20230912 ch1

Two Internet core networking technologies: Packet switching (P.S) and Circuit switching (C.S)

 

1. Packet switching

•  each pckt is delivered independently(연관성을 모른다! 지들 맘대로 순서없이 도착함) in core nw over different e2e path.
• In-order delivery cannot be guaranteed. Flexibility can be reserved (그 때 그 때 봐가면서 라우팅) 불나면 그 라우터 피해서 갈 수 있음
• 왜 pckt으로 쪼개냐? multiple user access in LAN : Max delay 가 줄어듦 (내가 안 쓰면 너가 쓸 수 있음, 동시처리효과 = parallel delay = e2e delayed) & efficient Data delivery in core nw. error correction 시간이 줄어듦 (오류난 패킷들만 다시 보내면 되니까)
• each pckt is transmitted when a link is available otherwise it waits in output buffer (= Queueing delay controled by congestion control, tcp)
  • one user can use a full link rate(Rbps) 
  • bit overhead(header :dst address) : 다음 유저는 누가될 지 정보가 담겨있음. router가 그때그때마다 판단 unlike circuit switching

 

2. Circuit switching

• resources reservation (e2e path 어떻게 갈 지 확보, link bandwidth 길이 확보)
• voice delivery : delay가 있거나 맘대로 확 data 보냈다가 쉬고 이러면 안됨.
• idle 해도 no sharing. dedicated! : 전화 끊기 전에는 내꺼 확보
• synchronous : 정해진 시간에만 신호 보내기

 

3. the differences between P.S. and C.S

• Transmission delay for transmitting a L-bit packet over a R bps link = (L-bit)/(R bps) = L/R sec
• Transmission delay for transmitting one message (=3 L-bit pkts) over a R bps link = 3 x (L/R) sec
• End-to-end transmission delay for transmitting one message (=3 L-bit pkts) over four R bps link is NOT (3 x (L/R)) x 4 sec since multiple packets belonging to the same message are transmitted in parallel in core network.

1) queueing delay @ packet switching : router에 packet이 들어오는 속도(arriving rate) > 나가는 속도

2) Basic operation of packing switching network : store and forward

1. Store (L bits) in an input buffer
2. checking the forwarding table to find the output link via which the packet will finally arrive at its destination computer.
3. forwarding the L-bit packet to the output buffer of the output link
4. may be waiting in the output buffer (this is queueing delay)
5. transmitted ( L/R sec delay incurred for transmission )

3) packet switching allows more users(who generate bursty&intermittent traffic) to use network!

 

4. Routing v.s. forwarding in the packet switching network

1. Routing is to determine the e2e path from a source host to a destination host by exchanging control messages among routers. 전체를 봐야 함, 판단 by routing protocol (2. n번 링크로 가야지!)
2. Forwarding is to switch a packet from an input port to an output port at ONE router using the forward table (Forwarding Information Base, FIB).  memory가 router's input에서 router's ouput으로 copy되는 현상