Hubs Bridges and Switches


The simplest way to interconnect LANs is to use a hub.  A hub is a simple device that takes an input (i.e., a frame's bits) an retransmits the input on the hub's outgoing ports.  Hubs are  essentially repeaters, operating on bits.  They are thus physical-layer devices. When a bit comes into a hub interface, the hub simply broadcasts the bit on all the other interfaces. In this section we investigate bridges, which are another type of interconnection device.


In contrast to hubs, which are physical-level devices, bridges operate on Ethernet frames and thus are layer-2 devices. In fact, bridges are full-fledged packet switches that forward and filter frames using the LAN destination addresses. When a frame comes into a bridge interface, the bridge does not just copy the frame onto all of the other interfaces. Instead, the bridge examines the destination address of the frame and attempts to forward the frame on the interface that leads to the destination.


Up until the mid 1990s, three types of LAN interconnection devices were essentially available: hubs (and their cousins, repeaters), bridges and routers. More recently yet another interconnection device became widely available, namely, Ethernet switches. Ethernet switches, often trumpeted by network equipment manufacturers with great fanfare, are in essence high-performance multi-interface bridges. As do bridges, they forward and filter frames using LAN destination addresses, and they automatically build routing tables using the source addresses in the traversing frames. The most important difference between a bridge and switch is that bridges usually have a small number of interfaces (i.e., 2-4), whereas switches may have dozens of interfaces. A large number interfaces generates a high aggregate forwarding rate through the switch fabric, therefore necessitating a high-performance design (especially for 100 Mbps and 1 Gbps interfaces).


The hub allows branching of the cable which then might form a loop-free tree of cables with the same appearance to the computers. The hub may shape and amplify the signal. The hub sends the incoming signal out on each of the lines that it did not come in on. I don’t know how smart it is upon packet collisions. It has no storage beyond about one bit. Ethernet theory calls this a Repeater and includes bridges in the hub category.
The bridge examines the 48 bit destination address for the packet and directs the packet only to the cable where the recipient resides. One packet causes less congestion thereby. A bridge may buffer enough bits to interpret the destination address. It could buffer a whole packet and queue it for the correct outgoing link. I don’t know if they do. Bridges learn who’s where by watching return addresses. Bridges may join into the Spanning Tree Protocol.
A switch will typically talk some protocol besides Ethernet, such as PPP over longer lines provided by the Telco. The switch knows about IP (internet protocol) packets within the Ethernet packets and routes according to 32 bit IP addresses. The switch will typically buffer and queue packets. It will to talk to the LAN with Ethernet as well.
“A router is a node that forwards IP packets not explicitly addressed to it.” The router talks PPP over very fast links such as fiber. It probably talks only to switches and other routers. Routers must be know to confine packets with IP addresses that are of local scope. Network Address Translation (NAT) happens in routers.