CSE 413: Computer Network Circuit Switching and Packet Switching Networks Md. Kamrul Hasan 09-03-2010

The network core: mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent through net in discrete “chunks” (packets) on shared media

The network core: Circuit Switching End-to-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required

processing delay at switch Circuit Switching Boston Switch It’s the method used by the telephone network A call has three phases: Establish circuit from end-to-end (“dialing”), Communicate, Close circuit (“tear down”). If circuit not available: “busy signal” LA Switch Caller Callee processing delay at switch propagation delay between caller and Boston switch DATA (1) (2) (3)

Circuit Switching: Multiplexing/Demultiplexing Frames Slots = 1 2 3 4 5 1 2 3 4 5 One way for sharing a circuit is TDM: Time divided into frames and frames divided into slots Relative slot position inside a frame determines which conversation the data belongs to E.g., slot 0 belongs to the red conversation Need synchronization between sender and receiver Lecture notes use the word “frame” for slot

The network core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) Consumers are charged on a per-minute basis 2 ways of dividing the link bandwidth into “pieces” frequency division multiplexing (FDM) time division multiplexing (TDM)

Circuit Switching: FDM and TDM 4 users Example: Frequency Division Multiplexing (FDM) frequency time Time Division Multiplexing. (TDM) frequency time

Numerical example How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? The link’s transmission rate = 1.536 Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit Figure it out … Solution: Bandwidth of circuit = 1.536/24 = 64 kbps Time to send: 640 kbits/64 kbps + 0.5s = 10.5s What would be different if we use FDM instead of TDM?

Common mistake/confusion : Question: A) Express transmission rate of 1Kbits/sec in bits/sec B) Express the file size of 1KBytes in bits Answer: A) 1000 bits/sec (in throughput, K = 103=1000) B) 1024 Bytes = 8192 bits (in data size, K = 210=1024) Electronic speeds/times: K = 103, M = 106, G = 109 Computer file/memory sizes: K = 210 , M = 220, G = 230 Common computer notation: b(bits) Kb, Mb, Gb B(Bytes) KB, MB, GB Better computer notation: b(bits) Kib, Mib, Gib B(Bytes) KiB, MiB, GiB

Packet Switching Used in the Internet Data is sent in Packets (header contains control info, e.g., source and destination addresses) Per-packet routing At each node the entire packet is received, stored, and then forwarded (store-and-forward networks) No capacity is allocated Host 1 Host 2 Node 1 Node 2 Header Data propagation delay between Host 1 & Node 2 transmission time of Packet 1 at Host 1 processing delay of Packet 1 at Node 2 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3

Packet Switching: Multiplexing/Demultiplexing Router Queue Multiplex using a queue Routers need memory/buffer Demultiplex using information in packet header Header has destination Router has a routing table that contains information about which link to use to reach a destination

Packet switching also show reordering Packets in a flow may not follow the same path (depends on routing as we will see later)  packets may be reordered Host C Host D Host A Node 1 Node 2 Node 3 Node 5 Host B Host E Node 7 Node 6 Node 4

The network core: Packet Switching all streams share network resources each packet uses full link bandwidth resources used as needed Resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link Bandwidth division into “pieces” Dedicated allocation Resource reservation

The network core: Packet switching Data transmitted in small, independent pieces Source divides outgoing messages into packets Destination recovers original data Each packet travels independently Includes enough information for delivery May follow different paths Can be retransmitted if lost

The network core: Functions of packet-switching networks Packet construction encode/package data at source Packet transmission send packet from source to destination Packet interpretation unpack/decode data from packet at destination acknowledge receipt

The network core: statistical multiplexing 100 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link D E statistical multiplexing  Sequence of A & B packets does not have fixed pattern; shared on demand. Compare: in TDM, each host gets same slot (periodically) in FDM, each host gets same bandwidth (continuously)

Differences Between Circuit & Packet Switching Circuit-switching Packet-Switching Guaranteed capacity No guarantees (best effort) Capacity is wasted if data is bursty More efficient Before sending data establishes a path Send data immediately All data in a single flow follow one path Different packets might follow different paths No reordering; constant delay; no pkt drops Packets may be reordered, delayed, or dropped

Network performance metrics End-to-end delay (nodal delay) : Total time from initiating “send” (from source) to completed “receive” (at destination) Throughput : Rate (bits/sec) at which bits are actually being transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time

Four sources of packet delay 1. nodal processing: check bit errors determine output link 2. queueing delay time waiting at output link for transmission depends on congestion level of router A B propagation transmission nodal processing queueing

Four sources of packet delay 3. Transmission delay: R=link bandwidth (speed in bits per second, i.e. “bps”) L=packet length (in bits) transmission delay = L/R 4. Propagation delay: d = length of physical link (in meters) s = propagation speed in medium (~2.5 x 108 m/sec) propagation delay = d/s Note: R and s are very different quantities! A B propagation transmission nodal processing queueing