If you encounter an error when trying to install a fresh operating system, it could be a problem with the date and time.
The other day a co-worker needed to borrow a Mac, so I grabbed an older MacBook Air from storage. I decided to wipe out the laptop’s flash storage and install a fresh version of OS X. I created a bootable USB flash drive installer, plugged it into the laptop, pressed the Option key as the machine booted, and then selected the USB drive as the boot disk.
I then ran Disk Utility to reformat the drive and then ran the installer to install OS X. It looked like things were moving along, until this appeared on the screen.
I created another boot disk using a different storage drive, in case it was a hardware problem, but I got the same error. I figured that the fact that the laptop had been in storage for a while had something to do with it, and it did.
Note: This fix should work with OS X Mavericks and El Capitan. It also worked for me with macOS Sierra, which shows a different error message (“The installer payload failed signature check” pops up near the end of the installation) than the one that appears for El Capitan and Mavericks.
How to fix the problem
The installer checks the date on the computer. If the date isn’t current, you get the error above. The fix involves correcting the date on your Mac.
If you have an older OS on the Mac
If you have a complete OS on the Mac already, boot into it. Fix the date in the Date & Time system preference (Apple menu > System Preferences). Reboot using the USB boot disk, and you should be able to proceed with the installation.
If you don’t have an OS on the Mac
If you are in a situation like I was, and you don’t have a complete OS on your disk (you reformatted it), you’ll find that there’s no way to access the Date & Time system preference when you use an external USB boot installer drive. The Apple menu doesn’t give you access to System Preferences. You have to use the Terminal to set the date and time.
If you use an external boot disk, you Mac starts up into OS X Disk Utilities. You can access the Terminal by clicking on the Utilities menu and selecting Terminal. Once the Terminal has launched, follow these steps.
- At the prompt, type
dateto see the date that’s on the Mac. For example, here’s the date of the MacBook Air I worked on.
- At the prompt, type
- If your Mac is connected to the internet, you can tell your Mac to check online for the current date and time. You can do this by typing at the prompt
ntpdate -u time.apple.comand then pressing Return.
Your Mac should now have the current date and time. If you enter
date again (as I did in the screenshot above) the information that appears will be current. You can now run the OS installer.
If you don’t have an internet connection
You can still use Terminal to set the date. Go into the Terminal as described above, then follow these steps.
- At the prompt, type
dateto see the date that’s on the Mac.
- Figure out the numerical representation of the date and time. The pattern you need to follow:
- Month: in two-digit format. For example, July is 07.
- Day: in two-digit format. For example, the 19th is 19. Another example: the 7th is 07.
- Time: in military format of hours and minutes. For example, 2:00 p.m. is 1400.
- Year: Last two digits.
- At the prompt, type
In my example, the numerical representation is 0719140016. This stands for July (07) 19, (19), 2 p.m. (1400), 2016 (16).
- At the prompt, type
date [numerical], with
[numerical]being the representation you figured out. In my example, I typed
date 0719140016. Then press Return.
The date should be set. You can check the date again by typing
date at the prompt. You can now run the installer.
This article is not dealing with technical terms and concentrates to deliver the content simple. If you are a programmer let me tell you, why you should prefer Framework ahead of Core PHP for your projects. If you are a business guy let me tell you, why it is so important to insist your software vendor to develop your web applications and websites with PHP Framework.
The comparison between Core PHP and PHP Framework can be related to Mathematics.
- To solve a complicated problem in scientific mathematics, you can either take a paper to work out, or you can use a scientific calculator to solve it.
- Working out mathematics in a paper is like coding in Core PHP, tapping a scientific calculator is like coding in Framework.
So what do I mean?
Core PHP vs PHP Framework
Core PHP – Maths With Paper
- Best student can solve the problem in few steps. Accuracy level – 75% to 100%.
- Average student may or may not solve the problem, he will write down few more steps to solve the same problem.
- Accuracy level – 50% to 75%.
- Poor student cannot solve the problems. Still he will write down, lot and lot of steps to solve the problem. Accuracy level – 0% to 50%.
Framework – Scientific Calculator
- Every student can solve the problem with 100% accuracy, once they learnt how to use the calculator.
- The predefined formulas in the calculator will provide you accurate results faster for any problem.
Problem With Core PHP
Core PHP becomes complicated, when people start writing their own logic in it.
