Section 1.3: Packet switched and circuit switched networks

routing protocols

are used to automatically set the forwarding tables. A routing protocol may, for exam- ple, determine the shortest path from each router to each destination and use the shortest path results to configure the forwarding tables in the routers.

output buffer

Each packet switch has multiple links attached to it. For each attached link, the packet switch has an output buffer (also called an output queue), which stores packets that the router is about to send into that link.

packets

To send a message from a source end system to a destination end system, the source breaks long messages into smaller chunks of data known as packets.

time to transmit the packet

if a source end system or a packet switch is sending a packet of L bits over a link with transmission rate R bits/sec, then the time to transmit the packet is L/R seconds.

time to send n number of packets to destination over 1 link

As before, at time L/R, the router begins to forward the first packet. But also at time L/R the source will begin to send the second packet, since it has just finished send- ing the entire first packet. Thus, at time 2L/R, the destination has received the first packet and the router has received the second packet. Similarly, at time 3L/R, the destination has received the first two packets and the router has received the third packet. Finally, at time 4L/R the destination has received all three packets!

telephone circuit

Before the sender can send the infor- mation, the network must establish a connection between the sender and the receiver. This is a bona fide connection for which the switches on the path between the sender and receiver maintain connection state for that connection. In the jargon of telephony, this connection is called a circuit. When the network establishes the circuit, it also reserves a constant transmission rate in the net- work's links (representing a fraction of each link's transmission capacity) for the duration of the connection.

FDM vs TDM

For FDM, the frequency domain is segmented into four bands, each of bandwidth 4 kHz. For TDM, the time domain is segmented into frames, with four time slots in each frame; each circuit is assigned the same dedicated slot in the revolving TDM frames.

queuing delays

If an arriving packet needs to be transmitted onto a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer. Thus, in addition to the store-and-forward delays, packets suffer output buffer queuing delays.

circuit switched vs packet switched

In circuit-switched networks, the resources needed along a path (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end systems. In packet-switched networks, these resources are not reserved; a session's messages use the resources on demand, and as a consequence, may have to wait (that is, queue) for access to a communication link.

end to end delay between several routers

Let's now consider the general case of sending one packet from source to desti- nation over a path consisting of N links each of rate R (thus, there are N-1 routers between source and destination). Applying the same logic as above, we see that the end-to-end delay is: N * (L/R)

bandwidth.

Specifically, the link dedicates a frequency band to each connection for the duration of the connection. In telephone networks, this frequency band typically has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). The width of the band is called, not surprisingly, the bandwidth. FM radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being allocated a specific frequency band.

how transmission delay works

The source begins to trans- mit at time 0; at time L/R seconds, the source has transmitted the entire packet, and the entire packet has been received and stored at the router. Only after the router has received all of the packet's bits can it begin to transmit (i.e., "forward") the packet onto the outbound link. Router must buffer the packet. At time 2L/R, the router has transmitted the entire packet, and the entire packet has been received by the destination. Thus, the total delay is 2L/R.

Packet forwarding

When a source end system wants to send a packet to a destination end system, the source includes the destination's IP address in the packet's header. When a packet arrives at a router, the router examines the address and searches its forwarding table, using this destination address, to find the appropriate outbound link. The router then directs the packet to this outbound link.

two fundamental approaches to moving data through a network of links and switches:

circuit switching and packet switching

For TDM, the transmission rate of a circuit

equal to the frame rate multiplied by the number of bits in a slot. For example, if the link trans- mits 8,000 frames per second and each slot consists of 8 bits, then the transmission rate of a circuit is 64 kbps.

packet switches

for which there are two predominant types, routers and link- layer switches)

store-and-forward transmission

the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.

packet loss

Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely full with other packets wait- ing for transmission. In this case, packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped.