Communication Networks (3)

Network-core

2 key functions:

• routing: determine source-destination paths taken by packets
• forwarding (switching): move arriving packets from router’s input link to appropriate router output link
  

Packet-switching: Store-and-Forward

• entire packet must arrive at router before it can be transmitted on next link
• end-end delay = $2L/R$ (assuming zero propagation delay)

Packet Switching: Queueing Delay, Loss

• if arrival rate > transmission rate,
  • packets will queue
  • packets can be dropped (lost) if memory (buffer) fills up

Alternative core: Circuit Switching

• FDM: Frequency Division Multiplexing
  • 
• TDM: Time Division Multiplexing
  • 

Internet Structure: Network of Networks

• IXP: Internet exchange point

Delay, Loss, Throughput in networks

• packet arrival rate to link (temporarily) exceeds output link capacity

• packets queue, wait for turn

• packet being transmitted (delay)

• packets queueing (delay)

• free (available) buffers: arriving packets dropped (loss) if no free buffers

Delay

• $d_{\text{proc}}$ nodal processing delay < msec
  • check bit errors
  • determine output link
• $d_{\text{queue}}$ queueing delay
  • time waiting at output link for transmission
  • $R$ : link bandwidth (bps)
  • $L$ : packet length (bits)
  • $a$ : average packet arrival rate
  • $La/R\to 0$ : avg delay small
  • $La/R\to 1$ : avg delay large
  • $La/R> 1$ : avg delay infinite
• $d_{\text{trans}}$ transmission delay
  • time to upload bits
  • $L$ : packet length (bits)
  • $R$ : link bandwidth (bps)
  • $d_{\text{trans}} = L/R$
• $d_{\text{prop}}$ propagation delay:
  • time to propagate bits from A to B
  • $d$ : length of physical link
  • $v$ : propagation speed
  • $d_{\text{prop}} = d/v$

Loss

Throughput

• throughput: rate (bits/time unit) at which bits transferred.
• bottleneck link: link on end-end path that constrains end-end throughput

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Internet protocol stack

layer	use
application	supporting network applications (FTP, SMTP, HTTP)
transport	process-process data transfer (TCP, UDP)
network	routing of packets from source to destination (IP, routing protocols)
link	data transfer between neighboring network elements (Ethernet, 802.11 (WiFi), PPP)
physical	bits on the wire

• Switch
  • level 2 device
  • connects devices on a computer network by using packet switching to receive and forward data to the destination device.
• Router
  • level 3 device
  • Routers perform the traffic directing functions between networks and on the global Internet.