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Mesh and Star

Discussion in 'Network+' started by Eggbert, May 26, 2008.

  1. Eggbert

    Eggbert Bit Poster

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    Hello, just a quick couple of questions...

    Firstly... Mesh topologies... This is more out of curiosity, but If I have a network of 12 computers conneted in a mesh topology, what exactly do the connections look like at the back of each machine? Do they use special network cards or some type of hub at each computer? (I realise mesh topologies are not used these days).

    Secondly, star topologies. What exactly does the "centre" point of the star topology physically look like? If a star bus, and a star ring topology use hubs and mau's, what exactly does a star topology use or look like?

    Cheers.
     
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  2. greenbrucelee
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    greenbrucelee Zettabyte Poster

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    With a star topology all computers are connected to each other via a hub. So the network cable goes from each PC to the hub.

    If one PC hoes down the network remains stable but if the hub goes down the entire network goes down.

    With a Mesh topology each PC is connected to each other, with a mesh topology if a cable stops working the data can go an alternate way to its destination.

    If a PC goes down it can effect the networks speed.

    Hope this helps
     
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  3. Eggbert

    Eggbert Bit Poster

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    Thanks for the reply Greenbrucelee, but not really what I was after. I understand the logical concept of the two, but if I were to physically see a mesh network, what would the network cards in each PC look like? Would it be a wierd type of network card with many coax or RJ45 plugs, or maybe a pc with lots of network cards installed.
     
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  4. greenbrucelee
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    greenbrucelee Zettabyte Poster

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    Just the usual network ports, in some cases there could be a couple of NICs in a PC with CAT 6 cables.
     
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  5. Ambushed

    Ambushed Nibble Poster

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    Or CAT5 (Most common)
     
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  6. Eggbert

    Eggbert Bit Poster

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    But how could 1 computer connect physically to 11 other computers using just a "couple of network cards"?

    Wouldn't the pc need at least 11 physical ports to do so? (as would the other 11 computers?):blink

    Also one thing I forgot to mention... If a star topology is simply a series of computers connected to a central hub, why is it called a star topology? as opposed to a "star bus" topology which is also just a series of computers connected to a hub.

    Thanks again
     
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  7. greenbrucelee
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    I am not sure what you mean by star bus topology, as far as I am aware its Bus, ring, star and mesh.

    this is what are star looks like, as you can see a cable come from each computer and connects to the hub, the hub has lots ports on it so lots of computers can be connected to it.

    the data transmits up and down each cable through the hub.
     

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  8. Eggbert

    Eggbert Bit Poster

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    But in that star topology presuming it is Ethernet, the central hub is a bus that has been shrunk into the hub, thus a hybrid "Star Bus". At least that is what Mike Meyers N+ and A+ books refer to it as.

    IBM Token Ring MSAU's are referred to as hybrid "Star Ring".

    I guess then "star bus/ring" can be used interchangeably with simply "star"?
     
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  9. ffreeloader

    ffreeloader Terabyte Poster

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    Mesh is normally used in reference to wireless networks where all computers are capable of making direct connections to all other computers.

    In a wired mesh network you have alternate routes to each node of the network so that if one route goes down there is another route available.

    As to the number of NIC's per computer in a wired mesh topology that depends on the number of nodes in the network, the number of alternate routes for data to travel, etc....
     
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  10. r.h.lee

    r.h.lee Gigabyte Poster

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    Eggbert,

    Before we embark on the topic of topologies (intentional choice of first three letters matching), there are two types of topologies: 1) Physical 2) Logical.

    The Physical topology can be thought of how the devices are cabled up to each other. Let's say you've got two Windows computers in a workgroup with a single FastEthernet NIC in each. You physically connect them together using a single Ethernet UTP crossover cable. So the physical topology would be a single "spoke" to borrow terminology from another name for star which is "hub and spoke." Now, let's say you had a third Windows computer in a workgroup. Now in order to connect the third computer to the network, you'd have to: 1) Add another FastEthernet NIC to Computer1 and Computer2 2) Get Computer3 with 2 FastEthernet NICs. Then you'd need three Ethernet UTP crossover cables to connect each computer to each of the others. So at this point, all three computers are now in a physical mesh topology. Now, let's say there's now a fourth computer. Since it's practically annoying to buy another NIC for each computer (an additional 3 NICs for Computer1, Computer2, Computer 3, as well as get Computer4 with 4 NICs on board) and buy another set of Ethernet cables (in this next growth stage, Computer 1 will need 1 additional cable to connect to Computer 4, Computer 2 will need 1 additional cable to connect to Computer 4, Computer 3 will need 1 additional cable to connect to computer 4, and Computer 4 will need an additional 3 cables to connect to all of the other computers, for a total of 6 additional cables). Now, this scenario goes crazier if you want to add another computer to the four computer mesh network because you'd need: 1) 4 addtional NICs for each of Computer 1 through Computer 4 2) a fifth computer with 5 NICs 3) 4 additional cables for Computer 1 through Computer 4 and 4 additional cables for Computer 4 alone for a total of an extra 8 cables. So here's a review of: 1) additional NICs 2) total NICs 3) additional cables 4) total cables

