Untitled

_{What complicates the connection of one lan
to another?}
_{network architecture (ethernet, token ring,...)}
_{media (cables, etc)}
_{network operating system (novell netware,
windows NT,...)}
_{operating system (DOS. OS/2, Unix,...)}
_{What are the lan connection devices?}
- _{repeaters, bridges, routers, brouters, gateways}
_{repeaters - function at ? layer of OSI model
(page 245)}
- _{repeat signal (regenerate, retime, amplify)}
- _{passes all signals between all attached
  segments}
- _{does NOT read destination address of data
  packets}
- _{allows for connection of different types
  of media}
- _{extends LAN distance}
- _{why use a repeater?}
  - _{Increase total length of media}
  - _{isolate key network resources on different
    lan segments}
_Bridges
- _{function at ? layer of OSI model}
- _{more discriminating than routers - only
  let addressed data over the bridge, routers let all data through}
- _{bridge types}
  - _{transparent (connect lans of similar data
    link formats)}
  - _{translating (connect lans of different data
    link formats - ethernet to token ring)}
  - _{encapsulating (bridges ethernet and token
    ring)}
  - _{source routing (designed for token ring
    lans)}
  - _{for rest see fig 6-14}
- _{why is a source routing bridge not a router?}
  - _{because the best path to the destination
    is chosen by the source device ( a lan attached PC) not by the
    bridge}
  - _{also, the seven hop limit of bridges}
- _{Bridge performance}
- _{filtering rate - filters accesses at a certain
  speed - token ring packets or ethernet frames per second (7000-60,000
  frames per second)}
- _{forwarding rate - the operation of placing
  the data on the internetwork media (700-30,000 per second)}
_Routers
- _{routers only examine data packets addressed
  to it - NOT destination address of every packet on the LAN}
  - _{where is this address? It is in the header
    portion of the data-link layer frame}
  - _{recall that the data-link layer is at layer
    2 of the OSI 7 layer model and it provides the required reliability
    to the physical layer transmission by organizing the bit stream
    into structured frames which add addressing (header) and error-checking
    (trailer) information. It is the network interface card on the
    PC that builds the data-link layer frames. Also, recall that the
    data-link layer is split into two sublayers to allow NetWare (for
    example) to run equally well over Token Ring or Ethernet NICs}
    - _{media access control (lower sublayer)}
      - _{interfaces with the physical layer (layer
        1) of the OSI model}
      - _{uses a token ring or ethernet frame (depending
        on the NIC)}
    - _{logical link control (upper sublayer)}
      - _{interfaces with the the next OSI layer up
        - network layer}
  - _{how does a router's address get into the
    header portion of the data-link layer frame?}
    - _{the source PC determined that the destination
      PC was not local}
    - _{the source PC built a data-link layer frame
      (for example ethernet) with the address of the default gateway
      router in the destination field in its header area}
    - _{the gateway router is queried to find the
      address of the best next router}
    - _{when the data-link layer frame hits the
      next router, the header information is discarded and replaced
      with either address information of the next router or address
      information of the destination PC}
  - _{how does the data-link layer packet finally
    arrive at the destination PC?}
    - _{if the IP address is determined to be on
      a given router's local network the data portion of the data-link
      layer packet is examined for the address of the destination workstation}
    - _{next, the data-link layer frame header information
      is replaced with a destination address of the destination PC}
- _{how do routers choose the best path for
  the packets destination}
  - _{number of intermediate hops}
  - _{speed/condition of communications circuits}
- _{protocol of network operating system (multiprotocol
  routers can take more time) note protocols on page 256}
- _{Routing protocols - look at page 257 fig
  6-19}
- _{note algorithm or method of figuring the
  best path to the destination PC distinguishes one router from
  another}