Wassim

We’ve seen in a previous post that IEEE ratified wireless networks as 802.11 standards. They are commonly known as WiFi by the general public (for Wireless Fidelity). But this cool name hides some interesting challenges.   Wireless signal concepts Wireless network frequencies operate in the spectrum of microwaves. They explore the electromagnetic signals to transport data. Like other technologies where data is transported by electrical signals (in copper cables) or light (Fiber Optic cables) as carriers, data in WiFi is transported by an electromagnetism carrier signal that flies in the air. WiFi standards use high frequencies. The IEEE 802.11b and 802.11g use a frequency of 2.4GHz. That’s more than two billion oscillations of the waveform per second. In radio communication, higher frequencies mean shorter length of the wave, thus shorter covered distance. The challenges of wireless networks Interference: wireless networks use electromagnetic waves to transport information...

This post is an overview of the most popular LAN technology: Ethernet. A bit of History At first, the DIX group developed Ethernet II, then IEEE 802.3 group adopted Ethernet and added to it some improvements. All Ethernet technologies after Ethernet II are defined by the IEEE 802.3 body, from the early 10Base-5 and 10Base-2 till today. Standards such as Fast Ethernet and Gigabit Ethernet came to existence. And nowadays we are talking about 10Gigabit Ethernet and even 40Gigabit Ethernet. IEEE 802.3 is a working group that standardized wired Ethernet, on both layer 1 layer 2 of OSI model, just like other IEEE working groups. Each IEEE working group specializes in one technology. For example, IEEE 802.11 works on WiFi. Extract from IEEE802.org. We see the list of the IEEE working groups, including the 802.3 working group. All LAN and MAN technologies (such as Broadband, Wimax,…) can be referred to as IEEE 802 LAN/MAN standards . Ethernet Cabling 10Base-T uses category 3 UTP cables th...

In this post we will cover the recovery procedure steps of a Cisco ACS aka Secure Access Server. I assume you have the ACS Recovery disc, that you have physical access to the appliance and a valid Cisco license key. If you lost the recovery disc, I think it is possible to download an ISO file from cisco.com and burn it into a CD-ROM. What does Cisco ACS do? Cisco Access Secure Server is a AAA server from Cisco that integrates with your IT infrastructure and provides authentication, authorization and accounting. It is sure a more robust TACACS+ alternative . What could cause an ACS to crash? because it is based on Linux, an improper power off of the appliance may corrupt the database. Using an unsupported Internet browser is also risky. I faced this scenario in the past and the GUI went crazy. So I had to restore the box completely. Thank God I had backups! Cisco ACS recovery steps First, I attach a keyboard and monitor to the Cisco ACS server. I Insert the recovery disc into the drive ...

We will talk about MAC protocols, Ethernet technology and CSMA/CD, all operating at the lowest two layers of the OSI model, which corresponds to the Link layer in TCP/IP. Shared Ethernet In 10Base-2 and 10Base-5 networks, hosts share the same medium of communication. Shared Ethernet Card – copyright wikipedia.org Hosts on a shared Ethernet network physically connect to the same medium. Questions arise regarding who and when to send data on the medium first. → Medium Access Control protocols -aka MAC protocols- are the protocols that regulate communication on shared medium. MAC protocols There are three main MAC protocol families: Packet-switched radio networks eg. Aloha protocol “whenever I have data to send, I simply send it on the medium” “if there is collision, I send at a later time”  –> the algorithm is simple and has random behavior. with higher loads on the medium, collision rate increases and hosts spend more time re-sending data than sending n...

In this post, we make our neurons work a bit in the world of communication systems. We learn what the communication process is, discover some terms then attack modulation techniques. I remember I learned most of the following theory in my network engineering studies at the university. The communication process The communication process can be summarized in the following diagram: copyright Communication Systems, Haykin. The source of information delivers what is called the signal message . This is the message we want to transmit. It can be text, computer data, sounds, images or video. At the transmitter, level, the message signal is converted into symbols , and symbols are then encoded into a form that is understandable by the communication channel. At the receiver, the received signal is a set of encoded symbols . These are decoded and used to re-generate the original message signal. However, it is only an estimate of the original message signal, and that’s due to the noise in t...