school of electrical engineering and telecommunications...

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10 March 2009 Tim Moors

School of Electrical Engineering and TelecommunicationsUNSW

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TELE 9751 Switching Systems ArchitectureTELE 9751 Internet Design and Equipment

ArchitecturesHandouts

Please pick up a copy of the handout as you enter.Handouts are located beside entries to the aisles.

All slides from lectures will be available on the course web page in PDF format.

2School of Electrical Engineering and TelecommunicationsUNSW

10 March 2009 Tim MoorsCopyright ©

TELE 9751Switching Systems Architecture /

Internet Design and Equipment Architectures

Session 1 2009Lecturer: Tim Moors

Source of figure unknownBonus mark if you can find the source

talks too fast!

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Outline (for today)Administrivia: Course outline

IntroductionNon-switched networksDefining “switch”Switch examples

Switching in other fieldsExternal perspective of switches

Switch classification 1: External perspective of switchesHistory of switching technologiesTerminology

Historical perspectiveRouters vs switches

Switching at various layersHierarchical networks



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Who cares?LUCKY: Several years ago, I asked my wife: "Does it bother

you that you don't know how the television works?" I mean, she just uses it,. . . She said, "I know how it works; you turn the switch and the thing comes on." I thought, "You know, she's right." There's these whole layers of understanding. There's a layer where you know how to turn a switch and make the TV come on.

-- Robert W. LuckyFrom full transcript of the Discover magazine Roundtable "Will Computers Replace Engineers?“ held on June 24, 2002, as part of IEEE’s INFOCOM Conference, www.comsoc.org/headlines/EngineerRTFINAL.pdf, accessed 2008dec11



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Sample evaluation questions

The aims of this course were clear to me

This lecturer communicated effectively with students (e.g. He/She explained things clearly).

I was provided with clear information about theassessment requirements for this course.

StronglyA

greeA

greeM

ildlyA

greeM

ildlyD

isagreeD

isagreeStrongly

Disagree

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Lecture shorthandSome abbreviations that you may see in lectures:

standard mathematics: => implies, ≈≊∝

↑↓ increases/decreases√ × advantages/disadvantagesc.f. compare withs.t. such thatwrt with respect toaka: also known asa la: in the manner ofb bits, B bytes, k 1000, K 1024

��

∃ there exists ∀ for all

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Ad breakWant to do a thesis/project on networking, e.g. as part of

MEngSc(Ext) or BE degree?

Several in the broad area of “network reliability” on offer

See http://www.eet.unsw.edu.au/~timm/thesis.html

You must have done well in networking course(s), possibly have industrial experience, confident programming

Email resume and academic records to [email protected] by end of week

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OutlineMotivation for (& definition of) switching

Non-switched networksFull meshBroadcast and select

Switched networks“Switch” defined

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Full mesh networks

• Each terminal directly connects to everyother terminal (that it communicates with)

× Uneconomical: Large number (N(N-1)/2, e.g. 15) of poorly utilized connections

× Unreliable: Single path between endpoints (unless nodes are willing to forward for others)

× Insecure: Endpoints control who can access their node. No capacity for partitioning or centrally managed policy.

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Broadcast and select networks

• Each terminal connects to a common shared medium.• Sources broadcast information.• Destinations select appropriate information.× Poor scalability: Shared medium is a bottleneck.

• As # of nodes ↑, transmission time spent arbitrating access (e.g. Ethernet collisions) also ↑.

× Poor security: Information is visible to all nodes.Endpoint control as per mesh.

× Poor reliability: Single failure point.× Difficult upgrade: Backward compatibility baggage, unless upgrade is

universal.

or

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Switched networksMost traffic is directed (broadcast=bad) and bursty

(mesh=bad)Switches

• Forward traffic only towards its destination(s)• Multiplex traffic from multiple sources

Advantages:√ Economical for large scale, e.g. 9 connections√ Smaller collision domains;

less time spent arbitrating access√ Relatively secure√ Reliable, e.g. choice of path√ Simple to upgrade ⇒

supports heterogeneity

Caveats:× Switches cost× Switches may get

congested or “block”× Switches introduce delay

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Functional definition of “Switch”“Switch”: Any device with multiple ports that aims to direct

unicast traffic only to one output port that leads to the destination.

Notes:“functional definition” – not a marketing “definition”“multiple ports” – multiple input ports alone would be a multiplexer;

multiple output ports alone a demultiplexer. Ports are aka interfaces.Multiple is best thought-of as 3 or more, in which case the switch must decide which

output port to send traffic to. A switch with just 2 ports (the routing part of many home “routers” is just that) is effectively a filter.

