Download Networking and Communication: Bus, switch, and network structures

Networking and Communication:
Bus, switch, and network structures
and protocols, I/O, Synchronization
Ray Tsai, Shui wing Yim
CS490 (Fall 2003)
10/20/03
Jeff Miller
Networking Basic
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Network - A network is a group of
computers connected together in a way
that allows information to be exchanged
between the computers.
Node - A node is anything that is
connected to the network. While a node is
typically a computer, it can also be
something like a printer or CD-ROM tower.
Segment - A segment is any portion of a
network that is separated, by a switch,
bridge or router, from other parts of the
network.
Topology - Topology is the way that each
node is physically connected to the
network. Common topologies include:
BUS
RING
STAR
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Bus - Each node is daisy-chained
(connected one right after the other)
along the same backbone.
Information sent from a node travels
along the backbone until it reaches
its destination node. Each end of a
bus network must be terminated with
a resistor to keep the signal that is
sent by a node across the network
from bouncing back when it reaches
the end of the cable.
Bus
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Ring - Like a bus network, rings have the
nodes daisy-chained. The difference is
that the end of the network comes back
around to the first node, creating a
complete circuit. In a ring network, each
node takes a turn sending and receiving
information through the use of a token.
The token, along with any data, is sent
from the first node to the second node,
which extracts the data addressed to it
and adds any data it wishes to send.
Ring
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Star - In a star network, each node is
connected to a central device called a
hub. The hub takes a signal that comes
from any node and passes it along to
all the other nodes in the network. A
hub does not perform any type of
filtering or routing of the data. It is
simply a junction that joins all the
different nodes together.
Start
Network Structure
There are two common types of home
networks: peer-to-peer and client/server.
Client/server networks have a centralized
administrative system that provides
information to all of the other devices.
Peer-to-peer means that each device can talk
directly to each other device on the network
without consulting a central system first. The
first computer that you install the software on
becomes the Application Server.
Clients and Servers
In general, all of the machines on the Internet
can be categorized as two types: servers and
clients. Those machines that provide services
(like Web servers or FTP servers) to other
machines are servers. And the machines that
are used to connect to those services are
clients. When you connect to Yahoo! at
www.yahoo.com to read a page, Yahoo! is
providing a machine (probably a cluster of
very large machines), for use on the Internet,
to service your request. Yahoo! is providing a
server. Your machine, on the other hand, is
probably providing no services to anyone else
on the Internet.
Switch
Switches are a fundamental part of most
networks. They make it possible for several
users to send information over a network at
the same time without slowing each other
down. Switches allow different nodes (a
network connection point, typically a
computer) of a network to communicate
directly with one another in a smooth and
efficient manner.
Switches usually work at Layer 2 (Data or
Datalink) of the OSI Reference Model,
using MAC addresses, while routers
work at Layer 3 (Network) with Layer 3
addresses (IP, IPX or Appletalk,
depending on which Layer3 protocols
are being used). The algorithm that
switches use to decide how to forward
packets is different from the algorithms
used by routers to forward packets.
Packet-based switches use one of three
methods for routing traffic:
 Cut-through
 Store-and-forward
 Fragment-free
Cut-through switches read the MAC address
as soon as a packet is detected by the
switch. After storing the 6 bytes that make up
the address information, they immediately
begin sending the packet to the destination
node, even as the rest of the packet is
coming into the switch.
In above example, if either switch A or
C fails, then the nodes connected to
that particular switch are affected, but
nodes at the other two switches can
still communicate. However, if switch
B fails, then the entire network is
brought down. What if we add
another segment to our network
connecting switches A and C?
In this case, even if one of the switches fails,
the network will continue. This provides
redundancy, effectively eliminating the
single point of failure.
But now we have a new problem. In the last
section, you discovered how switches learn
where the nodes are located. With all of the
switches now connected in a loop, a packet
from a node could quite possibly come to a
switch from two different segments. For
example, imagine that Node B is connected
to Switch A, and needs to communicate
with Node A on Segment B. Switch A does
not know who Node A is, so it floods the
packet.
The packet travels via Segment A or Segment C to
the other two switches (B and C). Switch B will
add Node B to the lookup table it maintains for
Segment A, while Switch C will add it to the
lookup table for Segment C. If neither switch has
learned the address for Node A yet, they will
flood Segment B looking for Node A. Each
switch will take the packet sent by the other
switch and flood it back out again immediately,
since they still don't know who Node A is. Switch
A will receive the packet from each segment and
flood it back out on the other segment. This
causes a broadcast storm as the packets are
broadcast, received and rebroadcast by each
switch, resulting in potentially severe network
congestion.
IP Addresses
To keep all of these machines straight,
each machine on the Internet is assigned a
unique address called an IP address. IP
stands for Internet protocol, and these
addresses are 32-bit numbers, normally
expressed as four "octets" in a "dotted
decimal number." A typical IP address
looks like this:
216.27.61.137
The four numbers in an IP address are
called octets because they can have
values between 0 and 255, which is 28
possibilities per octet.
Protocols
Once a client has connected to a service on
a particular port, it accesses the service
using a specific protocol. The protocol is
the pre-defined way that someone who
wants to use a service talks with that
service. The "someone" could be a person,
but more often it is a computer program like
a Web browser. Protocols are often text, and
simply describe how the client and server
will have their conversation.
Perhaps the simplest protocol is the daytime
protocol. If you connect to port 13 on a
machine that supports a daytime server, the
server will send you its impression of the
current date and time and then close the
connection. The protocol is, "If you connect
to me, I will send you the date and time and
then disconnect." Most UNIX machines
support this server. If you would like to try it
out, you can connect to one with the Telnet
application. In UNIX, the session would
look like this:
%telnet web67.ntx.net 13 Trying
216.27.61.137... Connected to
web67.ntx.net. Escape character is '^]'.
Sun Oct 25 08:34:06 1998 Connection
closed by foreign host.
Five Classic Components of a
Computer
Processor
1. Control
2. Datapath
Input
Memory
Output
Why have I/O?
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Interactive Applications
Long Term Storage
Swap for Virtual Memory
Different types of devices(different
performance)
Hide differences between devices
Sharing of devices among different
applications
I/O Devices
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Provide input and output to a computer’s
processor (AKA: peripherals)
Measuring Performance: response time
vs. throughput (data rate)
Examples:
Device
Function
Data rate (KB/sec)
Keyboard/Mouse
Input
.01/.02
Video Card/Printer
Output
60000/200
Modem/LAN Card
Input+Output
8/500-6000
Optical/Magnetic
Disk
Storage
2000-10000
Magnetic Disks
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Nonvolatile storage
Slower, but less expensive level of
memory hierarchy
Access Time = seek time +
rotational latency + transfer time +
overhead
Average rotation delay = 0.5/RPM
Communication between
processor/memory and I/O devices
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Buses – PCI, SCSI
Interface
1. Memory mapped I/O – Single Bus for memory and I/O
2. Separate I/O Bus, separate I/O Instructions
Device Controllers – deal with control of the I/O device, hides
complexity from processor, processor communicates with
controller
I/O Control – Polling (simple, uses processor time) vs.
Interrupts
Data Movement
1. Programmed I/O – processor touches the data (uses up
processor time)
2. DMA – processor sets up transfer, DMA controller transfers
data (complicates memory system)
Synchronization
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Definition – process of coordinating the
behavior of two or more processes,
which may be running on different
processors
1) Locking - Semaphores
2) Message Passing
References
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Computer Organization & Design: The
Hardware/Software Interface, Second
Edition by Patterson and Hennessy.
(Chapters 8-9)
http://computer.howstuffworks.com