadcast and Multicast Service (MBMS) for GPRS networks because it may require signal saturation for multicast and broadcast. For CDMA2000 or Wideband CDMA (WCDMA) networks, 3GPP2 Broadcast and Multicast Services (BCMCS) provide another approach to multicast for mobile, an overlay to mobile networks that provides multicast support, as in wireline networks, between the backbone and the distribution layer. Standards-based BCMCS has yet to be deployed, aside from a few individual efforts such as MediaFLO, a Qualcomm design. BCMCS multicast applications provide the replication of multicast packets in the mobile network at the point where paths diverge by routers enabled with Internet Group Management Protocol (IGMP) or other supporting multicast protocols, resulting in the most efficient delivery of data to multiple receivers.
MBMS and BCMCS approaches to multicast in mobile networks and their current development status will be examined in greater detail. Challenges remain for both, including the ability to move beyond best-effort delivery and network design that anticipates and compensates for frequent drops.
Congestion avoidance is another issue because the lack of TCP windowing (because TCP is point-to-point) and slow-start mechanisms can result in network congestion. Some multicast protocol mechanisms (such as asserts, registers, and shortest-path tree transitions) result in the occasional generation of duplicate packets. Various network events can also result in packets arriving out of sequence. Multicast applications should be designed to address all of these issues common to the mobile environment.
There is no single reliable multicasting protocol for 3G IP applications. Two limited multicast mechanisms have become a part of mobile networks. Reliable Multicast Transport Protocol (RMTP) enables designated receivers to collect status messages from nodes in a local RMTP domain and to provide repairs (such as retransmissions of missing data), if available. Another approach, Pragmatic General Multicast (PGM), bypasses User Datagram Protocol (UDP) and interfaces directly to IP sockets, providing reliable transport of multicast traffic with optional network assistance that is provided by a PGM-aware multicast router.
Solution
Cisco Multicast for Wireline and Wireless
A Cisco native IP or Cisco IP/MPLS backbone in wireline networks can support multiple multicast networks through multicast VPNs, which allow service providers to offer IP multicast services across any existing access technology, such as ATM, Frame Relay, or Ethernet. Instead of delivering multicast to multiple recipients through a point-to-point mesh of tunnels and multiple copies of the same multicast data across the network backbone, Cisco IP/MPLS multicast VPNs transmit a single multicast source stream and this stream is replicated in the network where paths diverge with routers enabled with intradomain multicast protocols such as PIM, IGMP, Cisco Group Management Protocol, and Pragmatic General Multicast (PGM); and interdomain multicast protocols such as Multiprotocol Border Gateway Protocol (MBGP), Multicast Source Directory Protocol (MSDP), and Source Specific Multicast (SSM).
Today's 3G mobile networks supply a finite amount of power to the downlink and achieve great efficiency from a closed loop power control design, where an uplink reports the signal quality and power requirements in a cellular transmission. Studies have concluded that using CDMA2000 or WCDMA technology to provide a required minimum 64 kilobytes of power for a multicast transmission over 85 percent of a wide 3G cell area, for example, would require 20 percent of the power of the entire network for one application, according to some estimates. Therefore, the saturation of a cell with multicast signals using this architecture would not be economical from bandwidth, power, and cost standpoints.
3GPP2 BCMCS
3GPP2 BCMCS is a novel multicast-for-mobile solution for CDMA2000 or WCDMA networks that includes the introduction of a flexible common radio channel suitable for point-to-multipoint and broadcast traffic (Figure 2). The benefit of multicast and broadcast on the air interface is that many users can receive the same data on a common channel without clogging up the interface with multiple transmissions of the same data.
Figure 2
BCMCS Broadcast
Registered mobile subscriber devices can communicate with the BCMCS Controller to obtain BCMCS session and header compression information and transport and application protocols. Notification of the availability of multicast content can be made either through Short Message Service (SMS) or Wireless Application Protocol (WAP) approaches. The BCMCS controller can function as a server to provide this information as well.
