Suppose that a network switch is a xGbit switch.
< xGbits? Perhaps by using an adapter? Cables? Software modification? > xGbit. Please correct me if I'm wrong.Which component(s) defines this speed? Is it the switch core?
Smaller switches have fixed ports, some may have a few modular slots, larger switches commonly use port modules (in groups of 24, 12, 8, ...).
Each port has its supported speeds with copper/twisted pair ports being almost universally downward compatible several grades from 1 Gbit/s to 10 Mbit/s, or 10 Gbit/s also supporting 1 Gbit/s or 100 Mbit/s. Optical ports most often support only a single speed, depending on the SFP module fitted. Additionally, only a subset of optical PHYs can be compatible with each other (e.g. 100BASE-FX and 1000BASE-SX use different wavelengths/colors).
Is it possible to decrease the speed to < xGbits?
Downward-compatible ports negotiate speed automatically to the highest mutually supported speed. Some managed switches support limiting the negotiated speed to e.g. 100 or 10 Mbit/s.
Perhaps by using an adapter?
An adapter actually being able to change the link speed is a bridge, or more commonly a switch.
Cables?
A specific speed requires a minimum cable grade, e.g. Cat-5e for 1000BASE-T. If the cable is below the required quality the link will be unreliable of even doesn't come up at all.
Software modification?
It's usually possible to force a specific link speed on a NIC or a switch. Due to autonegotiation limitations, this should always be done on both sides.
I guess that it's not possible to increase the speed to > xGbit.
You can certainly increase a link's speed from, say 1 Gbit/s to 10 Gbit/s using a bridge/switch in between. However, the usable bandwidth won't actually grow beyond 1 Gbit/s, so there's nothing gained. If a node requires more bandwidth you need to add more or faster network interfaces.
Of course, you can use the increased uplink speed to aggregate multiple 1 Gbit/s ports - a switch can feed ten 1 Gbit/s downlink ports with only a single 10 Gbit/s uplink without any congestion. This is an extremely common scenario - often the downlink capacity is "oversubscribed" when you provide more downlink bandwidth then the uplink can simultaneously support.
Most manufacturers use the switch port speed. So if the switch has gigabit ports, it's a "Gigabit switch."
As you might imagine, this is mostly a marketing strategy. The port speed has nothing to do with the forwarding rate (how fast the switch can forward data from one interface to another). This can be significantly slower than the port speed, especially on lower end switches.
Many (most) switches autonegotiate the port speed, and can reduce to 100Mb.
