Need to Understand the Hardware (Serial) Failover Vs Network Failover in F5 - LTM v11.3?

Hard-wired failover When you configure hard-wired failover, you enable failover by using a failover cable to physically connect the two redundant units. This is the default setting.

Network failover When you configure network failover, you enable failover by configuring your redundant system to use the network to determine the status of the active unit. You can use network failover in addition to, or instead of, hard-wired failover. When using network failover, you can configure the system to use either self IP addresses or management IP addresses for failover. For more information on specifying IP addresses for failover

By default, the way that a BIG-IP unit monitors the status of its peer, in order to detect that failover is required, is through a hard-wired connection between the two BIG-IP units. With proper configuration, however, you can cause each BIG-IP unit to monitor peer status by way of a TCP/IP network connection instead. For more information on types of failover connections, see Configuring the failover type.

Configuring High availability - http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip_nsm_guide_943/nsm_high_avail.html