The BCMCS Controller is responsible for managing and providing BCMCS session information to the BCMCS serving node through the authentication, authorization, and accounting (AAA) server in the serving domain, to the content server, and to the mobile user device (Figure 3). It serves the function of broadcast access key (BAK) distributor and may serve as BAK generator. It may also perform discovery operations to assist the mobile user device - commonly referred to as the mobile station - in finding desired content. The BCMCS Controller uses Extensible Markup Language (XML) in the HTTP payload to support BCMCS information acquisition requested by the user.
Figure 3
BCMCS Topology
The BCMCS content server makes multicast content available within an IP multicast stream. If the content server resides in the serving network, then it is not necessarily the creator or source of the content. The content server is the last application-level entity to manipulate (reformat) multicast content prior to the content reaching the BCMCS serving node. The content server may store and forward content from a provider or merge content from multiple content providers. If higher-level encryption is enabled, the BCMCS content server encrypts the stream while also serving as a multiple short-term keys manager.
The Subscriber Profile Manager is an application that updates the subscriber profile in the database with BCMCS-related access information. The AAA servers in the home and serving networks are RADIUS-compliant servers that authenticate and authorize users and store user service profiles. The AAA can allocate IP addresses for mobile subscribers and collect accounting records from mobile providers from the Subscriber Profile Database.
The Packet Data Serving Node (PDSN) communicates with the BSC to add and remove IP multicast flows. It may use IP multicast protocols to manage bearers supporting IP multicast flows between itself and the nearest router connecting back to the BCMCS content server. It also applies flow characteristics (such as header compression) received from the BCMCS Controller to the IP multicast flows and acts as a RADIUS client for the AAA functions.
The multicast router is not needed if the content is tunneled from the BCMCS Content Server to the BSN. The base station controller (BSC) and packet control function (PCF) are responsible for signaling, establishing, and removing bearer channels between the BSN and the mobile subscriber, and, if the link layer encryption is enabled, the BSC functions as a short-term keys manager. The BSC chooses the best bearer channel to the mobile subscriber based on such considerations as QoS and optimization of resources.
Multicast for mobile services using BCMCS operates the same in the backbone to the distribution layer as for wireline multicast. Instead of using IGMP or another intradomain protocol to provide multicast data to multiple receivers, in CDMA2000 or WCDMA networks the host signals the first-hop gateway (the PDSN) to join the multicast stream. The PDSN does join on the subscriber's behalf and initiates the broadcast in the designated multicast channel.
To demonstrate the viability of BCMCS, the cellular handset vendor Qualcomm has defined a personal BCMCS variant it calls MediaFLO that delivers full-motion video in Channel 55 at 700 MHz in a unidirectional link. Mobile users can tune into MediaFLO if they have the right security key and obtain premium multicast content. The service has been proposed not to compete with mobile operators but to demonstrate that BCMCS is inherently a viable solution for multicast data traffic. In another BCMCS variation, Verizon has also moved forward with multicast for mobile with its high-speed Evolution Data Only (EVDO) VCAST service for downloading short video clips on cellular phones.
3GPP MBMS
3GPP MBMS is the multicast technology in development within the 3GPP and is available for Global System for Mobile Communications (GSM) networks following the Universal Mobile Telecommunications System (UMTS) architecture. The first comprehensive description of standards is in 3GPP Release 6.
The MBMS architecture (Figure 4) inserts into a GPRS architecture with new elements such as Broadcast Multicast - Serving Center (BM-SC) and new procedures.
Figure 4
MBMS Architecture
MBMS is a downlink point-to-multipoint service with modes for multicast and broadcast data. The technology transmits a multicast and broadcast stream from a single source, which is then replicated to a group of mobile users. Past methods of sending data from a single source to multiple receivers include Cell Broadcast Service (CBS), which is text-based and operates at a low bit rate without any QoS; and unicast, which requires uneconomical power-sapping saturation of a cell area, as mentioned before.
