Multimedia streaming approaches for VANETs

Existing literatures on multimedia streaming approaches for VANETs are classified in Figure 3. We review these literatures in the following, and discuss their feasibility of multimedia streaming to a group of nodes in urban VANETs.

Figure 3. Classification of existing multimedia streaming approaches for VANETs.

2.1.1. Network coding based streaming

NCDD (Network Coding based Data Dissemination) [3] utilize random network coding techniques for data dissemination in VANETs. Each group node broadcasts its resource information to its 1-hop neighbors periodically. In addition, group nodes exchange coded pieces instead of original pieces. If a coded piece is linearly independent of the coded pieces in a node's local memory, then the node stores it. A node has to collect enough pieces then for decoding [2]. Note that network coding based approaches require group nodes periodically broadcast its collected pieces’ information and retrieves uncollected pieces. Broadcast packets are not always received by neighbor nodes, and the concurrent transmitting nodes may suffer from severe collision [4]. Furthermore, network coding based approaches may consume a lot of time to collect enough pieces to decode if group nodes are not dense enough [4]. Especially in urban VANETs, due to obstacles, it is difficult for group nodes to hear the broadcasting of pieces information from one another. CodePlay [22] improves the collision problem of network coding based approaches (e.g., [2][3]) by adopting a local push scheme that only selected nodes are allowed to push data packets to other nodes in the same road segment.

However, this technique still did not resolve the problem of group nodes not dense enough.

2.1.2. Hop-by-hop forwarding based streaming

In SMUG (Streaming Media Urban Grid) [6], a media stream is generated from a certain point (e.g. a roadside access point), and the stream is fed to SMUG-capable nodes and is distributed across a VANET [6]. Each node may dynamically be selected as a forwarder, and its transmissions are scheduled according to a TDMA scheme. Each forwarder is scheduled in a certain time slot to transmit, and neighboring forwarders would be assigned different time slots according to the proposed graph coloring technique so as to minimize the chance of collisions in adjacent areas [6]. However, if SMUG-capable nodes are not dense enough, it

would result in high packet loss, due to hard to meet any forwarder around. Besides, SMUG can only be applied in TDMA-based ad hoc networks, and it requires all nodes to follow its specific TDMA channel access scheme.

V3 [8] provides a scheme to retrieve the scene of a certain area to an interested vehicle.

The application scenario of V3 is that for a certain region on the road, the scene can be captured by one or more video sources, such as pre-deployed stations or vehicles passing by.

The interested vehicles, called receivers, continuously trigger the video sources to send the videos back. However, this scheme is not suitable for group communications, because each receiver establishes a path to a source, which is inefficient. Besides, the packet forwarding protocol in V3 only considers vehicles in a straight road, such as a highway. Therefore, V3 is not feasible to urban scenarios where a road map has many road intersections.

2.1.3. Cluster based streaming

Both VAPER (Vehicles Adaptive Peer-to-peer Relay Method) [5] and ZIPPER (Zero-Infrastructure P2P System) [4] form clusters among vehicles, and multimedia stream are relayed between clusters. Every vehicle periodically sends a beacon to neighbors to form clusters. There are a cluster head and also a cluster tail in a cluster. Each vehicle in the same cluster is one hop neighbor to each other. The main difference between VAPER and ZIPEER is that VAPER pushes a multimedia stream, while ZIPPER pulls a multimedia stream. In VAPER, the cluster head broadcasts a multimedia stream to its cluster members, and then the cluster tail relays the multimedia stream to the cluster head of the subsequent cluster. ZIPPER assumes a multimedia stream is composed of blocks, and a vehicle can retrieve blocks from other vehicles if available. If a required block is found, the block would be sent back.

However, both clustering schemes require all vehicles to form clusters and maintain the clusters all the time, regardless of whether cluster members want to receive multimedia

stream. And both clustering schemes consider only straight roads, such as highways. They did not consider urban scenarios where the map has many road intersections. That is, the clustering schemes can not be directly applied to urban VANETs.

2.1.4. Overlay based streaming

In overlay based streaming, the source node multicasts a multimedia stream to group nodes in an overlay. Various kinds of overlay multicast approaches over MANETs have been proposed [16][17][18][19][25], while in VANETs, only Qadri et al. [1][11][24] discuss video streaming using a static overlay, as far as we know. Due to high-speed, obstacle-prone and broken-link-prone characteristics, urban VANETs is very different from MANETs. For dynamic overlay approaches proposed for MANETs, most of them [16][18][25] aim to maintain low cost topology in terms of number of overlay links or physical links, but they cannot adjust the overlay in time for urban VANETs with high mobility. That is, they may cope with MANETs with low mobility; however, they are not feasible to urban VANETs with high mobility. Instead of maintaining low cost topology, OMHF [19] and ALMA [17] adjust an overlay quickly based on current overlay links’ quality. OMHF [19] uses the number of a node’s link failures to indicate its quality. If a parent’s link quality is lower than its child’s, their parent-child roles are exchanged. However, OMHF is not suitable for obstacle-prone urban VANETs. In obstacle-prone VANETs, the new parent may not be able to connect to the ancestor, and some children may not be able to connect to the new parent. ALMA [17] uses the overlay link delay as the estimated quality of an overlay link. If the link delay of a child to its parent exceeds a threshold, the child switches to another parent with shorter link delay.

However, if applying ALMA to urban VANETs, due to high packet loss and frequent disconnections, a child may switch to a parent with higher packet loss, although the link delay is low. Therefore, the existing approaches proposed for MANETs are not feasible to

multimedia streaming in urban VANETs. Feasible multimedia streaming in VANETs needs to consider the connectivity of children and parents as well as the stream’s packet loss rate and the end-to-end delay.

As to the related work in VANET, only Qadri et al. [1][11][24] discussed video streaming using an overlay, as far as we know. They evaluated the feasibility of video streaming using a static overlay in VANETs, and showed the improvement of applying video coding and error resilience. However, the overlay structure was static, even though nodes are mobile, which may suffer from inefficient overlay structures and frequent disconnections. In our work, we focus on dynamic overlay adaptation. In Table 1 the aforementioned approaches are compared, considering the feasibility of multimedia streaming for a group of nodes in urban VANETs.

Table 1. Comparison of existing multimedia streaming approaches for VANETs.

Hop-by-hop forwarding