Circuit switching and packet switching
| English | Chinese | Pinyin |
|---|---|---|
| circuit switching | 电路交换 | diàn lù jiāo huàn |
| packet switching | 分组交换 | fēn zǔ jiāo huàn |
| dedicated path | 专用路径 | zhuān yòng lù jìng |
| bandwidth | 带宽 | dài kuān |
| sequence number | 序号 | xù hào |
| router | 路由器 | lù yóu qì |
| multiplexed | 多路复用 | duō lù fù yòng |
| variable latency | 延迟 | yán chí |
The network built to survive its own destruction
- In 1964 Paul Baran was asked how a communications network could keep working after a nuclear strike. Every existing network failed the test, because a telephone call reserved one path and cutting that path ended the call.
- His answer was to stop reserving paths. Chop each message into small blocks, address every block, and let each one find its own way; if a link disappears, the next block simply goes round it.
- The idea was rejected as unworkable for years. It is now how essentially all data moves, and it is why the internet has no centre to cut.
- This lesson is circuit switching 电路交换 and packet switching 分组交换: how each works, what each costs, and which suits which traffic.
Circuit switching
- A dedicated path 专用路径 is set up between the two ends before any data is sent, held for the whole conversation, and released at the end.
- Benefits: the bandwidth 带宽 is reserved, so the rate is guaranteed and constant, and data arrives in order with a predictable delay.
- Drawbacks: it is inefficient, because during silences the reserved line sits idle and nobody else may use it, and it is slow to set up, because the path must be established first. One broken link cuts the circuit and the conversation ends.
- The classic example is the traditional telephone network.
Reserved for the whole call, used or not
In circuit switching:
Circuit switching reserves one path end-to-end (reserved bandwidth) — but it wastes the line during silences.
What is the main drawback of circuit switching?
Reserving the path guarantees the rate and the order, and that is exactly what makes it wasteful. The other three describe packet switching.
Packet switching
- The message is split into packets, each sent independently. Every packet carries the destination and source addresses and a sequence number 序号.
- Each router 路由器 makes a per-packet decision about the next hop, so packets may take different routes and arrive out of order. The destination uses the sequence numbers to reassemble the message, and missing packets can be requested again.
- Benefits: efficient, because one link is multiplexed 多路复用 across many conversations and carries data only when there is data; robust, because traffic reroutes around a failed link.
- Drawbacks: variable latency 延迟, since packets queue at routers, and packets can be lost, so reliability must be added above, by TCP.
Same message, many paths, reassembled at the end
A packet's journey across the internet
Step through packet switching. The message is split up, each packet finds its own way, and the destination puts them back together — which is why the internet is so efficient and hard to break.
A key advantage of packet switching is that it is:
Sharing links is efficient and rerouting makes it resilient. The trade-off is variable latency and possible loss (TCP fixes that).
Because each packet carries a sequence number, the destination can reassemble the message in the right order and re-request any missing packet.
Packets may arrive out of order or be lost; the sequence numbers (and TCP) put everything right.
What a router does
- A router joins two or more networks. For each packet it reads the destination IP address, looks it up in its routing table, and forwards the packet out of the appropriate interface towards the destination.
- It holds no memory of the conversation: two packets of the same message may leave by different interfaces if the table has changed between them.
- That per-packet decision is exactly what makes rerouting around a failure automatic.
Put the stages of sending a message by packet switching in order.
Split, address, route, arrive, reassemble. That is the five-mark answer to "explain how packet switching passes a message".
Worked example: describe packet switching
- Explain how packet switching is used to pass a message across the internet. [5]
- The message is broken into small packets. Each packet carries the destination address, the source address and a sequence number.
- Each packet travels through the network independently, with routers choosing the next hop for each one, so packets may take different paths and arrive out of order.
- The destination uses the sequence numbers to reassemble the message in the right order, and any missing packet can be requested again.
- Five sentences, five marks. Leaving out the sequence numbers or the reassembly loses two of them.
A router reads the destination address of every ____ and forwards it towards its destination.
The decision is made per packet, with no memory of the conversation, which is what lets traffic reroute around a failure automatically.
Side by side
| Aspect | Circuit switching | Packet switching |
|---|---|---|
| path | dedicated, reserved end to end | shared, chosen per packet |
| setup | slow, before any data | none |
| bandwidth use | inefficient, idle during silences | efficient, multiplexed |
| order | always in order | may arrive out of order |
| on a failure | the circuit is cut | reroute around it |
| delay | constant and predictable | variable |
| suits | constant-rate flows, voice | bursty flows, web and email |
Match each term to what it means.
Circuit = one reserved path; packet = independent packets reordered by sequence number — what the internet uses.
The modern internet uses:
The internet uses packet switching for its efficiency and resilience.
Worked example: which method for which traffic
- A phone call and a large file download share a network. Which switching method suits each, and why?
- The phone call needs a steady stream at a constant rate with low, predictable delay, and it would suffer badly if pieces arrived late or out of order. Circuit switching suits it: a path is reserved for the whole call and its capacity is guaranteed.
- The file download does not care about the order of arrival or short delays, since the file is reassembled before it is used, and it is bursty. Packet switching suits it: the link is shared with other traffic and used only when data is flowing.
- Name the method, name the property of the traffic, and connect them.
Which properties make circuit switching a good fit for a telephone call? Select all that apply.
Efficient sharing is packet switching's advantage, and it is the thing circuit switching gives up in exchange for the guarantees.
Marks that slip away
- Packets can arrive out of order, and the sequence numbers are what fix that. Both halves are needed.
- Circuit switching's drawback is the idle reserved line, not "it is old".
- The router's decision is made per packet, which is what makes rerouting automatic.
- Packet switching does not guarantee delivery on its own; TCP adds that above it.
You've got it
- circuit switching reserves a dedicated path before sending: guaranteed bandwidth and order, but idle during silences, slow to set up, and cut by one failure
- packet switching splits the message into independently routed packets carrying addresses and sequence numbers, reassembled at the destination
- a router decides the next hop per packet, so traffic reroutes around failures automatically
- packet switching is efficient and robust with variable latency; circuit switching suits constant voice flows, packet switching suits bursty data