# 1.3 The Network Core

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### 1.3.1 Packet Switching

* In a network application, end systems exchange **messages** with each other 
  * Can perform a control function or can contain data 
* 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** 
* Between source and destination, each packet travels through communication links and **packet switches** (**routers** and **link-layer switches**) 
* Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link 
  * L bits, R bits/sec, then time to transmit is L/R seconds 

#### **Store-and-forward transmission** 

* Most packet switches use this at the inputs to the links 
* Packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link 
* Total delay: 2L/R 

![End-to-end delay for N links each of rate R ](https://2097630930-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MVORxAomcgtzVVUqmws%2F-MZi8E4RJRoy2josZHwN%2F-MZiA-FHfif1PlLpKDMs%2Fimage.png?alt=media\&token=17b00297-b9b1-46f7-83d1-3b8ca395b1b6)

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#### Queuing Delays and Packet Loss 

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

![](https://2097630930-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MVORxAomcgtzVVUqmws%2F-MZi8E4RJRoy2josZHwN%2F-MZiAg8mURXaBI5Xj9bd%2Fimage.png?alt=media\&token=886bf147-63b5-4c2a-b23a-22a31cb3f185)

#### Forwarding Tables and Routing Protocols 

* How does the router determine which link it should forward the packet onto?
  * End system: IP address 
  * The source includes the destination IP address in the packet's header (hierarchical structure) 
  * Each router has a **forwarding table** that maps the destination address (or portions of it) to that router's outboud links 
  * How do forwarding tables get set? Are they configured by hand in each and every router, or does the Internet use a more automated procedure? 
    * **Routing protocols:** may determine the shortest path from each router to each destination and use the shortest path results to configure the forwarding tables in routers 

### 1.3.2 Circuit Switching 

* There are two fundamental approaches to moving data through a network of links and switches: **circuit switching** and **packet switching**.