What is eMBMS?
eMBMS offers LTE service providers an effective way to lower cost per bit when delivering the same content simultaneously to multiple end users. As the multicast standard for Long Term Evolution (LTE), Evolved Multimedia Broadcast Multicast Service (eMBMS) allows multimedia content to be sent once and received by many end users. This “one-to-many” distribution mode can be a valuable alternative to unicast when a large number of users are interested in the same content. For example, during live streaming of major sports or news events, unicast must send the same video to every user individually. But multicast takes advantage of the inherent broadcast qualities of wireless networks to send the video only once to reach an equal number of end users. In these types of scenarios, multicast makes more efficient use of the available spectrum and reduces cost per bit. At most, it will consume resources equivalent to that of the worst-performing link in a sector. Therefore, the most common uses for multicast are likely to include distributing video, music, software, news, weather, ads and other data to a mass audience. The content can be live or preloaded for later usage, which has the potential for additional cost savings.
More flexible spectrum usage
The eMBMS standard is supported in 3GPP R9, and initial deployments are expected to begin in 2012. With eMBMS, LTE networks will be able to support broadcast and multicast along with unicast, and the same frequency layer can be used for all these distribution modes. Therefore, to allocate resources appropriately, eMBMS standards have specified a high-level mechanism to reserve network resources for multicasting throughout a session — and to release them when it ends. That is, the necessary resource blocks will be “borrowed” from the total available spectrum for the duration of the eMBMS session, and then returned when it concludes. The response time for these reserve-and-release capabilities will depend on each vendor’s solution. But the key point is that eMBMS standards have established flexibility for spectrum usage and have eliminated the need for dedicated spectrum, which was an issue prior to 4G networks. For example, MBMS for UMTS and MediaFlo by Qualcomm both require dedicated resources. This makes it harder for Mobile Service Providers (MSPs) to meet customer demands for anytime, anywhere video consumption, and neither MBMS nor MediaFlo has been a huge success. Now eMBMS in LTE offers a more flexible option.
The underlying reasons for greater efficiency
In a wireless network, it might initially appear that unicast will use fewer resources than multicast when transmitting a particular video. In fact, the one-to-one and one-to-many modes each use roughly the same bandwidth (resource blocks) within a sector. That’s because at the physical layer, due to the nature of wireless transmissions, a one-to-one video transmission is actually broadcast throughout the entire sector — instead of reaching only a single customer, as occurs in wireline networks. As shown in Figure 1, the intended wireless recipient tunes in to the transmission, while other mobile devices are designed to ignore it, because it is not addressed to them.
In one-to-many transmissions, on the other hand, a large number of mobile devices can tune in and receive the video from a single transmission. So, if eight mobiles within a sector all want the same content, multicast can transmit it just once. But unicast would have to transmit it eight times — using on the order of eight times the resources that would be required in multicast.
An illustration of cost savings
If a MSP could consolidate demand for the same content — and use eMBMS to deliver it to multiple users at the same time — the cost per Mb could be significantly reduced, as shown in Figure 2. The extent of the cost reductions depends on two factors:
- The number of simultaneous users in a sector receiving the same content – In the cost model shown in Figure 2, this concurrency is called Sector Copy Factor (SCF), and modeling shows the savings that multicast enables when SCF equals 4 and 40.
- The fraction of the entire network traffic that is multicast – If a higher proportion of traffic is multicast, then the net spectrum required is lower. Less equipment will be required to operate the network for the same number of megabits delivered.
Figure 2 provides a relative cost comparison with and without eMBMS under different assumptions for the proportion of traffic being multicast. eMBMS-derived discounts were applied to the unicast cost data to calculate potential savings. The throughput for eMBMS was based on the worst-performing link in the sector, because, at most, multicast consumes resources equivalent to the worst link. Accordingly, eMBMS throughput is assumed to be 59 percent of the average unicast throughput, when using the same amount of spectrum.
Better reception and throughput
To provide better reception and throughput, eMBMS can operate on a single frequency across a group of cells, known as a Single Frequency Network (SFN). A specific set of frequency resource blocks is reserved for the SFN, so performance remains consistent when an end user moves from one cell to another. And because the cells are synchronized, interference is prevented. Users get better reception at cell edges, because their mobiles simply pick up the combined signal. Figure 3 shows how cells can be grouped administratively into clusters of SFNs, which can be useful for serving geographically fragmented markets.
When to use multicast
Broadcast is an appropriate distribution mode when an audience for the specified content is large but concentrated geographically. Unicast can be efficient when the audience is narrow and sparsely distributed. Multicast provides an alternative for distributing content to meet “clustered” demand. When considering which clusters can support a profitable business case, the multicast deployment decision can be assessed by market and by region. Mobile service providers are currently planning eMBMS solutions for live streaming, as eMBMS sessions can be set up dynamically and resources can be shared efficiently with unicast sessions. However, multicasting also has potential for use with other applications — particularly when it is combined with preloading of content, which can enhance the overall cost reductions.
Multicast in LTE networks
In an operational network, it is relatively easy to add multicast functionality with minimal equipment. The required elements to support eMBMS are shown in Figure 4, which provides an overview of the eMBMS logical architecture. A brief explanation of key functions follows this illustration.
Broadcast/Multicast Service Center (BM-SC): The BM-SC schedules an MBMS service, announces the service to user equipment, authorizes users, allocates bearer service identification, and initiates and terminates MBMS bearer resources. It may optionally be the direct interface point for content providers. This entity also terminates the SYNC protocol over the M1 interface that is required to synchronize the radio interface transmission of the same data from all eNBs in an MBSFN area. Multimedia Broadcast Multicast Service Single Frequency Network (MBSFN) area: This area consists of a group of cells coordinated to achieve an MBSFN transmission, which is a simulcast transmission technique that sends identical waveforms at the same time from multiple cells. This coordinated transmission is seen as a single transmission by a mobile device. Multimedia Broadcast Multicast Services Gateway (MBMS GW): This gateway’s function is to send IP multicast packets to all eNBs that are part of the eMBMS service. It performs MBMS session control signaling (session start/stop) toward the E-UTRAN using an interface to the mobility management entity (MME). Evolved Node B (eNB): eNB is an existing element in LTE. To support eMBMS, an eNB is upgraded to support SYNC protocol with BM-SC. The eNB joins IP multicast, terminates the multicast control channel and indicates multicast session start/stop to mobile devices (UE). Multi-cell/multicast Coordinating Entity (MCE): This is a logical function that could reside in another network element, such as an eNB. It performs admission control, allocation of radio resources throughout the MBSFN, and MBMS session control signaling — and makes decisions on radio configuration. Mobility Management Entity (MME): MME is an existing element in LTE and is involved in the signaling path to eNBs. The BM-SC signals to the eNB through MME. SYNC protocol: This helps the eNB identify the timing for radio frame transmission and detect packet loss. M2 and M3: These are signaling interfaces on the control plane, and M1 is a user plane interface to the eNB carrying IP multicast.
eMBMS multicast capabilities can provide valuable alternatives to unicast for distributing many types of live and non-live multimedia content. They take advantage of the inherent broadcast qualities of wireless networks to send content only once to reach multiple end users, thereby making more efficient use of the available spectrum and reducing cost per bit. In addition, eMBMS sessions can be set up dynamically — and share resources with unicast sessions — which eliminates the need for dedicated spectrum. To contact the authors or request additional information, please send an e-mail to email@example.com.