Packet switching encompasses a class of Wide Area Network (WAN) services as well as a basic technique for data transmission in networks.
Packet Switching WAN
Service providers (SPs) use packet switching, a WAN switching technique that allows network users to share bandwidth and is less expensive than dedicated connections like leased lines.
Key Characteristics of Packet-Switched WANs
Shared Bandwidth: By forwarding packets to one another, nodes in a packet-switched network share bandwidth. Similar to circuit switching, the network is made to resemble a leased line but charge less.
Virtual Circuits: Virtual circuits are essential to this architecture. By using a permanent virtual circuit (PVC), which functions similarly to a point-to-point link, packet switching allows users to share physical pathways while sending packets across the carrier network.
Bursty Traffic Model: When data transfers are intermittent rather than continuous, packet switching works well. A dedicated leased line is usually advised instead if data transfer is continuous.
Autonomous Packets: These networks allow messages to be conveyed in separate packets that are independently addressed. This allows the routers and switches that act as intermediary devices to send the messages via many channels that lead to the same destination. It’s possible for packets to arrive out of sequence.
Historical Context: Developed about 1970, the ARPANET was the first packet-switched network and the forerunner of the Internet.
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Packet Switching Technology
Metro Ethernet and Multiprotocol Label Switching (MPLS) are currently the most popular packet-switching connection choices in enterprise wide area networks (WANs). Examples of legacy technologies are ATM and X.25.
| Technology | Status/Description | Speed/Details |
|---|---|---|
| Metro Ethernet | Uses IP-aware Ethernet switches in the service provider’s cloud to offer converged services. It is one of the most common modern packet-switching technologies. | Easily integrates with existing Ethernet LANs. |
| MPLS | A data-carrying mechanism that emulates properties of a circuit-switched network over a packet-switched network. MPLS uses labels instead of destination IP addresses for faster forwarding inside the service provider’s network. | It can carry any payload, including IPv4, Ethernet, Frame Relay, and DSL traffic. |
| Frame Relay | A high-performance, packet-switched technology and a successor to X.25. It is cost-effective compared to point-to-point links and works best with bursty traffic. | Speeds typically range from 56 Kbps up to T3 (45 Mbps). |
| X.25 | An older, legacy WAN protocol. It was one of the first WAN protocols for packet switching and includes extensive error-checking capabilities, which makes it reliable but limits its bandwidth. | Largely deprecated for newer, faster protocols. |
| ATM | A cell-switched technology, often included within packet-switched technologies due to similarities. It uses fixed-size 53-byte cells instead of variable-sized packets/frames. | Designed for time-sensitive traffic (voice, video, data). |
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Packet Forwarding Techniques in Routers
Packet switching, commonly known as packet forwarding, is a crucial activity carried out by routers. A router can receive a packet on one interface and forward it on another interface to the switching capability.
In response to growing network demands, Cisco routers have modified their internal packet forwarding techniques:
- Process switching (Legacy): This was the original technique, which resulted in high latency and CPU cycle use as the router’s main CPU had to look up each packet in the routing table. Because of its inefficiency and complexity in contemporary networks, this technique is typically no longer employed.
- Fast Switching (Older): This middle-tier approach enhanced performance by storing destination information in a cache, eliminating the need for lookups for each packet after processing the first packet in a flow. This is also regarded as an outdated technique that is no longer in use.
- The most recent and standard packet-forwarding technique on Cisco routers is Cisco Express Forwarding (CEF) (Current Default). First-packet process-switched lookups are no longer necessary to CEF, a topology-driven, proprietary switching mechanism. Before any packets are even received, it generates unique cache tables called the adjacency table and the Forwarding Information Base (FIB). Because CEF is change-triggered, the cache is updated whenever the network topology shifts.
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Switching Mechanisms in LANs
Switches are the devices that move data within a Local Area Network (LAN). In hardware, LAN switching is regarded as a type of packet switching that takes place at the Data Link Layer (Layer 2).
Ethernet frames are forwarded via LAN switches, although the process is frequently explained using words that indicate the amount of processing done on the frame or packet before forwarding:
| Switching Method | Description | Error Checking/Latency |
|---|---|---|
| Store-and-Forward Switching | The switch copies the entire frame into buffers, calculates the Cyclic Redundancy Check (CRC), checks the data integrity, and only forwards the frame if it is error-free. | High latency (due to waiting for the entire frame) but high error detection. It is the default mode for most edge switching equipment. |
| Cut-Through Switching | The switch buffers just enough of the frame to read the Layer 2 destination MAC address (the first six bytes after the preamble) and immediately begins forwarding the data. | Lowest latency, but performs no error checking. It is sometimes called “on-the-fly packet switching”. |
| Fragment-Free Switching | The switch waits for the first 64 bytes (the collision window) to pass before forwarding the frame, checking to ensure that no fragmentation (collision) has occurred. | Provides better error checking than cut-through with practically no increase in latency. |
In contrast to circuit switching, which necessitates the establishment and maintenance of a specific physical circuit for the duration of the communication session, this notion divides data into discrete, autonomously routed packets. While packet switching is similar to sending numerous individual postal letters that can navigate a common system of roads and sorting offices, circuit switching is similar to reserving a dedicated railway track for a train.
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