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Character 1 - Introduction

Physical medium 物理介質​

  • Twisted-pair copper wire

  • Coaxial cable

  • Multimode fiber-optic cable

  • Terrestrial radio spectrum

  • Satellite radio spectrum

Physical media categories​

  • Guided media

  • Unguided media

Packet Switching​

Send message from source end system to destination end system

Long messages breaks into smaller chunks of data called packets

Packet Switches​

  • Router

  • Switch

Sending L bits with transmission rate R bits/s

Time to transmit the packet is L/R seconds.

Queuing Delays and Packet Loss​

Packet switch has output buffer / output queue

If link is busy, packet need wait in output buffer

  • Store-and-forward delays

  • Queuing delays (wait to be transmitted on link)

If output buffer completely full -> packet loss (dropped)

Forwarding Tables and Routing Protocols​

Differences Between Routing and Forwarding Tables | Baeldung on Computer Science

Routing table for router

Forwarding table for switch or bridge

Circuit switching​

電路交換要求必須首先在通訊雙方之間建立連接通道。在連接建立成功之後，雙方的通訊活動才能開始

在連接建立成功之後，雙方的通訊活動才能開始

通訊雙方需要傳遞的信息都是通過已經建立好的連接來進行傳遞的，而且這個連接也將一直被維持到雙方的通訊結束

Multiplexing in Circuit-Switched Networks​

  • Frequency-division multiplexing (FDM)

  • Time-division multiplexing (TDM)

Difference between TDM and FDM - GeeksforGeeks

FDM v.s TDM - iT 邦幫忙::一起幫忙解決難題，拯救 IT 人的一天

Packet Switching Versus Circuit Switching​

  • Packet switching not suitable for real-time services

  • Packet switching more efficient

  • Circuit switching waste the bandwidth if user inactivity

Example​

1 Gb/s link

Each user:

  • 100 Mb/s when active

  • Active 10% of time

Q: how many users can use this network under circuit-switching and packet switching?​

Circuit-switching: 10 users

Packet switching:

如果有 35 users, probability > 10 active at same time approximately 0.0004

probability <= 10 is 0.9996

二項分佈 (wiki), B(N, k)

拋硬幣，N 次試驗中有 k 次成功的概率, P(X = k)

X∼B(35,0.1)X \sim B(35, 0.1) P(X=k)=C(35,k)0.1k0.935−kP(X<=10)=P(X=0)+P(X=1)+...+P(X=10)=0.999575702P(X>10)=1−P(X<=10)=0.000424297\begin{align*} P(X = k) &= C(35, k) 0.1^k 0.9^{35 - k} \\\\ P(X <= 10) &= P(X = 0) + P(X = 1) + ... + P(X = 10) \\ &= 0.999575702 \\\\ P(X > 10) &= 1 - P(X <= 10) \\ &= 0.000424297 \end{align*}

可以做 Quantitative Comparison of Packet Switching and Circuit Switching ，題目參數會改

Four sources of packet delay​

  1. Processing delay
  • Check bit errors
  • Determine output link
  • Typically < microsecs
  1. Queueing delay
  • Time waiting at output link for transmission
  • Depends on congestion level of router
  • L: packet length (bits)
  • R: link bandwidth (bit transmission rate)
  • a: average packet arrival rate
  • Traffic intensity = L * a / R
  • La/R ~ 0: avg. queueing delay small
  • La/R -> 1: avg. queueing delay large
  • La/R > 1: more “work” arriving is more than can be serviced, average delay infinite
  1. Transmission delay
  • L: packet length (bits)
  • R: link bandwidth (bps)
  • d_trans = L/R
  1. Propagation delay
  • d: length of physical link
  • s: propagation speed in medium (~ 3x10^8 m/s)
  • d_prop = d/s
dnodal=dproc+dquene+dtrans+dpropd_{nodal} = d_{proc} + d_{quene} + d_{trans} + d_{prop}

ISO/OSI reference model​

  1. Application

  2. Presentation

  3. Session

  4. Transport

  5. Network Layer

  6. Data Link Layer

  7. Physical Layer

Network security​

Denial of Service (DoS)​

Attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic

Packet sniffing​

Records all packets

Wireshark is a (free) packet-sniffer

IP spoofing​

Send packet with false source IP address