First, realize streaming media multicast services for mobile users
As the laptop is constantly improving in terms of volume, weight and performance, more and more people have joined the ranks of mobile office. They hope to access networks, share resources, and services like desktop users, including many streaming services in wired networks, such as video conferencing, film and television, etc. The best way to use multicast when you need to transmit a stream of stream of streams of a node to multiple mobile nodes. Under this requirement, the flow of streaming media will become a reality in the mobile user in the next future.
With the in-depth study of the wireless broadband access network, the rate of wireless access has been greatly improved, such as the recently raised IEEE802.11b-based transmission standard Wi-Fi wireless network, it has been able to reach 11Mbit / S rate. Moreover, the streaming media has a large compression ratio, which is approximately 1/5 of MPEG I or MPEG II; also has a large compressed range, from several kbit / s to a few Mbit / s, so it can be adapted from narrow Broadband arbitrary network bandwidth.
However, mobile users' streaming media multicast services have not been implemented, and the reasons are mainly two points: First, mobile IP technology is still immature, and the multicast mechanism in mobile IP is not perfect; the second is due to the terminal host in the access network Move, streaming media to facing many operability issues in the mobile network. The discussion is discussed separately from these two points.
Second, the multicast support mechanism in mobile IP
Mobile IP support multicast is a more complex problem, although some solutions have emerged, but there are advantages and disadvantages, have not yet formed standards. Several basic mechanisms are compared below.
1. Mobile node is the multicast technology when the source terminal
In the multicast, the routing mechanism such as DVMRP and MOSPF, such as network addresses that need to be used to source nodes when calculating the route, and the mobile multicast router (MCR) believes that the multicast packet is sent from the primary network from the source node. If the mobile node (MN) is in the field network, the packet sent by the packet will arrive at many routers from an undesired link. The DVMRP will lose the packet, and MOSPF will forward the incorrect distribution tree, so these two routing mechanisms Data cannot be submitted to the destination. In order to avoid this problem, MN can be sent to its local agent (HA) by "tunnel" technology, and then forwarded by HA. Also, when the scheme is that the source address does not use its master address, and an address in the foreign network, such as fa, or a temporary address. This method is compared to directly efficient, but once Mn removes the foreign network, the message returned by the multicast is likely to give other mobile nodes. The CBT uses a single set of distribution trees, and the packet is only routed based on the destination address, so the MN is allowed to send multicast number packs in normal ways.
2. Multicast technology when the mobile node is the destination terminal
When the mobile node is a destination terminal, multicast technology can be discussed in three cases.
Local agent routing: MN can register a multicast group that it added, after receiving the message of the multicast group, the primary agent is transferred to the "Tunnel" technology to transfer these messages to the "Tunnel" technology Mn. This method is good with the interoperability of existing networks, and multicast is completely transparent. FA does not need any modifications, and its disadvantage is that the efficiency is relatively low.
. As shown in FIG. 1, the MN is directly registered directly, and the fa is responsible for receiving a multicast message and transmitting it to the local subnet. FA manages local group members through IGMP and communicates with any route mechanisms and other MCRs. The route of this solution is optimal, the highest communication efficiency, and the communication flow is also minimized. The main disadvantage is that it is required to provide MCR functionality, and this is often difficult to do.
Hybrid Routing: This often occurs in a subnet, and several MNs belong to a multicast group, but their main networks are different. As shown in FIG. 2, when the host sends a message to the multicast group, the message received by the MN is the same, so a HA can be served without having to serve the respective HA. This approach increases communication efficiency, reduces communication flow, but needs to be modified to external agent (FA) and local agent (HA), and interoperability is limited.
