  This paper presents the vision of multi-band communication networks (MBN) in
6G, where optical and TeraHertz (THz) transmissions will coexist with the
conventional radio frequency (RF) spectrum. This paper will first pin-point the
fundamental challenges in MBN architectures at the PHYsical (PHY) and Medium
Access (MAC) layer, such as unique channel propagation and estimation issues,
user offloading and resource allocation, multi-band transceiver design and
antenna systems, mobility and handoff management, backhauling, etc. We then
perform a quantitative performance assessment of the two fundamental MBN
architectures, i.e., {stand-alone MBN} and {integrated MBN} considering
critical factors like achievable rate, and capital/operational deployment cost.
{Our results show that stand-alone deployment is prone to higher capital and
operational expenses for a predefined data rate requirement. Stand-alone
deployment, however, offers flexibility and enables controlling the number of
access points in different transmission bands.} In addition, we propose a
molecular absorption-aware user offloading metric for MBNs and demonstrate its
performance gains over conventional user offloading schemes. Finally, open
research directions are presented.