“unicast traffic” – multicast traffic may be sent to multiple output ports leading to multiple destinations.

“aims to” – bridges may not be able when they are yet to learn the destination’s location

“one output port” rather than “the output port” – there might be choices; which port is the best is a routing decision.

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Relatives of “switches”A multi-port device that directs input traffic to all ports isn’t a switch.

Call it a hub, combiner, etc.

A router is a type of switch that deals with network layer headers.“a type of switch” => switch functions (fabrics, packet classification,

scheduling, buffer management etc) are used in routers.

We’ll consider detailed definitions of types of switches (routers, bridges, etc) shortly.

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Multidisciplinary switchingSwitching informationSwitching stuff

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Switching information• Data communications, integrated services networks • Telephone network

• Gave rise to Clos networks, SS7 signalling, etc• Interconnection networks for parallel processors

• Strong parallels with structured space-division networks (e.g. Banyan)

Figure 1-8 from A. Tanenbaum and M. v. Steen: 'Distributed Systems: Principles and Paradigms', 2002

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Switching “stuff”Useful sources of accessible analogies to help

understand networking:• Vehicular traffic – railway switching yards,

automotive traffic (→ congestion control)• Irrigation systems → fluid flow models

& Hurst parameter• Utility networks (water, sewerage,

electricity, gas ...) → reliability assessments

Photo from http://www.flickr.com/photos/dustpuppy/78871005/ licensed under Creative Commons Attribution 2.0

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OutlineExternal perspective of switches

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Switch classification 1:By modularity of implementation

Bounded systems: fixed, pre-determined configuration.

Stackable switches:intra-stack connection:high-speed port (e.g.

Gigabit Ethernet)Low Voltage

Differential Signaling (LVDS)

Chassis switches:

Increasing• cost• performance• flexibility

Image sources unknown

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Dominant manufacturers

Computingbackground(common in

access networks)

Consumer devices:D-Link, Netgear,

Linksys

Extreme

3Com

Cisco

Juniper, Avici

Telephonybackground(common in core networks)

NECLucentNortelMarconiAlcatelSiemensEricsson

Newer Chinese manufacturers: Huawei Technologies, ZTE

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CiscoOne of the pioneersEstablished Internet Operating System (IOS) that provides

consistent interface to their systemsPreaches IOS and products through certification programs,

e.g. CCNA, CCNP, CCIEGood support “networks”Expensive

Online tour of Carrier Routing System (CRS)-1 http://www.cisco.com/en/US/products/ps5763/index.html

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OutlineEvolution of networks

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History of switching technologies1876 Bell is first to patent the telephone; manual switchboards1892 Strowger automated telephone switch1937 Reeves invents Pulse Coded Modulation (digital transmission)1950s Research into switching networks (Clos, Batcher, etc)1965 Bell System introduces the 1ESS (Electronic Switching

System)early- Packet switching invented by Baran, Davies & Kleinrock1960s1969 ARPAnet contract awarded to BBN1973 Metcalfe invents Ethernet1970s Optical fibre transmission systems

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History of switching (continued)

1976 X.25 recommendation for public data networks1978 OSI Reference Model1982 Bell System introduces 5ESS switch1984 Cisco (dominant router vendor) founded1988+ ATM1993 WWW booms with NetscapeLate MPLS, diffserv, photonic networks, “active networks”, 1990s caching, Content Distribution Networks

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Trends in historySwitching techniques: circuit (originally), packet (1960s), more

circuits

What happens in core: switching only (to 1990s), active networks, caches, CDNs (later)

Content that is switched: Telephone, then data, then integrated (TV traditionally broadcast, not switched)

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Outline

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Historical perspective of terms

Switching (and hence switches) preceded routing.=> separation between “switch” (e.g. phone switch) and packet networks (using gateways, routers, etc)

In the 1990s, the “need for speed” led to new “switching” techniques => association between “switch” and “fast”.

1950s 1960s 1970s 1980s 1990s 2000+

telephoneswitching

packetswitching

Internetgateways

LAN bridgesrouters‡

brouters

ATMfast packet switching

photonicswitching

layer 4+switching

‡ The first RFCs to mention routers were RFC 898 (1984 ) and RFC 1009 (1987)

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Pronunciation of “routing”““Rōō’·ting” is what fans do at a football game, what

pigs do for truffles under oak trees in the Vaucluse, and what nursery workers intent on propagation do to cuttings from plants.

