Sunday, November 22, 2009

Quadrature Phase-shift Keying (QPSK)

Quadrature Phase-shift Keying

  • Sometimes known as quaternary or quadriphase PSK or 4-PSK, QPSK uses four points on the constellation diagram, equispaced around a circle.
  • With four phases, QPSK can encode two bits per symbol-twice the rate of BPSK.
  • This is used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed.
  • Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers.
  • With this interpretation, the even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature -phase component of the carrier.
  • BPSK is used on both carriers and they can be independently demodulated.

Binary Phase Shift Keying (BPSK)

BPSK is the simplest form of PSK.

  • It uses two phases which are separated by 180° and so can also be termed 2-PSK.
  • It does not particularly matter exactly where the constellation points are positioned
  • It is, only able to modulate at 1bit/symbol and so is unsuitable for high data-rate applications.


Modulation is the process by which a carrier wave is able to carry the message or digital signal (series of ones and zeroes).
There are three basic methods to this:-

  • Amplitude,
  • Frequency
  • Phase shift keying.

Higher orders of modulation allow us to encode more bits per symbol or period (time).

Wednesday, September 9, 2009

How DHCP works?

Technical details

Schema of a typical DHCP session
DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.
DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.
After the client obtained an IP address, the client may start an address resolution query to prevent IP conflicts caused by address poll overlapping of DHCP servers.

DHCP discovery
The client broadcasts on the local physical subnet to find available servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server on a different subnet. This client-implementation creates a UDP packet with the broadcast destination of or subnet broadcast address.
A client can also request its last-known IP address (in the example below, If the client is still in a network where this IP is valid, the server might grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP immediately. A non-authoritative server simply ignores the request, leading to an implementation dependent time out for the client to give up on the request and ask for a new IP.

DHCP offers
When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and sending a DHCPOFFER message across the network to the client. This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.
The server determines the configuration, based on the client's hardware address as specified in the CHADDR field. Here the server,, specifies the IP address in the YIADDR field.

DHCP requests
When the client PC receives an IP lease offer, it must tell all the other DHCP servers that it has accepted an offer. To do this, the client broadcasts a DHCPREQUEST message containing the IP address of the server that made the offer. When the other DHCP servers receive this message, they withdraw any offers that they might have made to the client. They then return the address that they had reserved for the client back to the pool of valid addresses that they can offer to another computer. Any number of DHCP servers can respond to an IP lease request, but the client can only accept one offer per network interface card.

DHCP acknowledgement
When the DHCP server receives the DHCPREQUEST message from the client, it initiates the final phase of the configuration process. This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the TCP/IP configuration process is complete.
The server acknowledges the request and sends the acknowledgement to the client. The system as a whole expects the client to configure its network interface with the supplied options.

Cisco Router Debug Commands

Cisco Command
Enable debug for RIP
debug ip rip
Enable summary IGRP debug information
debug ip igrp events
Enable detailed IGRP debug information
debug ip igrp transactions
debug ipx routing activity
debug IPX SAP
Enable debug for CHAP or PAP
debug ppp authentication
Switch all debugging off
no debug all
undebug all

Cisco Router Copy Commands

Cisco Command
Save the current configuration from DRAM to NVRAM
copy running-config startup-config
Merge NVRAM configuration to DRAM
copy startup-config running-config
Copy DRAM configuration to a TFTP server
copy runing-config tftp
Merge TFTP configuration with current router configuration held in DRAM
copy tftp runing-config
Backup the IOS onto a TFTP server
copy flash tftp
Upgrade the router IOS from a TFTP server
copy tftp flash

Cisco Router Basic Operations commands

Cisco Command

Enter privileged mode

Return to user mode from privileged

Exit Router
Logout or exit or quit

Recall last command
up arrow or

Recall next command
down arrow or Suspend or abort and 6 then x

Complete Command

Cisco Router Show Commands

Cisco Command

View version information
show version
View current configuration (DRAM)
show running-config
View startup configuration (NVRAM)
show startup-config
Show IOS file and flash space
show flash
Shows all logs that the router has in its memory
show log
View the interface status of interface e0
show interface e0
Overview all interfaces on the router
show ip interfaces brief
View type of serial cable on s0
show controllers 0 (note the space between the 's' and the '0')
Display a summary of connected cdp devices
show cdp neighbor
Display detailed information on all devices
show cdp entry
Display current routing protocols
show ip protocols
Display IP routing table
show ip route
Display access lists, this includes the number of displayed matches
show access-lists
Check the router can see the ISDN switch
show isdn status
Check a Frame Relay PVC connections
show frame-relay pvc
show lmi traffic stats
show frame-relay lmi
Display the frame inverse ARP table
show frame-relay map

