1InternetworkingSchool of BusinessEastern Illinois University © Abdou Illia, Spring 2007(Week 4, Tuesday 1/30/2007)2Learning Objectives Understand reasons for internetworking Describe types of LANs based on Access methods  Distinguish between internetworking devices3Internetworking ? Connecting separate Networks using internetworking devices (bridges, routers, hubs, switches, etc.) Reasons for internetworking:– LAN performance decrease due to growth => Break & Interconnect– Need to access resources available on another network– Need to share software & hardware between networks Will focus on– Interconnecting LANs to LANs– Interconnecting LANs to WANs24Different kinds of LANs In terms of their topology (logical design -How data moves around the network from station to station):– Bus topology– Star-Bus topology– Ring topology5Bus topologyUsually, use a linear coaxial cable (called a BUS) that multiple devices or workstations tap into.6Bus topology• Connecting to the BUS cable requires:• a patch cable, and • a simple device called a Tap (a passive device).Patch cable37Star-Bus topology• Most popular topology for LANs• Looks physically like a star, but acts logically like a bus• Star design built around a central collection device called Hub or Switch. All workstations attach to hub/Switch8Ring topology• Circular connection of workstations• One channel of information• Information flows in one direction around the ring9Ring topology• Implemented using a (physical) star design.• Star design built around a central collection point called a MAU (multistation access unit)• MAU passes the signal around in a ring fashion (logical ring)410Media Access Control Protocols used in LANs Ethernet or CSMA/CD– Most common form of LAN today.– Star-Bus is most common topology– Many standards (10Base2, 100BaseT, 1000BaseT, etc.) Token Ring– For LAN that use Ring topology– Offered at speeds of 4, 16 (and 100 Mbps).– More expensive components than CSMA/CD– Losing ground quickly to CSMA/CD.11Why Ethernet and Token ring? Created to answer two questions:– How to identify sending & receiving computers in LANs?– Which computer should send? At what time? Ethernet answer:– Adding Sender’s & Receiver’s MAC address to packets– Implement a rule known as CSMA/CD Token ring answer:– Adding Sender’s & Receiver’s MAC address to packets– Implement a Token passing method: a Token circulate in the ring. Only the computer that gets the token will send.12Internetworking Need to connect Networks using different protocols– e.g.: Ethernet LANs and Token Ring LANs– e.g.: LANs (Ethernet or Token Ring) and WANs Need to connect Networks using same protocols Need to break a LAN into segments to relieve traffic congestion513Internetworking Categories of devices used to interconnect networks:– Bridges– Routers– Hubs– Switches14Bridges A bridge can be used to connect two similar LANs, such as two CSMA/CD LANs or two Token ring LANs15Bridges A bridge can also be used to connect a CSMA/CD LAN and a Token Ting LAN616How Bridges operate ? The answer depends on the type of bridge. Three basic types:– Transparent Bridges– Source-routing Bridges– Remote Bridges17Transparent BridgesEach bridge has two connections (ports), and there is a routing table associated with each portThe transparent bridge observes all traffic and builds routing tables from this observation(This observation is called Backward learning) A transparent bridge is found with CSMA/CD LANs (Filtering & Forwarding functions).18A transparent Bridge• Observes each frame that arrives at a port• Extracts the source address from the frame• Places that address in the port’s routing table.719Recap: Transparent Bridges A transparent bridge observes all traffic and builds routing tables from this observation. This observation is called Backward learning. Each bridge has two connections (ports) and there is a routing table associated with each port. A bridge observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table.20Transparent Bridges• A transparent bridge can connect a CSMA/CD to a Token Ring LAN ¾ Convert one frame format to another¾ Forward frames1. The bridge removes the headers and trailers from one frame format2. Encapsulates the headers and trailers for the second frame format.Notice that a bridge is a 2-layer device21Summary Questions1. Name the two most known Medium Access Control Protocols found in LANs. How they differ?2. (a) What are the primary reasons for internetworking two or more networks ? (b) What are the basic functions of a bridge ?3. How does a Transparent bridge operate?4. Exercise # 4 on page 268-269 of book.822Summary Questions Consider the local area network shown below Suppose that the only traffic generated on the network since it has been physically set up is as follow: - Workstation 1 has sent a message to workstation 3- Workstation 2 has sent a message to Server 1- Server 1 has sent a message to workstation 3What would be the content of the bridging tables? Port A Table Port B Table23Summary Questions (Cont) Now, suppose that Workstation 1 sends another message. This time the message is sent to Workstation 2. What would happen? (Circle all correct answers)a) The hub will send a copy of the message to the bridgeb) The bridge will forward the message to the switchc) The switch will discard the received messaged) The bridge will check its Port A bridging table and will discard the received message.24Ethernet Uses CSMA/CD access method for data transmission on a network CSMA/CD* :1) All computers (“carriers”) listen (“sense”) for traffic on the LAN2) If no traffic, computer that wishes to transmit may transmit3) If collision occurs, computers must wait a random amount of time4) The computer with smallest random number send again first.* Carrier Sense Multiple Access with Collision