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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 600-601 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 600
SEC. 8.3
DISTRIBUTED SYSTEMS
569
Pentium
Windows
Middleware
Middleware
Middleware
Middleware
Application
Pentium
Linux
Application
SPARC
Solaris
Application
Mac OS
Application
Macintosh
Common base for applications
Network
Figure 8-27.
Positioning of middleware in a distributed system.
Networks
), which can be citywide, countrywide, or worldwide. The most impor-
tant kind of LAN is Ethernet, so we will examine that as an example LAN.
As our
example WAN, we will look at the Internet, even though technically the Internet is
not one network, but a federation of thousands of separate networks. However, for
our purposes, it is sufficient to think of it as one WAN.
Ethernet
Classic Ethernet, which is described in IEEE Standard 802.3, consists of a co-
axial cable to which a number of computers are attached. The cable is called the
Ethernet
, in reference to the
luminiferous ether
through which electromagnetic ra-
diation was once thought to propagate. (When the nineteenth-century British phys-
icist James Clerk Maxwell discovered that electromagnetic radiation could be de-
scribed by a wave equation, scientists assumed that space must be filled with some
ethereal medium in which the radiation was propagating. Only after the famous
Michelson-Morley experiment in 1887, which failed to detect the ether, did physi-
cists realize that radiation could propagate in a vacuum.)
In the very first version of Ethernet, a computer was attached to the cable by li-
terally drilling a hole halfway through the cable and screwing in a wire leading to
the computer. This was called a
vampire tap
, and is illustrated symbolically in
Fig. 8-28(a).
The taps were hard to get right, so before long, proper connectors
were used. Nevertheless, electrically, all the computers were connected as if the
cables on their network interface cards were soldered together.
Page 601
570
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
Computer
Ethernet
Switch
Computer
Ethernet
(b)
(a)
Vampire tap
Figure 8-28.
(a) Classic Ethernet. (b) Switched Ethernet.
With many computers hooked up to the same cable, a protocol is needed to
prevent chaos.
To send a packet on an Ethernet, a computer first listens to the
cable to see if any other computer is currently transmitting.
If not, it just begins
transmitting a packet, which consists of a short header followed by a payload of 0
to 1500 bytes.
If the cable is in use, the computer simply waits until the current
transmission finishes, then it begins sending.
If two computers start transmitting simultaneously, a collision results, which
both of them detect. Both respond by terminating their transmissions, waiting a
random amount of time between 0 and
T
μ
sec and then starting again. If another
collision occurs, all colliding computers randomize the wait into the interval 0 to
2
T
μ
sec, and then try again. On each further collision, the maximum wait interval
is doubled, reducing the chance of more collisions. This algorithm is known as
binary exponential backoff
.
We saw it earlier to reduce polling overhead on
locks.
An Ethernet has a maximum cable length and also a maximum number of
computers that can be connected to it.
To exceed either of these limits, a large
building or campus can be wired with multiple Ethernets, which are then con-
nected by devices called
bridges
.
A bridge is a device that allows traffic to pass
from one Ethernet to another when the source is on one side and the destination is
on the other.
To avoid the problem of collisions, modern Ethernets use switches, as shown in
Fig. 8-28(b).
Each switch has some number of ports, to which can be attached a
computer, an Ethernet, or another switch. When a packet successfully avoids all
collisions and makes it to the switch, it is buffered there and sent out on the port
where the destination machine lives. By giving each computer its own port, all
collisions can be eliminated, at the cost of bigger switches. Compromises, with just
a few computers per port, are also possible.
In Fig. 8-28(b), a classical Ethernet
with multiple computers connected to a cable by vampire taps is attached to one of
the ports of the switch.
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