Protocol Stacks

Data-communications protocols are built up in layers, known as Protocol Stacks. Figure 1 shows the structure of the TCP/IP protocol stack but AppleTalk and IPX employ the same kind of structure.

To understand how the layers hang together, consider fetching a web page. The user types in a URL which is placed into the payload of an HTTP request by the browser. The request is handed to TCP, which sticks a TCP header on the front and hands it to IP. In turn, IP sticks an IP header on the front and hands it to Ethernet. The Ethernet driver puts an Ethernet header on the front, a trailer on the end, and hands the frame to the card electronics for transmission on the wire.

Structure of Transmitted Information

Figure 2 shows the HTTP request in relation to all the accretions it picks up along the way. FrameSeer works with Frames; hence its name.

Unicast, Multicast and Broadcast

Data-communications networks are divided into manageable portions, connected together by communications devices that control how and where information is forwarded. When making forwarding decisions, communications devices often need to consider how the information is to be distributed. The information may be:

Unicast	Sent to a specific host.
Multicast	Sent to zero or more interested hosts. A host registers its interest in a particular multicast group via the Internet Group Management Protocol (IGMP).
Broadcast	Sent to all hosts in the local network.

Routers and Layer Three Switches

A router works at the Network Layer (OSI Layer Three). Routers forward packets. A packet is a unit of information to which a Layer Three Protocol header has been attached. For TCP/IP, that’s an IP header. Unicast packets are forwarded to whichever of the router’s interfaces will bring the packet closer to its ultimate destination. If a router has been programmed to forward multicast packets, it may need to forward a multicast packet out of zero, one, or more of its interfaces, depending upon how many downstream devices have requested the multicast service. In general, broadcast packets do not pass through routers. A Layer Three Switch is also a form of router. The distinction is mainly one of speed: Layer Three Switches are faster than Routers.

Bridges and Layer Two Switches

A Layer Two Switch (sometimes simply called a switch) works at the Data Link Layer (OSI Layer Two). It receives and transmits Ethernet frames. An Ethernet frame is a packet encapsulated inside an Ethernet header and trailer. A switch operates by limiting how unicast frames are forwarded. A switch keeps track of the hosts that are connected to its various ports. Once it has learned to which of its ports a particular host is attached, a switch only transmits unicast frames addressed to that host on that single port. Some switches also have the ability to limit how multicast frames are forwarded but, in general, multicast and broadcast frames are sent to all connected ports. A bridge is also a form of switch. Again, the distinction is primarily one of speed: switches are faster than bridges. An Airport Hub can be both a router and a bridge.

Hubs and Repeaters

A hub, or repeater, is the simplest kind of communications device. It operates at the Physical Layer (OSI Layer One). It simply repeats whatever information it receives on all of its connected ports and does not care whether the traffic is unicast, multicast or broadcast.

Collision Domains

The term Collision Domain refers to that part of a network where, if two or more devices attempt to transmit at the same time, an Ethernet collision will occur. All ports of a hub lie in the same Collision Domain but each port on a Layer Two Switch lies in a separate Collision Domain.

The term Ethernet Segment is (generally) equivalent to Collision Domain.

Broadcast Domains

The term Broadcast Domain refers to that part of a network where a broadcast packet will be relayed by interconnected Layer Two Switches and hubs. Each port on a router lies in a separate Broadcast Domain.

The terms Broadcast Domain and Subnet and Virtual LAN (vLAN) are equivalent. The distinctions are that a Broadcast Domain is a concept that applies to multiple protocols (eg AppleTalk, IPX) whereas a subnet is, in the strict sense, a TCP/IP term. The term Virtual LAN conveys the information that a device like a Layer Two Switch may be a member of multiple Broadcast Domains.