“Rou’·ting” is how one creates a beveled edge on a tabletop or sends a corps of infantrymen into full-scale, disorganized retreat.Either pronunciation is correct for routing, which refers to the process of discovering, selecting, and employing paths from one place to another (or to many others) in a network.”

– D. Piscitello and A. Chapin: Open Systems Networking: TCP/IP and OSIcited in Cheswick, W. and S. Bellovin: Firewalls and Internet security: Repelling the wily hacker, Addison-Wesley, p. 26, 1994

+ Australian slang!Truffle hunting photo from www.paristempo.com/art/06truf-pig.jpg

Or more succinctly: “there are two different ways to pronounce the word router, either as “rootor” or as “rowter,” and people waste a lot of time arguing over the proper pronuciation [Perlman 1999].” [Kurose and Ross, p. 475]

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Marketing classificationThe most widespread, and eventually you have to use it to purchase productsDesigned/evolved to earn revenue for manufacturers: It’s easy to upsell to a

bewildered customer

Router: A multiport device that uses network layer (e.g. IP) headers to decide which port to forward packets on

e.g. Cisco 7000 series routerSwitch: A multiport device that uses link layer (e.g. Ethernet) headers to decide

which port to forward packets one.g. Cisco Catalyst 2900 Series

This course deals with the design of both routers and switches, in the marketing sense.

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Concerns about marketing terms• Classification according to layer (switch=link, router=network) doesn’t say

anything about different functionality; just examing different header bits• Doesn’t this just shift the question to one of numbering layers?

e.g. Q: Is ATM a link layer or a network layer technology?A1: ATM is a link layer: You can send IP packets over it => ATM switchesA2: ATM is a network layer: It concatenates links to form a path between systems

connected to the ATM network. => ATM routers (term isn’t used despite definitions justifying it)

• What is a “layer 3 switch”?, e.g. Cisco Catalyst 4840Gor for that matter, a “switch router”, e.g. Cisco Catalyst 8500Answer: A fast router.

• And questions arising in other layers (apart from link/network):Layer 4: What is layer 4 switching? (A: switching affected by transport headers)

e.g. Cisco Catalyst 6500 Series Content Switching Module Layer 2: Do switches differ from bridges? Layer 1: What do we call a device that operates only at the physical layer (e.g. MEMS

photonic switch using mirrors)? Why are some such devices called “lambda routers”?

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The issue of speedA “router” may require more processing than a “switch”, so may

operate slower† (packets/sec) for a given technologyEthernet switch:

1. Use frame addresses to index a database, indicating which outgoing port to use.

2. Start forwarding to outgoing port (needn’t wait to check CRC)Router:

1,2: Ethernet processing (check destination address, check CRC, frame validity checks), and only once that is complete, pass the packet up to the network layer

+ 3. IP processing (check destination address, decrement TTL, packet validity checks, IPv4 Segmentation And Reassembly)

=> perception that routers are slower than (Ethernet) switchesHeaven forbid us marketing a device whose name has “slow” connotations!

→ “switch router” “layer 3 switch” = fast router (e.g. lots of hardware, start IP processing before receive Ethernet CRC).

† A router may process fewer data units per second than a switch, butcan make more informed forwarding decisions, finding better paths etc=> network performance may be better

Check CRC

SARCheck header

Check DA MACLinklayer X

Check DA MACLinklayer

Netlayer

X

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Classification by implementationPacket switches traditionally operated on datagrams: self-contained data

units.Routing/switching/forwarding decisions (e.g. which port, which queue,

etc) can be made:• Each time a datagram arrives. This causes appreciable load:

• processing to make these decisions• transmission capacity to convey information used for decision making

• At the beginning of a flow of packets. Store the state, and refer back to those decisions whenever subsequent packets arrive. Couldn’t this reduce the processing load?=> “Fast Packet Switching” (e.g. ATM):1. Set up state info in switches2. Transfer data3. Release state info in switches

e.g. “switches” contain more state information than “routers” & this state info is explicitly established and released for each flow/connection.

