CS-32 Data Communication and Computer Network Lecture notes on (Virtual circuit Vs Datagram subnet) For MCA 3RD Semester (PG Vocational) Prepared by Mrs. Shakti Pandey Department of Computer Science J.D. Women’s college, Patna Virtual circuit Vs Datagram subnet Computer networks that provide connection-oriented service are called Virtual Circuits while those providing connection-oriented services are called as Datagram networks. For prior knowledge, the Internet which we use is actually based on Datagram network (connectionless) at network level as all packets from a source to a destination do not follow same path. Let us see what are the highlighting differences between these two hot debated topics here: Virtual Circuits: 1. It is connection-oriented simply meaning that there is a reservation of resources like buffers, CPU, bandwidth etc. for the time in which the newly setup VC is going to be used by a data transfer session. 2. First packet goes and reserves resources for the subsequent packets which as a result follow the same path for the whole connection time. 3. Since all the packets are going to follow the same path, a global header is required only for the first packet of the connection and other packets generally don’t require global headers. 4. Since data follows a particular dedicated path, packets reach in order to the destination. 5. From above points, it can be concluded that Virtual Circuits are highly reliable means of transfer. 6. Since each time a new connection has to be setup with reservation of resources and extra information handling at routers, its simply costly to implement Virtual Circuits. Datagram Networks: 1. It is connectionless service. There is no need of reservation of resources as there is no dedicated path for a connection session. 2. All packets are free to go to any path on any intermediate router which is decided on the go by dynamically changing routing tables on routers. 3. Since every packet is free to choose any path, all packets must be associated with a header with proper information about source and the upper layer data. 4. The connectionless property makes data packets reach destination in any order, means they need not reach in the order in which they were sent. 5. Datagram networks are not reliable as Virtual Circuits. 6. But it is always easy and cost efficient to implement datagram networks as there is no extra headache of reserving resources and making a dedicated each time an application has to communicate. Comparison of Datagram and Virtual Circuit Subnet Datagram Virtual Circuit Connection None Required Setup Addressing Packet contains full source and Packet contains short virtual circuit destination address number identifier. State None other than router table Each virtual circuit number entered to Information containing destination network table on setup, used for routing. Routing Packets routed independently Route established at setup, all packets follow same route. Effect of Router Only on packets lost during crash All virtual circuits passing through Failure failed router terminated. Congestion Difficult since all packets routed Simple by pre-allocating enough Control independently router resource buffers to each virtual circuit at setup, requirements can vary. since maximum number of circuits fixed. Network Layer Interaction with the Data Link Layer In Figure 4 four different types of data links were used to deliver the data. When the network layer protocol is processing the packet, it decides to send the packet out the appropriate network interface. Before the actual bits can be placed onto that physical interface, the network layer must hand off the packet to the data link layer protocols, which, in turn, ask the physical layer to actually send the data. And as was described earlier, "Data Link Layer Fundamentals: Ethernet LANs," the data link layer adds the appropriate header and trailer to the packet, creating a frame, before sending the frames over each physical network. The routing process forwards the packets from end-to-end. The network layer processes deliver the packet end-to-end using successive data-link headers and trailers just to get the packet to the next router or host in the path. Each successive data link layer just gets the packet from one device to the next Figure: 5 Packet transferred from one Router to another Because the routers build new data-link headers and trailers (trailers not shown in figure),and because the new headers contain data-link addresses, the PCs and routers must have some way to decide what data-link addresses to use. An example of how the router determines which data-link address to use is the IP Address Resolution Protocol (ARP). ARP is used to dynamically learn the data-link address of an IP host connected to a LAN. In short, the process of routing forwards Layer 3 packets, also called Layer 3 protocol data units (L3 PDUs), based on the destination Layer 3 address in the packet. The process uses the data link layer to encapsulate the Layer 3 packets into Layer 2 frames for transmission across each successive data link .