[network planning] 1.3 packet switching and circuit switching in the network core

This article is based on 《 computer network —— top-down approach 》 A Book

Network core

After examining the network edge , Let's take a look at how the core of the network is made up .

Packet switching

In the network , End systems must exchange with each other message (message),（ Message refers to the data unit exchanged and transmitted in the network , That is, the site to send a one-time block of data . To send a message from the source to the destination system , The source will segment the message into smaller data blocks , be called grouping （packet). Between source and destination , Each packet passes through a communication link and Packet switch delivery .

1. Store and forward transmission

Most packet switches use store and forward transmission mechanism . This mechanism means that the switch receives an entire network packet , Start sending the packet to the output link .

Circuit switching

The representative of circuit switching is telephone , Circuit switching needs to apply for the resources required for connection in advance （ Such as caching 、 Link transmission rate ）, However, the requested resources will be used throughout the connection , Just like when we call, even if we don't talk , It will also continue to occupy telephone line resources . In contrast, packet switching does not occupy resources in the whole process , Instead, network resources are used on demand in real time , This will sometimes make it necessary to queue up to send messages .

Both comparisons

Circuit switching can always occupy network resources , Therefore, it is suitable for real-time services （ For example, live broadcast ）, But sometimes the two end systems do not transmit information to each other , But when the connection is still disconnected , It will lead to the waste of network resources , This is circuit switching Silent period , hypothesis QQ Circuit switching , If 3 Seconds to receive / Send a message , receive / Sending messages takes time 0.2 second , The rest of the 2.8 Seconds are wasted . Packet switching does not occupy separate network resources , There would be no such thing . in addition , Circuit switching requires complex signaling software and coordinated switch operation , So its implementation is more complicated .

Packet switching will be queued , Not suitable for real-time services （ For example, video conference , live broadcast ）, Because its end-to-end delay is unpredictable , But packet switching has no silent period , Therefore, it provides better bandwidth sharing than circuit switching , Higher utilization of network resources , And it is simpler and more efficient than circuit switching 、 Lower implementation cost . So now , Packet switching is becoming more and more popular