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Functional classification of verbsFunctional sense of the words:Routing: Determining how to get there: Which output port should be used to

get to the destination?Switching†: The process of going there: Moving information from input ports

to appropriate output ports.Automotive analogy:

Routing = Navigating, Switching (lanes) = driving the vehicle

The 2 functions can be physically separatede.g. ATM & MPLS: device that determines routes may be separate (e.g. it could be centralised & omniscient) from the devices that actually do the switching

This course deals with switching in the general sense.We care about achieving functionality, not with naming products.It does not deal with routing, neither algorithms (e.g. Bellman-Ford) nor protocols (e.g. BGP). (It does deal with routers.)

† Sometimes called “forwarding” to avoid confusion about switching being only part of the role of a switch.

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Bottom-line definitions“Switch”: Any device with multiple ports that aims to direct

unicast traffic only to one output port that leads to the destination.

Router: A switch that deals with network layer headers.“a type of switch” => switch functions (fabrics, packet classification,

scheduling, buffer management etc) are used in routers.

Bridge: A switch that deals with link layer headers.

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A variety of textbook definitionsSources:• Keshav• Peterson and Davie• Kurose and Ross• Tanenbaum• McDysan• Telecom Glossary 2000 [http://www.atis.org/tg2k/]

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Keshav’s definitionsSwitch: “A switch allows data arriving at any of its inputs to be

transferred to any of its outputs.” p. 6 & details in Chapter 8

Routing: “How can we determine the shortest path from a source to a destination, or the best tree along which to distribute data from a source to a set of destinations? This is the problem of routing” p. 7 & details in Chapter 11

See also Keshav’s Infocom97 panel presentation on “Routing vs. Switching” http://www.cs.cornell.edu/skeshav/talks/infocom97panel/

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Peterson & Davie’s definitions“the core job of a switch is to take packets that arrive on an input and

forward (or switch) them to the right output so that they will reach their appropriate destination. Knowing which output is the right one requires the switch to know something about the possible routes to the destination. The process of accumulating and sharing this knowledge, the second problem for a packet switch, is called routing.”

– L. Peterson and B. Davie: Computer Networks: A Systems Approach, Morgan Kaufmann, 1996, p. 150

and they go into depth about the distinction between bridges, switches, and routers on pp. 234-237

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Kurose and Ross3rd edition, Section 5.6 pp. 475-6“routers are store-and-forward packet switches that forward packets

using network-layer addresses. Although a switch is also a store-and-forward packet switch, it is fundamentally different from a router in that it forwards packets using MAC addresses. Whereas a router is a layer-3 packet switch, a switch is a layer-2 packet switch.”

Problems:× Tying definitions to layers (see earlier slide)× Recursive definitions:

switch → packet switch → layer 3 packet switch → router→ layer 2 packet switch

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Tanenbaum’s definitions“As an aside, some people make a distinction between routing and

switching. Routing is the process of looking up a destination address in a table to find where to send it. In contrast, switching uses a label taken from the packet as an index into a forwarding table. These definitions are far from universal, however.”– A. Tanenbaum: Computer Networks, 4th edition, Prentice-Hall, 2003, p. 415

Notes:“some people” but not Tanenbaum?The distinction here is the method used for classification, with routing

presumably being necessary when identifiers are large (globally unique)

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CCNA course materials (v3.0)Semester 1 module 10:"10.2.2 Routing versus switching

Routing is often contrasted with switching. ... The primary difference is that switching occurs at Layer 2, the data link layer, of the OSI model and routing occurs at Layer 3. This distinction means routing and switching use different information in the process of moving data from source to destination....Another difference between switched and routed networks is switched networks do not block broadcasts.”

Semester 3, module 4.2.7 “The features and functionality of Layer 3 switches and routers have numerous similarities. The only major difference between the packet switching operation of a router and a Layer 3 switch is the physical implementation.In general-purpose routers, packet switching takes place in software, usingmicroprocessor-based engines, whereas a Layer 3 switch performs packet forwarding using application specific integrated circuit (ASIC) hardware.”module 4.3.4: “Today, switches are also able to filter according to the network-layer protocol. This blurs the demarcation between switches and routers. A router operates on the network layer using a routing protocol to direct traffic around the network. A switch that implements advanced filtering techniques is usually called a brouter. Brouters filter by looking at network layer information but they do not use a routing protocol.”

