Notes On Chapter Eleven -- Extending LANs: Fiber Modems, Repeaters, Bridges, and Switches

• 11.1 Introduction
  
  
  • LAN technology is best suited to a network contained, say, with a single building.
    
    
  • However sometimes extensions of LANs are necessary. This chapter is about how to make extensions.
    
    
• 11.2 Distance Limitation and LAN Design
  
  
  • Distance limitation is fundamental to a LAN.
    
    
  • If an ethernet is too long two hosts may send a frame simultaneously before they become aware that a collision is occuring.
    
    
  • If a token ring becomes too long it requires too much time for information to get around the ring.
    
    
  • there can be so much signal loss as a network grows longer that frames sent from one end to the other cannot be detected.
    
    
  
• 11.3 Fiber Optic Extensions
  
  
  • One can connect an office LAN to a remote computer with a pair of optical modems and some optical fiber.
    
    
  • The modems take care of translating the signals properly so that the remote computer operates exactly as though it were connected to the LAN normally.
    
    
  • Distances of up to several kilometers are not a problem in this case because the fiber has low signal loss and high speed.
    
    
  
• 11.4 Repeaters
  
  
  • One can use ordinary coaxial cable to join two adjacent ethernet segments if a repeater is inserted at the join.
    
    
  • A repeater simply copies each signal received on one side and transmits it on the other side at full strength.
    
    
  • A repeater forwards all signals immediately. It does not buffer frames. It does not "understand" frames.
    
    
  • Hosts communicate across a repeater exactly the same way they communicate on the same segment.
    
    
  • A 10Base5 ethernet segment can be up to 500 meters in length. No more than four repeaters may separate any two nodes. Usually it is easy to stay within these limits when networking a building.
    
    
  • Although signal strength does not become a problem just because too many repeaters are in the network, CSMA/CD will not work reliably when there are too many repeaters between nodes. This is simply a matter of time delay. If a sending host does not "hear" a collision soon enough, it will not back off yet the packet may be unintelligible to the intended receiver.
    
    
  • The limits for 10Base2 and 10BaseT ethernet are lower, which is perhaps a reason why a network architect might choose to use 10Base5 technology instead of the newer cheaper 10BaseT.
    
    
  • One can connect two very distant ethernet segments using two special repeater/modem devices and an optical fiber connection between the repeater/modem devices. This arrangement is much like the one described previously for connecting a remote computer to a LAN. The tecnology is called Fiber Optic Intra-Repeater Link (FOIRL).
    
    
  • Unfortunately, repeaters repeat everything including collisions and noise. Repeaters fail to isolate "problem traffic" on a segment.
    
    
  
• 11.5 Bridges
  
  
  • One can use a bridge to connect two ethernet segments.
    
    
  • A bridge is different from a repeater.
    
    
  • A bridge has two NIC's , one for connecting to each of the two ethernet segments. (Repeaters don't have NIC's.)
    
    
  • Hosts communicate across a bridge exactly the same way they communicate on the same segment.
    
    
  • When a bridge receives a frame on one side that is addressed to a node on the other side, it transmits that frame on the other side.
    
    
  • A bridge waits until it receives a whole frame, and then commands the NIC on the other side to transmit. (Repeaters don't wait.)
    
    
  • A bridge forwards only correct frames, so therefore it does not forward noise or collisions. (Repeaters forward everything.)
    
    
  • Bridges help to keep problems on the net isolated because they do not forward noise.
    
    
• 11.6 Frame Filtering
  
  
  • Most bridges are adaptive, learning bridges: they grow a list associated with each of their NIC's.
    
    
  • The bridge examines the source address of each frame that arrives at one of its interfaces. It keeps a list of source addresses on side "A" and source addresses on side "B."
    
    
  • The bridge also examines the destination address of each frame that arrives.
    
    
  • The bridge always forwards broadcast or multicast frames.
    
    
  • If a frame arrives on side "A" and it is addressed to a specific host whose address is in the list of sources on side "A," the bridge does not forward the frame to side "B" -- because the destination is known to be on side "A" of the bridge. Otherwise, the bridge does forward the frame.
    
