Interconnection DevicesRepeaters, Hubs, Bridges, Switches & Routers

for LANs, CANs & WANs

CECS 474 Computer Network Interoperability

Tracy Bradley Maples, Ph.D.Computer Engineering & Computer ScienceCal ifornia State University, Long Beach

LANs, CANs & WANs

LAN (Local Area Network)Usually, a single LAN technology with multiple

segments connected via switches. One router connection to the Internet.

CAN (Campus Area Network)Usually, many switched LANs operating as sub-networks through VLANs (Virtual LANs) or sub-

netting. Some routers used.

WAN (Wide Area Network)Multiple routers interconnected by point-to-point links. Routers can also connect to local CANs and LANs.

Repeaters & Hubs A repeater is a network interconnection device with two interfaces that regenerates signals coming in one interface onto the other.

Repeaters:• Use no logic in regenerating signals, thus data and noise are both regenerated

by the repeater.• Can be used to extend the range of a single LAN. • Operate at Layer 1 (Physical Layer) of the TCP/IP Stack.

Repeaters & Hubs (Cont’d)

A hub is a network interconnection device with multiple interfaces that accepts an incoming signal from an interface and repeats it onto all other interfaces.

Physically, hubs are:• Small electronic devices.• Have connections for several computers (e.g., 4, 8, or 20).

Logically, hubs:

• Operate only on signals.• Propagate each incoming signal to all connections.• Are similar to connecting segments with repeaters.• Do not understand frames (Layer 2 packets) so cannot filter them.

Hubs have extremely low cost and are becoming obsolete.

Layer 2 Frames

A frame is a packet of data passed across the network at Layer 2 of the TCP/IP Stack.

At Layer 2, media access control (MAC) addresses are used to send messages from one computer to another. 

• MAC addresses are also known as physical addresses or hardware addresses.

• MAC addresses are not the same an IP addresses. IP addresses are software addresses that can be changed. MAC addresses are hardware addresses associated with the network interface card (NIC) and cannot be changed.

 Frames have two MAC addresses in their header: (1) the MAC address of the source computer, and (2) the MAC address of the destination computer.

Ethernet Frame Format

Bridges & Switches

A bridge is a network interconnection device (with only two interfaces) that forwards frames coming in from an interface to the outgoing interface corresponding to the MAC destination address in the frame. A bridge: 

• Is a hardware device.• Connects two LAN segments.• Forwards frames.• Does not forward noise or collisions from the incoming connection.• Learns addresses and filters frames based on those addresses.

 A bridge is used to connect two local-area networks (LANs) of the same type.

Bridges & Switches (Cont’d)

A switch is a network interconnection device (with multiple interfaces) that accepts a frame from an interface and forwards the frame to the interface corresponding to the MAC destination address in the frame.

A switch:• Is physically similar to a hub.• Is logically similar to a bridge.• Operates on frames.• Understands MAC addresses.• Only forwards frames when necessary.

Switches allow separate pairs of computers to communicate at the same time. Switches can be used in heavily loaded networks to isolate data flow and

improve performance.  Switches are the Layer 2 Ethernet device of choice.

Bridges & Switches (Cont’d)

Layer 3 Datagrams

An IP datagram is a packet of data passed across the network at Layer 3 of the network protocol stack.

At Layer 3 (and above), IP addresses are used to send messages from one computer to another. IP addresses are software addresses that can be changed according to the network subnet to which they belong. IP datagrams have two IP addresses in their header: (1) the IP address of the source computer, and (2) the IP address of the destination computer. 

Routers

A router is a network interconnection device that accepts an IP datagram from an incoming port and forwards the datagram to the outgoing link that corresponds to the IP destination address in the frame.

A router:• Forwards data depending on IP

addresses, not Hardware (MAC) addresses. 

• Isolates each LAN into a separate subnet, with separate IP addresses.

• Can route between different LAN technologies.

• Needs to be set up before they are used. Once set up, they can communicate with other routers and learn the way to parts of a network that are added after a router is initially configured. A Home Router

Routers (Cont’d)

TCP/IP PROTOCOL STACK & INTERCONNECTION DEVICE SUMMARY