The multicast source is transmitting data from the backbone to the Gateway GPRS Support Node (GGSN). Figure 5 shows the low efficiency of a multicast from the network core to the RAN and GSM EDGE RAN (GERAN) when MBMS is not used. Current standards efforts involve introduction of an MBMS control channel and MBMS traffic channel to get around power control in the cellular area at the radio interface level. MBMS can also improve efficiency from the source up to the SGSN and radio network controller (RNC)/BSC.
Figure 5
Multicast Traffic Path Before MBMS
The Serving GPRS Support Node (SGSN) performs network control functions (such as billing) for individual users and provides IP multicast transmissions to the radio network controller for IP Universal Terrestrial Radio Access Network (UTRAN) or the GERAN and associated base station controller equipment that manage radio resources. The SGSN maintains a single connection with the source of the MBMS multicast data and is always "multicast-aware."
GPRS Tunneling Protocol (GTP) is a Layer 3 tunneling protocol used between the SGSN and GGSN to tunnel multicast packets through the GPRS/UMTS network. The GGSN is the gateway between 3G networks and external networks. It terminates the external multicast protocol and allows MBMS GTP tunneling between it and the SGSN.
In 3GPP Release 6, two MBMS architectures are proposed. One sends multicast traffic into a unicast GTP tunnel (Figure 6) and the second into a multicast GTP tunnel (Figure 7). Variations of these approaches have also been proposed.
In Figure 6, the data multicast traffic is sent up to the GGSN(s), then the GGSN duplicates the IP multicast packets into each GTP tunnel in unicast (orange arrows) to all SGSN serving multicast receiver subscribers. IP multicast packets are then also duplicated and sent by SGSN to each radio network controller (RNC) part of the distribution path in unicast GTP tunnels (purple arrows).
Figure 6
MBMS Multicast Topology with Unicast GTP Tunnels
In Figure 7, the GTP tunnels are now multicasting the IP multicast traffic (sent to users over multicast tunnels) to improve distribution to core and RAN nodes. However, RNC will have to duplicate the traffic to every base station where a user subscribed to the multicast session.
Figure 7
MBMS Multicast Topology with Multicast GTP Tunnels
The MBMS architecture seeks to provide efficient use of RAN and backbone resources, with the primary focus on radio interface efficiency. It must support different levels of QoS, and should reuse, if possible, existing 3GPP network components and protocols, to minimize changes to existing infrastructures. Backward compatibility of the MBMS services to 3GPP Release 99 is being considered. Billing data and security should be available on a per-subscriber basis and when the subscriber is receiving multicast data using a MBMS service, notification about a new or ongoing data transfer from other MBMS services should be possible.
To make MBMS a real option for mobile operators, multicast applications using MBMS should be able to tolerate packet loss and duplication cased by transmission loss and other factors. The MBMS should not place excessive signaling requirements on the network.
In Europe, Nokia and Texas Instruments have introduced Digital Video Broadcast - Handheld (DVB-H) broadcast technology, a combination of conventional video and IP. DVB-H uses components in the existing Digital Video Broadcasting - Terrestrial (DVB-T) topology, the current standard in digital video broadcasting. Though DVB-T was not designed for mobile devices, as antenna technology has improved, DVB-T mobile services have become feasible, leading to extensive commercial trials. DVB-H transmits multicast and broadcast data in bursts, allowing the receiver to be off during inactive periods for significant power savings. This MBMS-like solution is currently being used for applications that are downloaded during the evening, or during another inactive period, for later viewing.
Conclusion
Cisco can guide operators on the core network and the RAN aspects of evolving multicast solutions. Cisco already provides mobile operators with multicast features for locating the handset when it is in a dormant state. Multicast for mobile is supported in the WiMAX Forum architecture, an edge-to-edge IP architecture to facilitate the deployment of broadband wireless networks based on the IEEE 802.16 standard. It has also been defined by the Standard Panels Working Group (SPWG) and the Network Working Group (NWG).