One can bring the output in few lines of code, where the other can take a few hundred of lines to bring the same. Both of them cannot read each other’s code. So the problem starts here, that is inconsistency.
Why Choose Framework?
Framework assures reliability, consistency and a big time-saver. It has rich set of functionalities, so you don’t need to reinvent the wheel again and again. You will have almost all the functionalities to develop a PHP web application. Since it has been developed in OOPS, you can extend the existing functionality and create your own to have a full control over the application. Framework will not let you to write bad code, unless you purposely do it. When you work as a team, integrating your entire module becomes very easier, also it helps a lot in understanding each other’s code.
When you start developing a project, there are a lot of things, which you have to take care about, but we know only half of the things in it. Framework does everything for you, so you can be assured that your application is clean and safe. Inputs can be sanitized easily. MVC is one of the key functionality of Frameworks, separation of logics from views is a very good practice.
We all know that the client will surely come back to us one day to enhance the website with lot of his innovative requirements. If the project was done in Core PHP, you will have to say no for 50% of his new requirements, or simply you can tell him that the project is expired. But if the project was done is Framework, the beauty of Framework can be witnessed here. All you have to do is a cakewalk and give the updated project back to the client. I can assure this to you with the personal experiences I had.
Is Core PHP That Bad?
No, not at all. Core PHP helps you to understand the logics behind framework. Your logical thinking can be improved with Core PHP. The Core PHP becomes bad only when it goes to a bad programmer’s desk. Don’t dive into Framework without learning or coding in Core PHP. Please make sure that you read the full documentation before you start coding in Framework, writing Core PHP inside Framework has become common nowadays, it’s an insult to Frameworks.
We suggest our clients to prefer frameworks for their web applications. Choosing a framework depends upon the requirement of the project, will explain you about the functionalities and unique features of frameworks and how should you pick a framework in upcoming blogs.
Source: PAMIR WEB HOSTING
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Following are some important terms, which are frequently used in context of Computer Networks.
|1. ISO||The OSI model is a product of the Open Systems Interconnection project at the International Organization for Standardization. ISO is a voluntary organization.|
|2. OSI Model||Open System Interconnection is a model consisting of seven logical layers.|
|3. TCP/IP Model||Transmission Control Protocol and Internet Protocol Model is based on four layer model which is based on Protocols.|
|4. UTP||Unshielded Twisted Pair cable is a Wired/Guided media which consists of two conductors usually copper, each with its own colour plastic insulator|
|5. STP||Shielded Twisted Pair cable is a Wired/Guided media has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Shielding also eliminates crosstalk|
|6. PPP||Point-to-Point connection is a protocol which is used as a communication link between two devices.|
|7. LAN||Local Area Network is designed for small areas such as an office, group of building or a factory.|
|8. WAN||Wide Area Network is used for the network that covers large distance such as cover states of a country|
|9. MAN||Metropolitan Area Network uses the similar technology as LAN. It is designed to extend over the entire city.|
|10. Crosstalk||Undesired effect of one circuit on another circuit. It can occur when one line picks up some signals travelling down another line. Example: telephone conversation when one can hear background conversations. It can be eliminated by shielding each pair of twisted pair cable.|
|11. PSTN||Public Switched Telephone Network consists of telephone lines, cellular networks, satellites for communication, fiber optic cables etc. It is the combination of world’s (national, local and regional) circuit switched telephone network.|
|12. File Transfer, Access and Management (FTAM)||Standard mechanism to access files and manages it. Users can access files in a remote computer and manage it.|
|13. Analog Transmission||The signal is continuously variable in amplitude and frequency. Power requirement is high when compared with Digital Transmission.|
|14. Digital Transmission||It is a sequence of voltage pulses. It is basically a series of discrete pulses. Security is better than Analog Transmission.|
Following are some major differences between OSI Reference Model and TCP/IP Reference Model, with diagrammatic comparison below.