    
• 11.7 Startup and Steady State Behavior of Bridged Networks
  
  
  • Note that "when in doubt" the bridge forwards the frame. Therefore a bridge will forward all frames until it has learned about some sources.
    
    
  • There is an advantage to the "smarts" of a bridge -- isolation of local traffic. However the CPU of the bridge has to work very hard -- it needs to processes every frame that arrives on either side.
    
    
• 11.8 Planning a Bridged Network
  
  
  • If segment A is connected to segment B by a bridge, then a pair of hosts can communicate on segment A at the same time that another pair of hosts is communicating on segment B.
    
    
  • When planning a network of segments connected by bridges, one should try to put hosts that need to communicate frequently together on the same segment. This will tend to foster a situation in which many communications between pairs of computers can go on simultaneously. There will tend to be more throughput on such a network -- more bits per second of communication among hosts. Also hosts will tend to spend less time waiting for a communication to begin.
    
    
  • Keep in mind, however, that when a frame is not local, it will not in general get forwarded in a way that "funnels" it to the destination hosts. In a spanning tree arrangement, the non-local frame is forwarded onto all segments except it will not be forwarded to the descendents of the destination's segment.
    
    
• 11.9 Bridging Between Buildings
  
  
  • Because a bridge connects to a LAN exactly like a computer connects, a bridge can be connected to a far distant segment using a pair of fiber modems as described in section 11.3.
    
    
• 11.10 Bridging Across Longer Distances
  
  
  • The delay on a segment has to be small so that collision detection will work properly.
    
    
  • It's not possible for one host to "collide" with another host on the other side of a bridge.
    
    
  • A bridge acts just like any of the other hosts when it puts a frame on a segment. A bridge participates in CSMA/CD.
    
    
  • Therefore if two segments are separated by a bridge, it will not cause incorrect operation of the network if frames moving from one segement to the other are delayed for a very long time.
    
    
  • We can therefore join two very distant ethernet segments by putting a leased line or satellite uplink between two bridges, and connecting one segment to each bridge. There should always be two bridges in such arrangments, to avoid forwarding packets unnecessarily across the (low bandwidth) remote link in either direction.
    
    
  • Since the leased line or uplink may be very slow compared to the LAN speed, the bridges used for this kind of connection may be equipped with a large capacity to buffer (queue) packets.
    
    
• 11.11 A Cycle of Bridges
  
  
  • Any number of ethernet segments can be connected together using bridges. This provides at least one solution to the problem of building a large LAN.
    
    
  • It is possible to join segments with bridges in such a way that cycles are formed: routes that go through two or more segments and return to the starting segment.
    
    
  • Bridge software has to be designed carefully so that frames will not be forwarded around cycles ad infinitum. For example, how do we avoid the forwarding of broadcast packets around a cycle?
    
    
• 11.12 Distributed Spanning Tree
  
  
  • If a network contains a cycle of bridged segments, then we can avoid forwarding cycles by instructing some bridges not to forward packets.
    
    
  • When a bridge boots it performs an algorithm that tells it whether it would create a cycle if it forwards packets. If the answer is yes then that bridge will not forward packets.
    
    
  • The bridges that do forwarding form a spanning tree of the graph whose nodes are segments and whose edges are bridges.
    
    
• 11.13 Switching
  
  
  
  • Computers connect to an ethernet switch in the same manner that they connect to an ethernet hub. However, the hub simulates a single shared ethernet segment. The switch simulates a set of bridged segments. The "forwarding" is very selective. There is a simulated bridge between each pair of segments. Each host is on one segment alone.
    
    
  • If 2M computers are connected to a switch, up to M pairs can communicate through the switch simultaneously.
    
    
• 11.14 Combining Switches and Hubs
  
  
  • Switches are expensive so it is typical to connect a hub to each port in a switch and then connect computers to the hubs. This arrangement is similar to a set of bridged LAN segments.
    
    
• 11.15 Bridging and Switching with Other Technologies
  
  
  • Other LAN technologies besides ethernet can make use of fiber modem connections to distant hosts. It makes sense with token ring or FDDI.
    
    
  • FDDI switches exist.
    
    
• 11.16 Summary