Basic Working of Switch

In this section, we will see about basic switching. The LAN switching technology is  L2 (data-link layer) of the OSI model. 

The "switching means forwarding frames. In the data communication, all switching devices (namely the switches) implement two basic tasks: 

• Frame forwarding: forward the frames received from the input media to the corresponding output media.
• Address learning process: construct and maintain the switching address table to maintain the switch operation. The following describes the details of the two basic operations.

Frame Forwarding

The switch forwards frames according to the MAC address. When the switch forwards frames, the switch works based on following rules:

1. If the destination MAC address of the frame is broadcasting address or multicasting address, the frame is forwarded to all ports of the switch (except the source port of the frame).
2. If the destination address of the frame is a unicast address, but the address is not in the address table of the switch, the frame is forwarded to all ports (except the source port of the frame).
3. If the destination address of the frame is in the address table of the switch, forward the frame to the corresponding port according to the address table.
4. If the destination address and the source address of the frame are in the same network segment, the frame is discarded and switching is not performed.

Address Learning Process

In the address table of the switch, one entry is composed of one MAC address and the resident switch port number. The generation of the address table is through the dynamic self-learning, namely, when the switch receives a frame, the source address and the input port are recorded in the switching address table. 


Below flow chart illustrates the forwarding and learning of the received frames. 

When a frame reaches from a specific port, the switch gets the conclusion according to the two items: from port X, the workstation specified by the frame source address domain can be reached. 
Therefore, the switch can update the forwarding database for the MAC address. To allow the change of the network topology, each item of the database is configured with a life timer. 


When a new item is added to the database, the timer is started. The default value of the timer is 30 seconds. If the scheduled time is up, the item searches the database to check whether any item with the same address field value and frame address exists. 
If such item exists in the database, the content of the item is updated. Reset the timer. If such item does not exist in the database, add a new item in the database.
The address in the new item is the MAC address of the received frame.The port number is the port of the received frame, the timer value is set to the original value.


This was the basic information about working flow of switch. In coming posts we will see about VLAN Technology. 

Hope this information will be useful for beginners. For any feedback, Plz comment with your mail id and name. For new users you can use Name/URL option.  

3G Theory

Today I would like to discuss about 3G theory part and would like to correlate with data communication and connectivity options for Routers.

3G is third generation in mobile communication standards. Right now 3G is in launching phase in India. 3G is as comparatively superb than 2G. 3G, everybody knows we will get more downloading speed and fast access to internet.

What is use for 3G ? What a normal person can get benefit ? The simple answers everybody know are –

Video calling , watching youtube on phone,  good speed for downloading and using apps on phone..

Here you are.. When you are watching youtube on your phone, that means you are access internet at good speed. That means I can say, I can connect  my phone to my pc using nokia pc suite or any other software and access internet. Yes we can access internet using phone  as connectivity option with our laptop/PC.

                Here the idea comes in for one more connectivity option for ISP, Right now many of service provider are planning 3G as connectivity option and majorly , those ISP who have good coverage in metro cities are deployed  this as backup connectivity option for current customer to provide good SLAs.

I think , now it’s more clear that we can use 3G as media , it can anything USB or Simcard for connectivity.  As vendors saw this as new growing market, everybody came out with 3G products like SOHO routers, CPE routers, etc.

                These routers are compatible for 3G as connectivity option. In my previous post, I explained about 3G configurations. But I think I was suppose to write this theory part first , to make everyone understand about 3G configurations. It just a dialup media , same as you connect a USB to your laptop and dial it to ISP cloud and you get a IP and surf internet.Same process for router, Router also Dial first and get IP and access internet. Routers helps to share internet to local LAN users and can enable standard function using 3G as connectivity option.  

I can say, Now a days Router uses 3G as connectivity option like SIM card or USB to provide high speed internet connectivity for multiple
users over the 3G mobile phone networks. The widespread availability
of 3G networks and the reduced cost of mobile broadband has led to a revolution in the way that businesses and consumers connect to the internet. Many of ISP are using this as 3G as backup for MPLS customer also. Here using cellular media concern customer connect to ISP cloud in respective vrf and it can access other sites. ( we will discuss this in later posts)

Router uses a single 3G SIM card or USB and provides wired and wireless Wi-Fi connectivity as well as providing additional services such as IPsec VPN and VOIP over 3G network.One case study I already shared.

Some important facts –

3G has many flavours , WCDMA and CDMA 2000 is majorly used.

CDMA 2000 includes - 1xRTT-only, EVDO-only and hybrid.

WCDMA – its majorly gsm media.

Before doing configuration confirm the flavour, Then it will be easy to configure in routers. Since both has bit configuration difference. You can check in configurations examples –

I can brief you –

• WCDMA needs correct APN name, majorly no need of username and password. Authentication happens based on APN.
• CDMA2000 needs username and password must for authentication.

Here I want share some important information about 3 G interfaces  and its communication process, –

             

             The 3G interface consists of an external modem  that you plug into the router. The card provides two logical components from a routers point of view.First, there is a control channel used by the router to talk to the modem for device setup and call control. It appears as line x in IOS. You use a chat script and AT commands to talk to the modem.

           Second, there is the data interface, Cellular X. After a connection has been established, all IP-related information and data flow goes via this interface. Its is designed like a dial on demand interface (BrI, for example), so you can put dial related stuff directly there, or use a Dialer interface that gets bound to the Cellular interface. The latter provides the advantage of an always-on setup through dialer persistent.

            The modem itself acts as a ppp server to the router (there is no PPP between the modem and the 3G cloud). It attaches via a profile to the ISP, and the APN of the ISP assigns you the information required for Internet access. The router will talk to the modem via ppp and the modem supplies the information received from the ISP (IP address, gateway, name servers etc.).

You can check coverage with sh cellular 0 all

This explanation is about 3G connectivity process.  

I hope this post will help for beginners, who are new to 3G and want to setup 3G as connectivity option in Router.  Respective configuration examples are already posted in 3G label.

For any query and feedback, Plz comment using your Name and mail id.. for easy way use Name/URL option..