revision

HND YEAR 2
[OS REVISION]
Ethernet Hubs/Switches

Carl
Monday, the day of doom!

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HUBS
A hub connects individual devices on an Ethernet
network so that they can communicate with one
another. The hub operates by gathering the
signals from individual network devices, optionally
amplifying the signals, and then sending them
onto all other connected devices. You should use
a
hub or a switch on your Ethernet network if the
network includes more than two clients, servers,
or peripherals.
While you can connect dozens of clients,
peripherals, and servers via hubs, your network
performance may degrade if too many devices try
to communicate within one area of the
network. You can improve performance by adding
switches, bridges, or routers to the network.
Each switch port, bridge port, or router port
regulates traffic so that devices on the port are
protected from the interfering signals of devices
on other ports.
Most hubs operate by examining incoming or
outgoing signals for information at OSI level 1, the
physical level.

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SWITCHES

Like a hub, a switch is a device that connects
individual devices on an Ethernet network so that
they can communicate with one another. But a
switch also has an additional capability; it
momentarily connects the sending and receiving
devices so that they can use the entire bandwidth
of the network without interference. If you use
switches properly, they can improve the
performance of your network by reducing
network interference.
Switches have two benefits: (1) they provide each
pair of communicating devices with a fast
connection; and (2) they segregate the
communication so that it does not enter other
portions of
the network. (Hubs, in contrast, broadcast all data
on the network to every other device on the
network.)
These benefits are particularly useful if your
network is congested and traffic pools in
particular
areas. However, if your network is not congested
or if your traffic patterns do not create pools of

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local traffic, then switches may cause your
network performance to deteriorate. This
performance
degradation occurs because switches examine the
information inside each signal on your network
(to determine the addresses of the sender and
receiver) and therefore process network
information
more slowly than hubs (which do not examine the
signal contents).
Most switches operate by examining incoming or
outgoing signals for information at OSI level 2,
the data link level.

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COMPARISON

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INODES
Each file is represented by a structure, called an
inode. Each inode contains the description of the
file: file type, access rights, owners, timestamps,
size, pointers to data blocks. The addresses of
data blocks allocated to a file are stored in its
inode. When a user requests an I/O operation on
the file, the kernel code converts the current
offset to a block number, uses this number as an
index in the block addresses table and reads or
writes the physical block. This figure represents
the structure of an inode:

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DIRECTORIES
Directories are structured in a hierarchical tree.
Each directory can contain files and
subdirectories.
Directories are implemented as a special type of
files. Actually, a directory is a file containing a list
of entries. Each entry contains an inode number
and a file name. When a process uses a
pathname, the kernel code searches in the
directories to find the corresponding inode
number. After the name has been converted to an
inode number, the inode is loaded into memory
and is used by subsequent requests.

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