## 2 Background
### Multi-Channel Operation
#### 2.2.1 The European MCO Standard  
It was first decided in 2008 to set aside a 30 Mhz wide band for safety-related C-ITS systems [19]. This ranges from 5.875 GHz to 5.905 GHz and is also referred to as ITS-G5, while a 20 MHz band is also reserved in the range of 5.855-5.875 GHz, called ITS-G5B, primarily reserved for non-safety applications and services. Later, the existing allocations were extended by an additional 20 MHz wide band in the range of 5.905-5.925 GHz, which was named ITS-G5D, thus creating the full 70 MHz wide spectrum [20]. In addition, another higher band between 63 GHz and 65 GHz has been assigned for future ITS services [19; 21]. The spectrum is divided into seven channels in total. ETSI has introduced a name for each channel to distinguish the channels logically [17]. SCH0 is the primary control channel. Its physical address is 180. The current Day 1 applications use this as the only channel. It is also repeatedly referred to as the CCH or control channel. SCH1-6 channels are so-called service or offload channels, meaning that if the primary CCH channel is congested, SCH channels can be used to allocate more radio resources [17]. The CCH, SCH1, and SCH2 channels are explicitly reserved for safety-related communications in the ITS-G5A band, while SCH3- and SCH4 are reserved for all other non-safety messages in the ITS-G5B band [2]. A 23 dBm/MHz limit has been set for both CCH and all SCHs, except for SCH2, where a stricter 13 dBm/MHz limit is standardized. This was necessary because of the adjacency interference issue, as SCH2 is located right between the two most frequently used channels [17]. It is important to note that, regardless of the configuration, the ETSI regulation requires that one channel be always set to the CCH, as safety-related messages (e.g., DENM) change hands there and, therefore, must be notified by the vehicle's on-board unit [20]. As for channel-use strategies, three mechanisms are distinguished which deal with analyzing channel utilization and their filling. For a given C-ITS application with an MCO architecture, we can decide which design is the most optimal for our application [17]:  
* **Sequential filling:** Sorts the channels in order of priority and uses the higher-priority channels first. When they are full, it uses the next channel in sequence. * **Load balancing:** In this case, it tries to distribute packets uniformly across all available channels so that each channel is nearly saturated, but more than one channel can be used simultaneously [17]. * **Elastic:** In this mechanism, conditions dictate how packets are distributed between channels. Hence, channel occupancy can vary across the board. Additional actions have been introduced in ETSI TR 103 439 V2.1.1 concerning the location of triggered messages. This is to ensure that messages from different applications only go out on a specific channel so that other C-ITS systems will know which channel is used to receive the message they expect [17]. Moreover, the separation of priorities is necessary because a more important message cannot be lost or moved to a lower-priority channel if the route is full. After all, this could lead to a loss of efficiency in the C-ITS system and decreased safety level of the whole V2X communication. Four different priorities for Day 1 applications in the ITS-G5 environment are distinguished based on the messages' TC (Traffic Class) field [17]. Priority zero is considered the most important, but the priority of each message may differ on different channels. Suppose that the saturation of the highest-priority channel is increased. In that case, it drops the low-priority packets to the next lower-priority channel, and so on, ensuring that the main channel always has available space for the most crucial message types. Typically, messages have at least one secondary channel available to use as a backup if their primary channel becomes congested [17]. In general, high-priority DENMs are the most important ones with TC 0.