In the figures, the control bits are shown in "L1.5 red" as a reminder that they are defined in- and used by protocol Layer 1.5. The main feature that L1.5 offers users is to allow the bandwidth of a UniPro link to be increased by using 2, 3 or 4 lanes when a single lane does not provide enough bandwidth. To the user, such a multi-lane link simply looks like a faster physical layer because the symbols are sent across 2, 3 or 4 lanes. Applications that require higher bandwidth in one direction but require less bandwidth in the opposite direction, can have different numbers of lanes per direction. Starting in UniPro v1.4, L1.5 automatically discovers the number of usable M-PHY lanes for each direction of the link. This involves a simple discovery protocol within L1.5 that is executed on initialization. The protocol transmits test data on each available outbound lane, and receives information back from the peer entity about which data on which lane actually made it to the other end of the link. The mechanism also supports transparent remapping of the lanes to give circuit board designers flexibility in how the lanes are physically wired. Starting in UniPro v1.4, L1.5 has a built in protocol called PACP (PA Control Protocol) that allows L1.5 to communicate with its peer L1.5 entity at the other end of an M-PHY-based link. Its main usage is to provide a simple and reliable way for a controller at one end of the link to change the power modes of both the forward and reverse directions of the link. This means that a controller situated at one end of the link can change the power mode of both link directions in a single atomic operation. The intricate steps required for doing this in a fully reliable way are handled transparently within L1.5. In addition to the L1.5 link power management the PACP is also used to access control and status parameters of the peer UniPro device. The mechanisms in L1.5 guarantee the following to upper layer protocols:  The main task of UniPro's Data Link layer (L2) is to allow reliable communication between two adjacent nodes in the network - despite occasional bit errors at the Physical layer or potential link congestion if the receiver cannot absorb the data fast enough. L2 clusters 17-bit UniPro L1.5 symbols into packet-like data frames (the term packet is reserved for L3). These data frames start with a 17-bit start-of-frame control symbol, followed by up to 288 bytes of data (144 data symbols) and followed by an end-of-frame control symbol and a checksum.