## 2 A Primer on Multi-Link Aggregation in IEEE 802.11 be
### Asynchronous vs. Synchronous Transmissions on Multi-Link  
As we mentioned in Section 2.1, aligning data transmissions can avoid the hindrance problems in the non-STR case. Based on this discussion, two kinds of transmission schemes are considered: asynchronous transmission and synchronous transmission. Moreover, we can classify synchronous transmission into two ways: with a single accessing link and with multiple accessing links. The asynchronous transmission inherits the conventional channel access mechanism in legacy IEEE 802.11, whereas the asynchronous transmission is the modified one to support the non-STR case in a multi-link environment. Figure 3 shows three illustrative examples of the multi-link transmission methods.  
Figure 2: Considerations on designing multi-link distributed coordination function (DCF) for non-simultaneously transmit and receive (STR) case. (**a**) Transmission hindrance due to in-device interference and (**b**) reception hindrance due to in-device interference.  
Indeed, asynchronous transmission [5], which is illustrated in Figure 3a, has been approved to be mandatorily supported in IEEE 802.11 be. When the MLD is under the STR condition, it means that the stations within the same MLD can transmit and receive frames simultaneously in a distributed manner. As a result, each station on the same MLD can perform a medium access process over the links independently and autonomously. Hence the asynchronous multi-link transmission can be regarded as two independent conventional channel access unless the MLD has saturated traffic, similar to today's dual/tri-band AP operations.  
On the other hand, if the MLD is under the non-STR condition, it means transmission and reception cannot be performed at the same time at any moment. With the alignment of ending times of transmission within the same MLD, as shown in Figure 3b,c, the case of transmission and reception at the same time can be avoided.  
How to organize the synchronous channel access is an open issue. Firstly, as shown in Figure 3b, there could be a single accessing link that performs channel access [5; 6; 7]. In this method, only one link will be designated as a channel access link, and thus a contention window only exists on the channel access link. Thus, MLDs and single-link stations will have the same opportunity to access the channel. As a result, a channel will be fairly allocated both to MLDs and single-link stations. Naturally, network throughput will underperform the multiple accessing links.  
Figure 3: Illustrative examples of multi-link transmission. (**a**) Asynchronous multi-link transmission, (**b**) synchronous multi-link transmission with a single accessing link, and (**c**) synchronous multi-link transmission with multiple accessing links.  
On the other hand, as shown in Figure 3c, channel access can be done on multiple links [4, 5, 8, 9, 10]. Contrary to the synchronous multi-link transmission with a single accessing link, all the stations within the same MLD can participate in the channel access through each contention window. That means MLDs will have an almost doubled chance to access the wireless medium. So far, it is under discussion about finding a midway design point that provides moderate gains over the conventional single-link environment while ensuring channel access opportunity to legacy devices in the same BSS.  
By the way, several simulation results [11, 12] have been presented to compare the performance between channel access methods. However, mathematical models are required to evaluate and optimize performance for synchronous and asynchronous channel access methods in various situations. [PERSON]'s model [13, 14, 15] is a well-known approach to model this DCF, which is the primary medium access control technique. Since this model cannot be applied directly to the multi-link situation, some modifications are required to apply the multi-link case to the model.