Cisco Router Configuration Commands

*Cisco Command*

Set a console password to cisco
Router(config)#*line con 0*
Router(config-line)#*password cisco*
Set a telnet password
Router(config)#*line vty 0 4*
Router(config-line)#*password cisco*
Stop console timing out
Router(config)#*line con 0*
Router(config-line)#*exec-timeout 0 0*
Set the enable password to cisco
Router(config)#*enable password cisco*
Set the enable secret password to peter.
This password overrides the enable password and is encypted within the
config file
Router(config)#*enable secret peter*
Enable an interface
Router(config-if)#*no shutdown*
To disable an interface
Set the clock rate for a router with a DCE cable to 64K
Router(config-if)*clock rate 64000*
Set a logical bandwidth assignment of 64K to the serial interface
Router(config-if)*bandwidth 64*
*Note that the zeroes are not missing*
To add an IP address to a interface
Router(config-if)#*ip addr*
To enable RIP on all 172.16.x.y interfaces
Router(config)#*router rip*
Disable RIP
Router(config)#*no router rip*
To enable IRGP with a AS of 200, to all interfaces
Router(config)#*router igrp 200*
Disable IGRP
Router(config)#*no router igrp 200*
Static route the remote network is, with a mask of,
the next hop is, at a cost of 5 hops
Router(config)#*ip route 5*
Disable CDP for the whole router
Router(config)#*no cdp run*
Enable CDP for he whole router
Router(config)#*cdp run*
Disable CDP on an interface
Router(config-if)*#no cdp enable*

What is Software Patch?

*Understanding Patches*
When vendors become aware of vulnerabilities in their products, they often
issue patches to fix the problem. Make sure to apply relevant patches to
your computer as soon as possible so that your system is protected.

*What are patches?*
Similar to the way fabric patches are used to repair holes in clothing,
software patches repair holes in software programs. Patches are updates
fix a particular problem or vulnerability within a program. Sometimes,
instead of just releasing a patch, vendors will release an upgraded
of their software, although they may refer to the upgrade as a patch.

*How do you find out what patches you need to install?*
When patches are available, vendors usually put them on their websites for
users to download. It is important to install a patch as soon as possible
protect your computer from attackers who would take advantage of the
vulnerability. Attackers may target vulnerabilities for months or even
after patches are available. Some software will automatically check for
updates, and many vendors offer users the option to receive automatic
notification of updates through a mailing list. If these automatic options
are available, we recommend that you take advantage of them. If they are
available, check your vendors' websites periodically for updates.
Make sure that you only download software or patches from websites that
trust. Do not trust a link in an email message—attackers have used email
messages to direct users to malicious websites where users install viruses
disguised as patches. Also, beware of email messages that claim that they
have attached the patch to the message—these attachments are often viruses
(see Using Caution with Email Attachments for more information).

Interview question's for desktop engg

1. Name another way of joining a PC to domain?
2. A manager cannot print to a network printer but all other users can, what do you do?
3. What’s the printer registry file name?
4. Name 5 reasons why a user might not be able to logon to the network?
5. Explain how DHCP works?
6. A user has a laptop that runs fast on the network (at work) but slow at home? Explain what could be the problem.
7. Explain how to share outlook calendars and the type of permissions you need to assign?
8. A user can log on to the network but has no internet connection? Explain what could be the problem.
9. A user’s application on their desktop is not working; explain what you would do to resolve this problem?
10. What is the difference between a UNC and drive mapping?
11. How do you import external data into an Excel spreadsheet?
12. What is a mail merge and how does it work?
13. Explain what TCP/IP is and how it works?
14. A user operating his laptop from home finds it very slow, but when using the laptop at the office is very fast. What could the possible reasons be while it was slow at home?
15. A Manager takes his laptop abroad and operates it with no problem using his mobile. Once he gets back the UK his modem no longer works. What could the possible reasons for this be?
16. What could be the cause of an error stating that NTLDR is missing when trying to boot?
17. What is the file extension for an Excel Add-in file?
18. What is the difference between a .PST file and an .OST file?
19. How would you obtain the MAC address from a machine?
20. A blackberry user is abroad and is not receiving e-mails; he notices that he has gprs in lowercase on his device. What can be done to fix this problem?
21. Name 5 reasons why a user might not be able to logon to the network?
22. In word how would you alter auto recovery to save every 5 mins instead of every 10 ?
23. If a customer’s on blackberry stopped syncing their calendar wirelessly, what would you do?
24. If a customer deleted a mail from his blackberry, but it did delete from his mailbox, how would you resolve this?
25. The deputy equities manager demands access to a legal shared drive, as he does not have the correct permissions, it is late at night and you are the only one left in the office what would you do? Follow on: if the decision makers are unreachable, what would you do? if the deputy equity manager tells you a multi-million pound contract depends on him getting access to this shared drive, what would you do ? if he then gets the equities manager who starts shouting at you and threatening to get you the sack, what would you do ?

How to disable USB Pendrive /USB Storage Device using Server Group Policy

How to disable USB Pendrive /USB Storage Device using Server Group Policy?

follow the 4 steps below.

