Network core

why the network exist ? 

: switching messege between hosts 

 

network core 

-no host, only exist router and switch 

- interconnected routher's mesh

packet-switching(=forwording) : network forwording packet from one routhers to another routhers  ex) 옥천허브 

 

network core function 

routing : route from satrt to destination  ->routing algorithm 

-which switch to go , global action , routing algorithms are diverse 

 

 

forwarding : which port to go , local action , 

-if packet arrive to router's input port then it searches the forwarding table made by routing algorithme and decide witch output port to send

 

over the picture , the port number is 1,2,3 

 

*switch = packet forwarding divice 

*network have a lot of hosts so a switch is needed

 

packet-switching : store and forward

packet transmission delay : the time it takes for a switch to transmit packet  to the output link 

- L(packet length)/R(bandwidth)

 

store and forward: all packets could arrive the router before transmition to the next link 

=> that is the packets could saved the swiche's memory before the transmition  

 

packet-switching: queueing

 

= line up 

 

- expecting transmition to output link , the packet  is saved the switche's memory (copy the packet to the switch's queue)

- queueing delay = wating time in front of the packet served 

- if switch's memory is fulled , the packet is droped that is the packet is lost

 

alternative to packet switching: circuit switching 

- a one solution of the packet loss

-because  there exsist a personal circuit , it dosen't have  queuing delay and packet loss

- the link is used by only two host so it dose not cuse queuing 

- usually the circuit is used in the tellephone lines  and  if there is no call then the circuit is not used

 

packet switching VS circuit switching 

how many people use this network? 

 

circuit -switching :

- if circuit is splited into ten slices, the  user is 10 users

- but if no call is made , then the circuit is not used 

- actually the circuit cannot  be splited into  a lot of slices realistically

- but the performance is guaranteed

 

packet-switching :

- many host share a circuit

- it is used  because of expandability 

- but queueing delay and drop problem still exist

- no performance guarantee

-> today it is supplemented by s.w

 

isp 

host is connected to the internet by isp

 

* initial setup is expensive , maintenance costs are row , living essentiality

but entry barriers are so high, profit structure is stable 

 

* global isp : linked by an ocean cable