 Hello and welcome to a lecture on Networking Devices 3, Routers and Getway. Learning Outcomes By the end of this session, students will be able to describe the operation of internet working devices like routers and getways. Before starting this topic, you may pause here the video and find out the answer to this question. What are the limitations of bridge? These are some limitations of bridge which are overcome by the router. What are those? Let's see. Linking of two dissimilar networks. Router can overcome the limitation of routing data selectively and efficiently. It can filter and it can route as well. Enforcement of security is an additional feature of a router over bridge and routers resist to broad cost storms. Router is a three layer relay which operates in the network layer of OSR model and it works on or it responds and take action on network layer frames. Network layer frames are generally referred to as data packets. Router forwards data packets between the computer networks. Router performs the traffic directing functions on the internet. Internet is nothing but a group or collection of number of computer networks and the data packet goes through the number of computer networks before it reaches to the destination. The router reads the network address information in the data packet to determine the ultimate destination path of the data to be transferred on a computer network. Router use the information in its routing table and using that information it directs the packet to the next network on its journey. Router isolates LANs into sub networks and it can manage and control the network traffic. In figure number one you can see the communication of data packet through the router. Here it is clear that router works in the three layers that is network layer, data link layer and physical layer. These are the bottom three layers of OSR model. Then what is router made of? Router has many of the same components as your computer. It has CPU, it has memory, it has input-output interfaces. Mostly these input-output interfaces are network interfaces beyond these network interfaces. Some administrative interfaces are there along with operating system and router is a small computer with specialized hardware and operating system specially designed for forwarding the packets to a network. Let us see the router hardware. Figure number shows the schematic diagram of router here and these are some internal blocks of your router. The router has input ports and sometimes referred to as input buffers as well as output buffers. It has N number of input buffers for N number of output buffers. It has routing processor and switching fabric. Input ports are used to receive the packets and output ports are generally used to transmit the packaging fabric, connect the input port to the correct output port while routing processor maintains and execute the data directing functions. We will discuss each in detail network interfaces. Routers have modular network interfaces. These network interfaces can be added, removed or replaced as need changes. Some routers can accept network interfaces of different types, means using the same network interface we can connect the Ethernet LAN as well as token ring LANs. Each network interface consists of input buffer, output buffer and routing processor. These routing processors are separately provided in high end routers. For low end routers the routing processor is shared. The second part of a router is input buffers. Input buffers are provided one per network interface means for single network interface we will be having a single input buffer and these input buffers are used to store incoming packets before they are processed. The input port performs physical and data link layer functions of the router. Here in figure 3 you can see the generalized block diagram of the input buffer. Whenever the packet comes it is processed by a physical layer processor then data link layer processor and it is queued here in this buffer and this queue holds the packet before forwarding the packet to the switching fabric. The incoming packets or network interface are placed into the input buffer and the input buffers are nothing but very high speed memory for packet queuing before they are processed. The packet is stored here until the routing processor is available. The network interface may have a routing processor which would have a copy of forwarding or routing table to prevent simultaneous access to the network interface and it would use forwarding table and with the help of the forwarding table it can configure the switching fabric to forward the packet to the correct output buffer. The next part of a router is the routing processor. The routing processor carries out the functions of the network layer. The routing processor performs the two major functions. The first function is to maintain and exchange the routing or forwarding table with other routers in the network. This exchange of routing data involves computing the forwarding table from the data packets received by other routers as well. The second function is it uses the forwarding table to configure the switching fabric to forward the packet to the correct output port or output buffer. Routing processor is nothing but a software which runs on a CPU of your router. There are two types of processors. First is of the shelf CPU. It is inexpensive but the performance of this is low as they are not optimized or designed for the type of operation a router typically needs to perform. The second type is application specific integrated circuits. It is expensive to design and implement because it needs more time and money. But these application specific integrated circuit processors are optimized for typical routing operations. And high end routers use these processors to achieve higher performance levels. The next part of a router is switching fabric. Switching fabric moves the packet from input buffer into the correct output buffer. The routing processor decides the correct output buffer with the help of the forwarding or routing information or routing table. There are three major types of switching fabrics. The first is in-memory switching fabric. The packets are input into the routing processor's memory and out into the correct output buffer. The second type is bus based switching fabrics. The packets move along a shared bus. The bus is similar to a network bus and this bus leads the packet to the correct output buffer. The third type of switching fabric is crossbar and it is very simple and very efficient as well. The third type is crossbar switching fabric. This switching fabric moves the packet through a grid of redundant buses. If any of the bus fails, the alternate part exists so that the forwarding can continue. In this figure, number four, you can see the crossbar switching fabric. It has n number of input and n number of output. With the help of this redundant bus grid, it can forward the packets. The last part of a router is the output buffers. The output buffer executes the same task as the input port of buffers but in reverse order. The switching fabric gets the packet to the right output port. However, port's network may not be immediately available for transmission. Once the processor outs the packet to the correct output buffer, it is placed in this queue and then whenever the network is available, that queued packets are data link processed, then it is processed by a physical air processor and they are out on the network. Let us see what are the parameters or how to improve the performance of a router. There are several methods to improve the router performance. The first is to use the application-specific integrated circuits, means specially designed integrated circuits. These circuits are optimized for routing operations and application-specific integrated circuits include much routing functionalities otherwise these are executed as a software. Many routing functions can execute in parallel, adding new functionality without decreasing the throughput if we go for application-specific integrated circuits. The second method is to use efficient switching fabric. We can use bus or cross bar based switching fabrics reduce the need for in-memory processing and it can improve the performance as well. Another networking device we will discuss is a gateway. Here in figure you can see the communication through a gateway. Gateway operates up to the top level that is application layer, a gateway works above the network layer up to the application layer which is shown in the figure. Gateway is also known as a layer 7 relay. Gateway is a key stopping point for a data on its way to or from its way to the other network means the gateway can filter out the data in many different ways depending on the parameters provided. Gateway is a networking device which connects the network which use different protocols. Gateway is referred as a protocol conversion device as well. The application level gateways can verify the content of application layer packets such as email before sending it to the other side of the network. Gateway expands the functionality of router by performing data translation and protocol convergence. This protocol conversion and data translation ability make gateway appropriate for use in firewalls. These are the references.