Wide Area Networks

fullgorgedcutΔίκτυα και Επικοινωνίες

24 Οκτ 2013 (πριν από 4 χρόνια και 15 μέρες)

131 εμφανίσεις

Chapter 10

Wide Area Networks

Part III: Wide Area Networks and Internetworking
Technologies


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

2


Topics Addressed in Chapter 10


What is a WAN?


Business rationale for WANs


WAN common carrier services


WAN hardware


WAN topologies


WAN data link protocols


WAN software


Transaction control processes in WANs


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

3


What is a WAN?


A wide area network (WAN) is one of the oldest kinds of data
communication networks


A WAN is a distributed network that covers a broad geographic area; a
WAN typically consists of multiple networks at geographically
distributed locations that are interconnected


Relative to LANs or MANs (see Figure 10
-
2) a WAN typically covers
a wider geographic area (see Figure 10
-
1) and operates at lower speeds


The Internet is the largest WAN that has been created


Many organizations leverage the Internet backbone to connect
geographically distributed sites


Some links in the Internet backbone have transmission speeds higher than
those found in LANs and WANs



Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

4


Figure 10
-
1

Figure 10
-
2


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

5


Business Rationale For WANs


Major motivations behind the development of early WANs were to:


Enable data transfers and communication among geographically dispersed
locations


Overcome computing capacity limitations at particular locations


Enable users at one location to leverage resources at other locations


Additional business reasons for today’s WANs include the ability to
leverage WAN infrastructure to:


Support groupware


EDI and other B2B applications


Reduce information float


Realize economies of scale


Provide an “electronic presence”


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

6


WAN Common Carrier Services


Communication media used to connect geographically
dispersed sites are typically supplied by common
carriers


Table 10
-
1 summarizes the major switched and leased
carrier services utilized in WANs in the U.S.


Today’s WANs employ the full range of carrier
services including mobile communication services


PBX, Centrex, VoIP, and VPNs are other important
facets of today’s WANs


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

7


Table 10
-
1


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

8


WAN Hardware


Virtually all data communication discussed in the text is utilized in
WANs including internetworking hardware, Web servers, firewalls,
and hardware used in voice
-
oriented networks. Other important WAN
hardware include:


Hosts


Terminals


Multiplexors


Concentrators


Front
-
end processors


Port concentrators


Port selectors


Cluster controllers


Several of these are illustrated in Figure 10
-
4


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

9


Figure 10
-
4


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

10


Hosts


In WANs, hosts have traditionally been
mainframes or midrange systems


In large WANs there are likely to be multiple
hosts interconnected by WAN links


Like severs in LANs, WAN hosts enable network
users to access application programs and database
management systems


Interconnections among WAN hosts enable users
at one location to access network resources at
other locations


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

11


Terminals


A
terminal

is an input/output device that can be connected
to al local or remote host


A variety of terminals are found in today’s WANs
including:


Microcomputers


Network computers (NCs)


aka
thin clients


Video display units (VDUs)


Display
-
only devices


Point
-
of
-
sale (POS) terminals


Automatic teller machines (ATMs)


Portable terminals


Touch
-
tone telephones


Sensor devices


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

12


Terminal Capabilities


Terminals have a wide range of capabilities


Traditional terminal classifications include:


Dumb terminals


Smart terminals


Intelligent terminals


Terminals also differ in terms of:


Input and output capabilities


Cost


Speed


Display attributes


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

13


Terminal Configurations and
Line Disciplines


Two options exist for attaching terminals to hosts:


Point
-
to
-
point

connections
in which there is a direct
communication path from one machine to another; this is most
feasible for local connections or for limited remote connections


Multipoint connections
in which several terminals share a
communication link (see Figure 10
-
7)


Line disciplines

(communication protocols) are needed to
control which station on a point
-
to
-
point or multipoint
connection is allowed to use the link. Line disciplines
include:


Contention


Polling
(see Figures 10
-
5 and 10
-
6)


Selection



Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

14


Figure 10
-
7


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

15


Multiplexors


Multiplexing

technologies allow multiple signals to be
transmitted over a single communication link;
multiplexors

(muxes) allow several devices to share a common circuit


Multiplexors combine data from all incoming lines and
transmits them via one line to another multiplexor which
separates the data and distributes it among outgoing terminal
lines


Figure 10
-
8 illustrates a generic multiplexor configuration


To terminal users, the multiplexor appears to function as though
there were several physical lines to the host instead of just one


Multiplexors avoid the need to have multiple connections
between two locations


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

16


Figure 10
-
8


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

17


Kinds of Multiplexors


A communication link is divided among several
users in two basic ways:


Frequency division multiplexing

(FDM)

which
separates the link’s available bandwidth into sub
-
channels, one for each incoming line (see Figure 10
-
9)


Time division multiplexing (TDM)

which separates link
into time slots. Each incoming line is given a time slot
for transmitting a byte (or bit)


see Figure 10
-
10(a)




Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

18


Figure 10
-
9


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

19


Figure 10
-
10(a)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

20


Statistical TDM


A major shortcoming of TDM is revealed when attached
lines do not have data to transmit


Times slots allocated to idle lines go unused (see Figure 10
-
10b
and Figure 10
-
11)


Communication circuit is not used to its fullest extent in this
circumstance


A statistical time division multiplexor (STDM) improves
on TDM efficiency by transmitting data only for lines with
data to send by reallocating time slots so that idle lines take
up none of the carrying capacity of the communication
circuit (see Figure 10
-
10c and Figure 10
-
12)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

