Final answer:
The Extended Service Set (ESS) allows for client device roaming in an 802.11 network, enabling movement across multiple access points while maintaining a continuous network connection.
Step-by-step explanation:
The service set in 802.11 that allows for client roaming is the Extended Service Set (ESS). This is because an ESS is made up of multiple overlapping Basic Service Sets (BSS) connected by a distribution system, which typically is an ethernet backbone. This design enables a client device to move seamlessly from one BSS to another within the same ESS, maintaining a continuous network connection, which is crucial for roaming. In contrast, an Independent Basic Service Set (IBSS) is a peer-to-peer network and does not support roaming, and a Basic Service Set (BSS) is a single access point with no roaming capability outside its range. The Personal Basic Service Set (PBSS), part of the WiGig (802.11ad) standard, is not designed for roaming across different access points.