Notes On Chapter Eight -- LAN Technology And Network Topology

• 8.1 Introduction
  • This chapter discusses LAN hardware and topology
    
    
• 8.2 Direct Point-to-Point Communication
  
  
  • The early "networks" were point-to-point links between computers.
    
    
  • Advantages: The pair of hosts sharing a link could agree on using any type of modem and any communication protocols they liked. Security and privacy were relatively easy to enforce.
    
    
  • Disadvantages: When each pair of computers is connected by a link, the number of links required is proportionate to the square of the number of computers. N hosts ==> N(N-1)/2 links. This is not practical when the number of computers becomes large.
    
    
• 8.3 Shared Communication Channels
  
  
  • Inexpensive shared networks were invented in the 60's and 70's so that people could build cost-effective small local networks -- these came to be known as LAN's.
    
    
  • Shared networks don't work well for WAN's. Hosts have to take turns using the network and this requires a lot of communication among the hosts. It is difficult to support high bandwidth communications on long-haul networks, and of course the long distances introduce delay.
    
    
  • point-to-point links incur less communication overhead and work out to be more efficient for long-haul networking.
    
    
• 8.4 Significance Of LANs And Locality Of Reference
  
  
  • In computer networks most traffic is local -- in other words most of the time hosts are communicating with hosts that are nearby. This phenomenon is termed "physical locality of reference."
    
    
  • LAN's are very important because of locality of reference.
    
    
  • There is also temporal locality of reference. Hosts that have communicated recently are likely to communicate again soon.
    
    
• 8.5 LAN Topologies
  
  
  • 8.5.1 Star Topology
    
    
    • all computers attach to a central point (hub).
      
      
    • Advantage: a cut cable affects only one host.
      
      
    • Disadvantage: hub failure brings down the whole network.
      
      
  • 8.5.2 Ring Topology
    
    
    • computers are connected in a loop, First to Second, Second to Third, and so on until the Last is connected to the First.
      
      
    • Advantage: ring networks operate efficiently at maximum capacity.
      
      
    • Disadvantage: the loss of one link brings down the whole network.
      
      
  • 8.5.3 Bus Topology
    
    
    • all computers on the network share a single link -- the bus -- an ethernet cable for example.
      
      
    • Hosts have to cooperate to decide which host gets to use the bus next.
      
      
    • Advantage: Only one "wire" is required so the network is inexpensive.
      
      
    • Disadvantage: the whole network goes down if the cable is cut.
      
      
  • 8.5.4 The Reason For Multiple Topologies: each topology has advantages and disadvantages. None is the all-round best for all purposes.
    
    
• 8.6 Example Bus Network: Ethernet
  
  
  • 8.6.1 History Of Ethernet
    
    
    • Invented by Robert Metcalfe at Xerox Corps Palo Alto Research Center.
      
      
    • Coaxial cable ether
      
      
    • Cable segments are limited to 500 meters -- about 1500 feet.
      
      
    • Hosts must tap into the cable at a minimum separation of 3 meters.
      
      
    • Ethernets operate at speeds of 10, 100, and 1000 Mbps.
      
      
  • 8.6.2 Ethernet Transmission and Manchester Encoding
    
    
    • Ethernet data is encoded as rising (1-bit) or falling (0-bit) voltages.
      
      
    • The "edge" has to be transmitted in the middle of a time slot.
      
      
    • Changes to the voltage that occur at the boundaries of a time slot do not "count" as data. However such changes must take place in order to send the previous bit value again.
      
      
    • The Manchester scheme is synchronous -- the receiver has to know where the centers and edges of the timeslots are.
      
      
    • To synchronize with the receiver, the sender transmits a preamble of 64 alternating 1's and 0's
      
      
  • 8.6.3 Sharing On An Ethernet
    
    
    • A sender's electrical signals propagate at about 70% of the speed of light and are soon sensed by all hosts on the segment.
      
      
    • Interference between the signals of two senders would make both signals unintelligible to the receiver.
      
      
    • Only one host sends at a time.
      
      
• 8.7 Carrier Sense On Multi-Access Networks (CSMA)
  
  
  • The mechanism used on an ethernet to coordinate transmssion is called carrier sense on multi-access (CSMA). A host wanting to transmit "listens" on the ether and waits until there is no electrical (carrier) activity before beginning to transmit.
    
    
• 8.8 Collision Detection And Backoff With CSMA/CD
  
  
  • Even with CSMA, collisions can occur if two hosts start to send concurrently.
    
    
  • Senders must listen on the ether for sign of a collision (interference). This is called collision detect (CD).
    
    
  • After a collision the hosts each choose a random delay less than some maximum d and attempt to retransmit after that delay.
    
    
  • If there is another collision immediately after the first, the host chooses its next random delay between 0 and 2d. If collisions continue, the host will continue to double the maximum delay.
    
    
• 8.9 Wireless LANs And CSMA/CA
  
  
  • There are wireless LAN's in which each host transmits on the same frequency. They use a form of CSMA/CD called carrier sense multi-access with collision avoidance (CSMA/CA).
    
    
  • A difference from CSMA/CD is required because in a wireless LAN, not all pairs of hosts will be able to "hear" each other. Therefore a host cannot always detect a "collision" with another host by "listening."
    
    
  • If X wants to send a frame to Y, X first sends out a short control message.
    
    
  • If Y gets the control message, it sends out another control message saying it is ready to receive.
    
    
  • All hosts within range of Y hear the control message sent out by Y. X sends its frame while all other hosts within range of Y refrain from transmitting.
    
    
  • Control messages can collide, and this is handled with exponential backoff.
    
    
• 8.10 Another Example Bus Network: Local Talk
  
  
  • Macintosh Local Talk is a kind of bus network. (obsolescent now -- supplanted by "Ether Talk." Apple Talk is now so old that probably the only place you will still find it is in California's public schools. :-7 Now Ether Talk is obsolescent too.)
    
    
  • Local Talk uses a version of CSMA/CA
    
    
  • Local Talk has more limited range than ethernet and much lower bandwidth -- 230.4 Kbps -- only 2.3% of the bandwidth of ethernet.
    
    
  • Local Talk was cheap, easy to install, and it was easy to get network printers.
    
    
• 8.11 Example Ring Network: IBM Token Ring
  
  
  • Token rings pass bits around the ring.
    
    
  • Only one host at a time is allowed to transmit -- the host currently in possession of a special bit pattern called the token.
    
    
  • Hosts pass the token around. The host with the token can send one frame and then send the token afterwards.
    
    
  • The sent data gets all the way around the ring. This is how the sender verifies that the frame got through without error.
    
    
  • Passing of the token is done very rapidly by low-level hardware without intervention of the host CPU.
    
    
• 8.12 Another Example Ring Network: FDDI
  
  
  • FDDI uses a pair of counter rotating-rings.
    
    
  • The effect is "self-healing" if there is a break in the connection.
    
    
• 8.13 Example Star Network: ATM
  
  
  • ATM is an acronym for Asynchronous Transfer Mode
    
    
  • Each hosts connects to an ATM switch which acts as a kind of hub.
    
    
  • When host X needs to send a frame to host Y, the switch connects them electronically.
    
    
  • The data rate for an ATM switch is upwards of 155 Mbps. Typically computers are attatched to the switch with optical fiber else that connection might tend to be a bottleneck.
    
    
• 8.14 Summary