3. The problems and solutions faced when streaming media in mobile networks
1. Mobile between different networks
In the autonomous system across multiple business providers, people cannot simply use the original multicast because the core router may not support multicast. However, based on the user layer or network layer, the island can be connected by using the IETF protocol of the UDP Tunnel Multicast Protocol (UMTP) and the IETF protocol without explicit tunnels, will be based on Multicast application extends to a network that does not support multicast. When the mobile host moves from a network that supports multicast network to another, the UMTP or AMT tunnel must be established in advance in the intermediate node that supports multicast routing. This approach can be easily implemented by installing a gateway agent between the two network boundations and the active UMTP tunnel.
2. Network mobile detection
The new wireless access point, subnet, or domain can be found in different layers. In the position transformation of the mobile host (MH), the first movement detection occurs at the second layer, where the customer determines whether it is converted to a new wireless access point based on the received radio signal strength. As long as it switches to a new wireless access point, the customer needs to calculate whether it is in a new subnet or domain at the Layer 2. Using a third layer trigger mechanism (such as route broadcast or IP broadcast ICMP broadcast), you can determine if the customer is in different subnets. If the customer participates in a real-time communication session, you can use the application layer detection mechanism as the server broadcast. However, compared to the use of the Layer 2 mechanism, the transformation notification can be obtained faster.
3. Join / leave the waiting time
Get the wait time of network parameter configuration time and IGMP join / leave / leave / departure increase the loss of instantaneous data and waste. In order to maintain minimum loss and wait time in customer move, the conversion time should be minimized by taking certain trigger mechanisms, and provides almost instantaneous multicast streaming transmission. The trigger technology of the multimedia stream can be implemented in several layers, such as the second layer, the third layer, and the application layer. At the same time, it is necessary to avoid waste of bandwidth associated with the leaves.
4. Load balance
In a content distribution network, there are multiple local content servers. When the mobile customer moves between cells, the local content server needs to be transferred to each other to transfer multimedia content to mobile customers. Especially when a server is seriously loaded, adjacent servers need to point multimedia content to mobile customers belonging to the multicast group. The positioning of the Global Positioning System (GPS) can be selected for a candidate server.
5. Content buffer
By buffering this processing method, the stream content is dynamically replicated, closer to the user, can provide better quality. For a cascading session, the source uses multicast to reduce the use of bandwidth in the network, but before the multicast start, this will introduce the customer's delay. The grade buffer reduces the bandwidth of the wait time and the stream of streams to the customer. Thus, when the hierarchical buffer server waits for a multicast stream, it reduces the latency of the initial client by transmitting the prefixed by the requested stream. When the mobile host moves from one cell to another, and when accepting a new multimedia agent service during this process, the agent's prefix buffer increases the speed of the fast trigger technology. A typical stream buffer agent can be provided using an RTSP server. However, there is still a problem with the buffer, such as transmission loss, transform loss, buffer, access bill, authorization, and copyright protection.
6. Safety
When the source is a multicast media stream, the source may encrypt a specific stream (audio or video) based on program type and audience characteristics. Between customers in the content provider, the local platform, and the business environment, there is a safe content allocation to facilitate a correct business model. Group key management and media encryption are two very important factors of mobile multicast. Due to the one-to-many nature of the application, the key distribution can be made by using the key management structure using the central concentration, and the key management construct is supplemented by SAP-based security associations. Media encryption can be provided in different layers, such as WEP, IPSec, and transport layer based on 802.11. As a result of repeated transformation, a mobile customer is easy to receive an influence of heterogeneous access (such as 802.11, cdma1xrtt) and IP address changes, so it is difficult to maintain the second layer and the third layer based on secure connection. In the mobile multicast environment, security RTP (SRTP) is superior to standard IPsec encryption because it avoids the package of the head, nor does it require tunnel establishment time related to IPSec.
Four, conclusions
This paper simply analyzes the use of multicast technology to provide streaming media services for mobile users to be feasible and have broad prospects. And the difficulties encountered in this service have been introduced and analyzed, namely, mobile IP multicast mechanisms, and problems and solutions facing moving in mobile networks. I believe that as research is further in-depth, this service will be achieved.