    Code:
    Number of Computers | Additional NICs | Total NICs | Additional Cables | Total Cables
    --------------------+-----------------+------------+-------------------+-------------
    1                   | 1               | 1          | 0                 | 0
    2                   | 1               | 2          | 1                 | 1
    3                   | 4               | 6          | 5                 | 6
    4                   | 6               | 12         | 6                 | 12
    5                   | 8               | 20         | 8                 | 20
    
    This is why at a certain point, like 3 computers, it makes sense to switch over to (pun intended) a physical star topology. That's because all you'd need would be 3 computers, 3 NICs, 3 cables, and 1 switch. That's better than the full mesh topology that would have required 3 computers, 6 NICs, and 6 cables.

    Here's some attempts at some ASCII art to show the different topology types...
    Code:
    Bus
    
         [ ]       [ ]     [ ]
    ------+----+----+----+--+------
              [ ]       [ ]
    
    Mesh
    
            [ ]---------[ ]
             |\         /|
             | \       / |
             |  \     /  |
             |   \   /   |
             |    \ /    |
             |     X     |
             |    / \    |
             |   /   \   |
             |  /     \  |
             |/        \ |
            [ ]--------[ ]
    
    Star
    
    
                 [ ]
                  |
                  |
                  |
     [ ]---------[ ]-----------[ ]
                  |
                  |
                  |
                 [ ]
    
    Now that physical topologies have been covered, let's move on to logical topologies. As we know, Ethernet from a physical topology perspective, should be in the shape of a physical star. However, logically Ethernet operates like a logical bus topology. That's because the logic inside an Ethernet hub is just the same as previous Ethernet technologies such as 10Base2 or "thinnnet" and 10Base5 or "thicknet." Now, Token Ring is also in a physical star topology but logically it is a ring. That's because of how the MSAUs used in a Token Ring LAN operate internally as well as other MSAUs in the same Token Ring LAN segment.

    Based on the scenario you've given, for a physical mesh topology and 12 computers, physically each computer would have 11 NICs and 11 x 11 = 121 cables connecting each computer to ALL of the rest.

    The "centre" point of a physical star topology is the networking device, in the case of Ethernet it would be the hub or switch, and in the case of Token Ring it would be the MSAU. So to show this graphically, I'll label the nodes in the above physical star topology diagram.

    Code:
    Ethernet Physical Star Topology
    
    
                         [Computer]
                             |
                             |
                             |
     [Computer]---------[Hub/Switch ]-----------[Computer ]
                             |
                             |
                             |
                         [Computer ]
    
    Token Ring Physical Star Topology
    
    
                     [Computer]
                          |
                          |
                          |
     [Computer]---------[MSAU ]-----------[Computer ]
                          |
                          |
                          |
                     [Computer]
    
    I hope this helps.
     
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  11. greenbrucelee
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    that explains things well to me r.h lee thanks, rep given :D
     
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  12. robbo1962

    robbo1962 Byte Poster

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    Based on the scenario you've given, for a physical mesh topology and 12 computers, physically each computer would have 11 NICs and 11 x 11 = 121 cables connecting each computer to ALL of the rest.

    a nice description r.h.lee but there is a formula to work out how many cables you would need for a mesh set up. cables needed = nodes x (nodes - 1 ) / 2 which in this case gives you 12(12-1) /2= 66 . Gary
     
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  13. r.h.lee

    r.h.lee Gigabyte Poster

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    robbo1962,

    I'm starting to hear voices of my past engineering and mathematics professors but what is the derivation of your "cables needed = nodes x ( nodes - 1) / 2" equation?
     
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  14. robbo1962

    robbo1962 Byte Poster

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    Its not my own theory just one i remember from a text book, you say 12 computers need 11 x 11 cables to connect = 121 cables but this formula duplicates cables as computer 1 needs 11 cables to connect to every one but computer 2 will only need 10 as it is already connected to computer 1 and so on and so on. Given this you need the numbers 11 to 1 which = 66, not sure how the formula i stated earlier is relavant to this but it does work. Gary
     
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  15. BosonMichael
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    Derivation? :blink Uh... you have a certain number of nodes (nodes), and you can't connect a node to itself (nodes -1), and you've got two routers at each end of the cable (/2). It's not rocket science. :D
     
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  16. Eggbert

    Eggbert Bit Poster

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    Thanks a lot for this r.h.lee, that's exactly what I wanted. I really appreciate the effort you put into that!!!

    :super
     
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