[컴퓨터네트워크] 1. Introduction (2)
Access networks
1) Access network: DSL(digital subscriber line)
2) Access network: cable-based access
- cable modem: digital signal --> analog signal
- HFC(Hybrid Fiber Coax): data and TV is transmitted at different frequencies over shared cable distribution network.
: Homes share access network to cable headend.
<-> DSL: dedicated access to central office. - Fiber node
- CMTS(Cable Modem Termination System) --> CATV, Internet
- Asymmetric(비대칭적): downstream transmission > upstream transmission
3) Access network: FTTH(fiber to the home)
- CO provides optical fiber path directly to the home.
- CO(Control Office) = OLT(Optical Line Terminal)
- Home = ONU(Optical Network Unit)
: OLT(Optical Line Terminal) --> PON(Passive Optical Network) --> ONU(Optical Network Unit)
▶ Two optical-distribution network (ODN) architecture
- AON(Active Optical Network)
- PON(Passive Optical Network)
4) Access network: 5G fixed wireless
▶ 5G for FWA(Fixed Wireless Access)
- 낮은 초기 도입 비용(rollout cost), 낮은 운영 비용(OPEX)
- Reliable, cost-effective approach
- Gbps broadband experience
Access network: home network
▶ LAN(Local Area Network) <-> WAN(Wide Area Network)
- Wireless access point
- Router: firewall(나쁜 데이터 방지), NAT(외부에서 보내는 IP주소를 하나만 쓰도록 함) 기능도 있다.
- cable / DSL modem
- headend / CO(Central Office)
- Internet
Ethernet (Enterprise access networks)
: edge network, institutional mail/web servers --> Ethernet switch --> Institutional router --> institutional link to ISP(Internet)
▷ Used in companies, universities ...etc
▷ Mix of wired, wireless link technologies, connecting a mix of switches and routers.
- Ethernet: wired access
- WiFi: wireless access point
Wireless access network
1) Wireless access network - Indoor: WiFi
- Shared wireless access network connects end system to router via base station called "access point".
- WLANs(Wireless Local Area Networks)
2) Wireless access network - Outdoor: cellular
- Wide-area wireless access: cellular
Host: sends packet of data
▶ Host sending function
- Takes application message.
- Breaks into smaller chunks(packets), of length L bits.
- Transmits packet into access network at transmission rate R.
The network core: circuit switching
※ Core: 목적지 access network
- Circuit switching
- Packet switching
▶ Circuit switching
: End-to-end (source --> destination) resources reservation made before communication and maintained during call.
- In the diagram, each link has 4 circuits.
- Dedicate resources
- Guaranteed performance.
- Circuit segment는 통화에 의해 사용되지 않을 경우, 비활성화된다. - 사용: telephone networks.
Circuit switching: FDM and TDM
▶ FDM(Frequency Division Multiplexing)
: Optical(시각적인), electromagnetic(전자기의) frequencies devided into frequency bands.
--> Each call allocated its own band, can transmit at maximum rate of that narrow band.
▶ TDM(synchronous Time Division Multiplexing)
: Time divided into slots.
--> Each call allocated periodic slots, can transmit at maximum rate of frequency bands, but only during its time slots.
Alternative core - packet switching
: Hosts break application-layer messages into packets.
--> Forward packets from one router to the next.
- Each packet transmitted at full link capacity.
Packet switching: store-and-forward
▶ Store-and-Forward
: L bits per packet --> The entire packet must arrive at router before it can be transmitted on next link.
- Transmission delay: packet 1개를 모두 link에 transmit 하는 데 걸리는 시간
**Transmission delay = L/R
Packet Switching: queueing delay, loss
▶ Packetqueueing and loss
: If arrival rate (in bps) to link exceeds transmission rate of link for period of time:
- packets will queue, and wait until the transmission link becomes available.
- packets can be dropped(lost) if memory(buffer) in router fills up.
Two key network-core functions in a packet switching network
▶ Forwarding: local action
: Packet - router's input link --> appropriate router output link.
- Forwarding table: header value + output link
▶ Routing: global action
: Determine source-destination path taken by packets.
- Routing protocol: 네트워크 정보를 모은다.
- Routing algorithms: Forwarding table을 작성한다.
Packet switching vs. Circuit switching
- Packet switching allows more users to use network.
▷ Is packet switching a "winner"?
- 장점: Packet switching is better for bursty data.
- Resource sharing
- No call setup - 단점: Excessive congestion is possible.
- Packet delay/loss due to buffer overflow.
▷ How to provide circuit-like behavior?
: Bandwidth guarantees traditionally used for audio/video applications.
Internet structure: "network of networks"
- End systems or hosts connect to Internet via access ISPs(Internet Service Providers)
- residential, enterprise(company, university, commercial) ISPs. - Access ISPs in turn must be interconnected.
- Global ISPs(Internet Service Providers)
- IXP(Internet eXchange Points)
- peering link: Global ISPs 끼리 연결한다.
Cf) IXP vs. peering link
- IXP는 여러 네트워크가 모여 트래픽을 교환할 수 있는 인프라 또는 장소를 나타내며, 트래픽 교환을 위한 중심 장소입니다.
- Peering Link는 두 개 이상의 네트워크 간에 직접 연결된 링크를 나타내며, 네트워크 간 트래픽을 교환하기 위한 구체적인 연결을 나타냅니다.
--> IXP는 여러 Peering Links를 호스팅하거나 그 안에 존재할 수 있으며, Peering Links는 트래픽 교환을 위한 IXP의 구성 요소 중 하나입니다.
▶ PoP(Point of Presence)
: A group of routers in the provider network, where customer ISPs can connect into the provider.
--> high-speed link from a third-party telecommunications provider.
▶ Multi-home
: ISP여러 개에 연결하는 것
▶ Regional ISP
- Regional ISP에도 peering link가 있다.
Internet structure: Content Provider Networks
▷ At center:
- tier-1 commercial ISPs
: national & international coverage. (ex. Level 3, Sprint, AT&T, NTT) - Content provider network
: private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs. (ex. Google, Meta)
Summary of Internet structure
▷ Consists of tier-1 ISPs + Lower-tier ISPs.
--> Lower-tier ISPs connect to higher-tier ISPs, and higher-tier ISPs interconnect with one another.
- Users and content providers are customers of lower-tier ISPs, and lower-tier ISPs are customers of highter-tier ISPs.
- Major content providers have their own networks and connect directly into lower-tier ISPs.