21


Figure 10
-
10(b)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

22


Figure 10
-
10(c)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

23


Wavelength Division
Multiplexing (WDM)


Wavelength division multiplexing (WDM)

is a relatively new
multiplexing technique for optical transmissions over fiber optic cables


Traditionally, a single laser operating at a single wavelength has been
used to transmit signals over a fiber optic cable. WDM multiplexors,
however, leverage multiple lasers operating at multiple wavelengths to
transmit several simultaneous signals


WDM enables carriers to increase transmission capacity without
having to install more fiber optic cables


Dense WDM (DWDM)

combines WDM and TDM to further increase
the amount of data that can be transmitted over a single fiber


Today, more than 10 gbps can be transmitted over each DWDM circuit
and more than 40 DWDM circuits can be created on each fiber


Experts expect DWDM technologies to achieve 128 simultaneous 10 gbps
circuits in a few years


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

24


Multiplexor Configurations


Multiplexors can be added in a daisy
-
chain fashion (see
Figure 10
-
13)


A variety of multiplexors are found in today’s WANs
including:


Inverse multiplexors

(see Figure 10
-
14)


Data/voice multiplexors


T
-
n (T
-
1, FT
-
1, and T
-
3) multiplexors


Frame relay multiplexors


IDSN multiplexors


Local/short distance multiplexors


E.g. Fiber optic multiplexors


RS232 multiplexors


DSL routers and multiplexors



Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

25


Figure 10
-
14


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

26


Other WAN Line Sharing
Technologies


Other line sharing technologies used in
WANs include:


Concentrators
(see Figure 10
-
15)


Front
-
end processors (FEPs)
(see Figure 10
-
16)


Port concentrators

(see Figure 10
-
17)


Port selectors

(data switches)


Cluster controllers

(see Figure 10
-
18)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

27


Figure 10
-
16

Figure 10
-
18


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

28


WAN Topologies


Network topologies define how network nodes are
connected


WAN topologies vary widely, however, major forms
include:


Star networks

(see Figure 10
-
19)


Hierarchical networks

(see Figure 10
-
20)


Interconnected (plex or mesh) networks

(see Figure 10
-
21)


Hybrid networks


Table 10
-
2 compares network topologies on several
important criteria including cost, reliability, and
expandability


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

29


Figure 10
-
19


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

30


Figure 10
-
20


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

31


Figure 10
-
21


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

32


WAN Data Link Protocols


Two fundamental categories of WAN data link
protocols underlie WAN line disciplines (such as
contention and polling):


Asynchronous protocols


Often used in transaction processing to connect POS terminals
to hosts


Synchronous protocols


Used for high
-
volume data transfers between hosts and for
downloading/uploading files between hosts and terminals


These are illustrated in Figure 10
-
23 and compared in
Table 10
-
3



Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

33


Asynchronous Transmission


Asynchronous transmission (async) is the oldest data link
protocol


It is derived from technologies first used in the telegraph and
telephone industries. Supporting technology is well
-
developed and
low cost and many suitable applications perpetuate its use


Data is transmitted one character at a time (see Figure 10
-
22)


Receivers must be able to recognize the arrival of data and often,
to check it for errors


Senders and receivers must agree on message termination.
Schemes for this include:


Interrupt characters


Character count termination


Time
-
out intervals



Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

34


Synchronous Transmission


Synchronous protocols fall into three groups:


Character synchronous

protocols (see Figure 10
-
25)


BISYNC (Binary Synchronous Communications) is an
example (see Figure 10
-
26)


Byte count synchronous

protocols (see Figure 10
-
27)


DDCMP (Digital Data Communication Message Protocol) is
an example


Bit synchronous

protocols (see Figure 10
-
29)
Examples include:


SDLC (Synchronous Data Link Control)


HDLC (High
-
Level Data Link Control)


LAPB (Link Access Procedure, Balanced)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

35


Figure 10
-
25

Figure 10
-
27

Figure 10
-
29


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

36


WAN Software


A generic software configuration within a WAN
host is depicted in Figure 10
-
31. Major aspects of
this environment include:


Application software


Operating system (see Table 10
-
4)


I/O drivers


Access methods


Database management system (DBMS)


Teleprocessing monitor (or transaction control process)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

37


Figure 10
-
31


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

38


Table 10
-
4


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

39


Access Methods


Access methods provide connection,
disconnection, and data transfer services to
applications


Because an access method separates application
programs from terminal access logic, access
methods can be used with or without
teleprocessing monitors (see Figure 10
-
33)


VTAM (Virtual Telecommunications Access
Method) supports several alternative terminal
-
application connections


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

40


Figure 10
-
33


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

41


Teleprocessing Monitors


Teleprocessing monitors serve as a switch
between applications and terminals


Figures 10
-
34 and 10
-
35 illustrate
alternative teleprocessing monitor
configurations


Some teleprocessing monitors support
single
-
threading
, others
multithreading
(see
Figure 10
-
36)


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

42


Figure 10
-
36


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

43


Transaction Control Process


Key aspects of transaction control processes in
teleprocessing monitors include:


Maintaining context


Memory management


Transaction routing (see Figure 10
-
37)


Transaction logging


Capturing security statistics


Managing message priorities


Providing an operation interface for network managers


Application re
-
activation after failure


Deleting inactive processes


Additional activities are listed in Table 10
-
5


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

44


Figure 10
-
37


Business Data Communications (6e)

Copyright © 2003 by Pearson Education, Inc.


Stamper and Case


10
-

45


Table 10
-
5

Chapter 10

Wide Area Networks

Part III: Wide Area Networks and Internetworking
Technologies