MBMS is contained in UMTS Release 6 and practical mobile MBMS applications are expected to be operational by the third quarter of 2007, with the first mobile terminals supporting MBMS expected by 2008. Some estimates claim that 30 percent of terminals and networks could support MBMS by 2010.
For both MBMS and BCMCS architectures, Cisco supports multicast with the usual IP multicast technologies between the content source and the packet gateway. As the architectures evolve, Cisco technologies will provide alternative options for delivering multicast to mobile subscribers using both 3GPP and 3GPP2 networks.
Cisco Commitment to Next-Generation Mobile Networks
Cisco has a clear vision of the next-generation IP network, where network functions, services, and applications converge for greater flexibility and efficiency. Mobile operators can trust in the ongoing development activities of thousands of Cisco engineers worldwide, along with Cisco worldwide partners, award-winning 24-hour onsite and virtual technical support, and an ongoing commitment to network customers of all kinds. The end-to-end intelligence of Cisco IOS® Software, with extensive management, control, traffic engineering, security, and other features, is continually increasing the possibilities for mobile networks of all kinds.
The Cisco IP Next-Generation Network (IP NGN) vision and architecture for mobile operators provides a superior platform for converged services; support for flexible billing and service plans; a migration path to an IP foundation; interoperability with different radio access technologies; and open and distributed support for multiple-vendor implementations.
Cisco is uniquely positioned to provide mobile operators with immediate network relief and long-term business success with intelligent solutions and approaches that help service providers connect customers with services, services with networks, and networks with each other (Figure 8).
Figure 8
Cisco IP Next-Generation Network for Mobile Operators - Layers
Mobile operators need an application layer that interfaces with the customer, a secure network layer that creates and delivers the services, and between them both a service control and virtualization layer that orchestrates the delivery, operations, features, and billing of the service itself. Connecting all layers and making the resulting communications as efficient and productive as possible are facilitated by intelligent networking, which simplifies the complexity of operating such a network by making it more resilient, integrated, and adaptive. Intelligent networking brings together all three layers, creating a whole greater than the sum of its parts - an IP NGN for mobile operators.
The key elements of the Cisco Next-Generation Mobility Framework for Mobile Operators include:
Cisco IP/MPLS provides a common transport and integrated management platform for the convergence of packetized voice, video, and data services over existing ATM, time-division multiplexing (TDM), and Frame Relay networks.
Cisco Mobile Exchange, the intelligent mobile Internet edge solution set that links the RAN to IP networks and their value-added services, simplifies and enhances service delivery independent of underlying access technologies with functions supporting service selection and control, flexible billing models, security, Mobile IP, and load balancing.
Cisco IP RAN optimization technology, which takes IP all the way to the cellular site and interoperates with leading wireless vendors, can dramatically lower backhaul costs, improve cell site maintenance, and help operators to more easily add 3G radios to a cell site.
The Cisco IP Transfer Point (ITP) platform, which increases SS7 signaling efficiency and lowers circuit costs by moving signaling traffic to IP, can help mobile operators dramatically decrease the costs of services like SMS, mobile number portability, custom ring tone downloads, and other resource-intensive new applications.
Cisco Mobile Voice solutions include high-capacity, carrier-class voice gateways that offer standards-based support for voice over IP (VoIP) and voice over ATM (VoATM) services; and wireless trunking that replaces traditional TDM trunking with aggregated VoIP transport between mobile switching centers.
Cisco Public/Private Wireless LAN solutions offer access points, access zone routers, and Cisco Mobile Exchange for service selection, content billing, and other features for consumer, enterprise, and service provider markets.
For More Information
For more information about multicast for mobile:
IP Multicast Technology White Paper: http://www.cisco.com/en/US/tech/tk828/tech_white_papers_list.html
"IP Multicast - A Scalable Transport for Network-Based Broadcasts" Case Study: http://www.cisco.com/en/US/about/ciscoitatwork/case_studies/routing_dl1.html
3GPP2 Standards: http://www.3gpp2.org/Public_html/specs/S.R0030-A_v1.0_012004.pdf