|OSI(Open System Interconnection)||TCP/IP(Transmission Control Protocol / Internet Protocol)|
|1. OSI is a generic, protocol independent standard, acting as a communication gateway between the network and end user.||1. TCP/IP model is based on standard protocols around which the Internet has developed. It is a communication protocol, which allows connection of hosts over a network.|
|2. In OSI model the transport layer guarantees the delivery of packets.||2. In TCP/IP model the transport layer does not guarantees delivery of packets. Still the TCP/IP model is more reliable.|
|3. Follows vertical approach.||3. Follows horizontal approach.|
|4. OSI model has a separate Presentation layer and Session layer.||4. TCP/IP does not have a separate Presentation layer or Session layer.|
|5. OSI is a reference model around which the networks are built. Generally it is used as a guidance tool.||5. TCP/IP model is, in a way implementation of the OSI model.|
|6. Network layer of OSI model provides both connection oriented and connectionless service.||6. The Network layer in TCP/IP model provides connectionless service.|
|7. OSI model has a problem of fitting the protocols into the model.||7. TCP/IP model does not fit any protocol|
|8. Protocols are hidden in OSI model and are easily replaced as the technology changes.||8. In TCP/IP replacing protocol is not easy.|
|9. OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them. It is protocol independent.||9. In TCP/IP, services, interfaces and protocols are not clearly separated. It is also protocol dependent.|
|10. It has 7 layers||10. It has 4 layers|
Diagrammatic Comparison between OSI Reference Model and TCP/IP Reference Model
TCP/IP means Transmission Control Protocol and Internet Protocol. It is the network model used in the current Internet architecture as well. Protocols are set of rules which govern every possible communication over a network. These protocols describe the movement of data between the source and destination or the internet. These protocols offer simple naming and addressing schemes.
Overview of TCP/IP reference model
TCP/IP that is Transmission Control Protocol and Internet Protocol was developed by Department of Defence’s Project Research Agency (ARPA, later DARPA) as a part of a research project of network interconnection to connect remote machines.
The features that stood out during the research, which led to making the TCP/IP reference model were:
- Support for a flexible architecture. Adding more machines to a network was easy.
- The network was robust, and connections remained intact untill the source and destination machines were functioning.
The overall idea was to allow one application on one computer to talk to(send data packets) another application running on different computer.
Description of different TCP/IP protocols
Layer 1: Host-to-network Layer
- Lowest layer of the all.
- Protocol is used to connect to the host, so that the packets can be sent over it.
- Varies from host to host and network to network.
Layer 2: Internet layer
- Selection of a packet switching network which is based on a connectionless internetwork layer is called a internet layer.
- It is the layer which holds the whole architecture together.
- It helps the packet to travel independently to the destination.
- Order in which packets are received is different from the way they are sent.
- IP (Internet Protocol) is used in this layer.
Layer 3: Transport Layer
- It decides if data transmission should be on parallel path or single path.
- Functions such as multiplexing, segmenting or splitting on the data is done by transport layer.
- The applications can read and write to the transport layer.
- Transport layer adds header information to the data.
- Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.
- Transport layer also arrange the packets to be sent, in sequence.
Layer 4: Application Layer
The TCP/IP specifications described a lot of applications that were at the top of the protocol stack. Some of them were TELNET, FTP, SMTP, DNS etc.
- TELNET is a two-way communication protocol which allows connecting to a remote machine and run applications on it.
- FTP(File Transfer Protocol) is a protocol, that allows File transfer amongst computer users connected over a network. It is reliable, simple and efficient.
- SMTP(Simple Mail Transport Protocol) is a protocol, which is used to transport electronic mail between a source and destination, directed via a route.
- DNS(Domain Name Server) resolves an IP address into a textual address for Hosts connected over a network.
Merits of TCP/IP model
- It operated independently.
- It is scalable.
- Client/server architecture.
- Supports a number of routing protocols.
- Can be used to establish a connection between two computers.
Demerits of TCP/IP
- In this, the transport layer does not guarantee delivery of packets.
- The model cannot be used in any other application.
- Replacing protocol is not easy.
- It has not clearly separated its services, interfaces and protocols.
It is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.
FUNCTIONS OF APPLICATION LAYER:
- Mail Services : This layer provides the basis for E-mail forwarding and storage.
- Network Virtual Terminal : It allows a user to log on to a remote host. The application creates software emulation of a terminal at the remote host. User’s computer talks to the software terminal which in turn talks to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals and allows user to log on.
- Directory Services : This layer provides access for global information about various services.
- File Transfer, Access and Management (FTAM) : It is a standard mechanism to access files and manages it. Users can access files in a remote computer and manage it. They can also retrieve files from a remote computer.
The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.
FUNCTIONS OF PRESENTATION LAYER:
- Translation : Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
- Encryption : It carries out encryption at the transmitter and decryption at the receiver.
- Compression : It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.