1.) Take the following text, copy it, and paste it into a text
document. Then, save it as USBSTOR.ADM.
CATEGORY "Custom Policies"
KEYNAME "SYSTEM\CurrentControlSet\Services\UsbStor"
POLICY "USB Mass Storage Installation"
EXPLAIN "When this policy is enabled, USB mass storage device
permissions can be changed by using the drop down box.
Selecting 'Grant Permission' will allow USB mass storage devices to be
installed. Selecting 'Deny Permission' will prohibit
the installation of USB mass storage devices.
IF REMOVING THIS POLICY: Reset to original setting and let policy
propegate before deleting policy."
NAME "Deny Permission" VALUE NUMERIC 4
2.) Open a group policy management console (gpedit.msc), and right
click on "administrative templates" under "Computer Configuration".
Select "Add/Remove Templates".
3.) Browse to the text document you just saved and click OK. You'll
now see "Custom Policies" under "Administrative Templates". Right
click on it, select "View", then select "Filtering". Uncheck the
bottom box, labeled "Only show policy settings that can be fully
4.) Click ok. Now you'll see the USB policy available for use under
the custom policy heading. From there, you can enable or disable it
just like any other policy.

Server Consolidation

The Challenge: The unimpeded growth of data centers over the last decade along with the popularity of distributed applications has resulted in a proliferation of servers, storage systems, and operating systems. Most of these applications are on isolated, single-function physical servers that are over-sized and under-utilized. The result:
Single function servers and application silos are wasting capacity.
Administration costs are exorbitant due to the increased complexity and lack of manageability of the data center environment.
Managing, patching, and securing servers is cumbersome, error-prone and time consuming.
Our Solution: Virtual Iron streamlines and automates server consolidation and reduces complexity throughout the data center. The software creates an optimized "virtual" data center that provides dynamic infrastructure built on industry standards and is flexible to changing business needs.

The Virtual Iron platform allows a physical server of any size (from a single CPU to a 32-way multi-processor) to be partitioned into multiple virtual servers that work identically to a physical server. Each virtual server can expand as demand dictates by allocating capacity that is available on the physical server. With this capability, Virtual Iron delivers a robust server consolidation solution:
A virtual computing platform that supports multiple virtual servers, up to 32 running concurrently, and multiple workloads per physical server; and adapts in real-time by delivering capacity based on the resource demands of the applications.
Efficient server virtualization with minimal performance overhead; optimized to take full advantage of hardware-assisted virtualization built into today’s processors from AMD and Intel.
Support for running unmodified 32- and 64-bit x86 guest operating systems including Windows XP, Windows Server 2003, RHEL4, and SLES 9.
Comprehensive policy-based management of virtual infrastructure capable of handling a wide range of applications and workloads.
Reduced risk of performance drag due to automated capacity management.
Up to 10X improvement in resource utilization

Terminal Services Gateway (TS Gateway)

Terminal Services Gateway (TS Gateway) is a role service in the Terminal Services server role of Windows Server® 2008 that allows authorized remote users to connect to resources on an internal corporate or private network, from any Internet-connected device. The network resources can be terminal servers, terminal servers running RemoteApp programs, or computers with Remote Desktop enabled.
TS Gateway uses Remote Desktop Protocol (RDP) over HTTPS to establish a secure, encrypted connection between remote users on the Internet and the internal network resources on which their productivity applications run.

What does TS Gateway do?
TS Gateway provides many benefits, including:
•TS Gateway enables remote users to connect to internal network resources over the Internet, by using an encrypted connection, without needing to configure virtual private network (VPN) connections.
•TS Gateway provides a comprehensive security configuration model that enables you to control access to specific internal network resources.
•TS Gateway provides a point-to-point RDP connection, rather than allowing remote users access to all internal network resources.
•TS Gateway enables most remote users to connect to internal network resources that are hosted behind firewalls in private networks and across network address translators (NATs).. With TS Gateway, you do not need to perform additional configuration for the TS Gateway server or clients for this scenario.
Prior to this release of Windows Server, security measures prevented remote users from connecting to internal network resources across firewalls and NATs. This is because port 3389, the port used for RDP connections, is typically blocked for network security purposes at the firewalls. TS Gateway transmits RDP traffic to port 443 instead, by using an HTTP Secure Sockets Layer/Transport Layer Security (SSL/TLS) tunnel. Because most corporations open port 443 to enable Internet connectivity, TS Gateway takes advantage of this network design to provide remote access connectivity across multiple firewalls.
•The TS Gateway Manager snap-in console enables you to configure authorization policies to define conditions that must be met for remote users to connect to internal network resources. For example, you can specify:
•Who can connect to network resources (in other words, the user groups who can connect).
•What network resources (computer groups) users can connect to.
•Whether client computers must be members of Active Directory security groups.
•Whether device and disk redirection is allowed.
•Whether clients need to use smart card authentication or password authentication, or whether they can use either method.

•You can configure TS Gateway servers and Terminal Services clients to use Network Access Protection (NAP) to further enhance security. NAP is a health policy creation, enforcement, and remediation technology that is included in Windows® XP Service Pack 2 (SP2), Windows Vista®, and Windows Server 2008. With NAP, system administrators can enforce health requirements, which can include software requirements, security update requirements, required computer configurations, and other settings.

Terminal Services Gateway (TS Gateway)
TS Gateway is new features on Windows Server 2008. You must install a Windows Server 2008 to do this.

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