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Switching at various layersLower layer switchingHigher layer switching

Transport layer switchingApplication layer switching

Outline

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Lower Layer SwitchingPhysical: all-optical networks: Wavelength Division

Multiplexing, MicroElectroMechanical Systems (MEMS)Data link: bridgingNetwork: routing Most common layers for switching

T. Sridhar: "Layer 2 and Layer 3 Switch Evolution", Internet Protocol Journal, 1(2):38-43, Sep. 1998

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Higher-layer (4+) switchingThe switches that we’ve considered so far

implement all functions of the layers that they use for switching:

• Layer 2 (link): MAC & framing• Layer 3 (network): routing

Another type of switch (common at higher layers) only implements a subset (possibly null) of the functions of a layer, but is influenced by the information sent by that layer.

i.e. it depends on what protocol is used at that layer, but it doesn’t implement all of the functions of that protocol.

congestioncontrol

errorcontrol

flowcontrol

multiplexing /demultiplexing

SMTPemail

port 25

HTTPweb

port 80

TCP

accesscontrol

framing

error checkMAC

TCP ports identify software processes, and are different from switch ports which are hardware entities.

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Transport layer switchingStrict interpretation: Transport layer fields affect direction of propagation

(i.e. which output port).Switching above network layer processing. Switching between processes, e.g. for load balancing on a web server: might construct what clients perceive as a singular “server” by placing a switch between the Internet & a server farm.

might use the source port number to determine which machine receives the request: odd → machine 1, even → machine 2

(Strictly, you could argue that end-systems implement a form of layer 4 switching because they forward segments to the appropriate process, as indicated by their port numbers.)

Loose interpretation: Transport layer fields only affect type of service, i.e. treatment within the switch. Lower layer fields alone may determine direction. e.g. Network layer switch (IP address => direction) that gives telnet (TCP port 23) priority over FTP data (TCP port 20)

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Application layer switchinge.g. consider a web service, handling HTTP GET requests• Users → machines: Could use cookies (identifiers included in

requests) that identify users to direct them to a specific machine (helps in providing consistent state between consecutive requests)

• Objects/services → machines: Could direct GET requests for different information to specialised machines (less content each => higher cache hit rates etc):• image requests (file with .JPG extension) to one machine• HTTPS to machine with crypto hardware• cgi-bin/ to another• ...

Figure from W. Mangione-Smith and G. Memik: “Network Processor Technologies Tutorial”

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Hierarchical networksa. A flat view of a network/internetwork is of links that interconnect nodes.b. We can also consider nodes as being interconnected by networks, which

in turn consist of interconnected nodes or even networks.c. Hierarchical view shows networks with varying distances from terminals.

a. b. c.

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Benefits of hierarchical switching√ Heterogeneous access networks

Elements of hierarchy may differ by virtue of who runs/owns them, what technology they use, physical location, etc

√ Localise problems√ Localised traffic needn’t burden core

Spatial locality – how much usually leaves a workgroup switch to the next level of the hierarchy?

√ Align network topology with geography →

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More benefits of hierarchical switching

Align network topology with geography:√ Distribute management/administration of network√ Different operators for different levels of the hierarchy:

Local area: private institutional networkMetropolitan area: public network providers

Few provide physical infrastructure: Telstra, OptusMultiple provide service: infrastructure

providers+ISPs Wide area: many provide physical infrastructure and

service

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Hierarchy within an organisationLevels often referred to as 1. Access 2. Distribution3. Core / backbone

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Examples of network hierarchy1. The Bell Telephone system

(before divestiturein 1984, after which it lost its regular structure)

Regional offices (Class 1)

Sectional offices (Class 2)

Primary offices (Class 3)

Toll offices (Class 4)

End offices (Class 5)

… … ……

Local switch

Transit switch

Local loops

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Examples of network hierarchy2. The Internet

UNSW(ISP 1)

Unwired(ISP 2)

AARnet(NSP 1)

Optus(NSP 2)

Telstra(NSP 3)

NSP = Network Service ProviderISP = Internet Service Provider

iiNet(ISP 3)

BigPond(ISP 4)

Reach Internet2 +“Dot bombs”:Global Crossing, UUnet, ...

(nswrno)

ISPs

NSPs

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Exercise: Use traceroute to view which networks packets traverse to reach their destination.

Many servers available through www.traceroute.org

Hierarchical switching in the Internet

unsw.edu.au↓

aarnet.net.au↓

pnw-gigapop.net↓

ucaid.edu↓

nox.org↓

mit.edu

www.telstra.net↓

reach.com↓

bbnplanet.net↓

mit.edu

unsw.edu.au↑

aarnet↑

pnw-gigapop↑

jp.apan.net↑

kreonet.re.kr↑

ucaid.edu↑

nox.org ↑

mit.edu

e.g.:

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Path from UNSW to www.irtf.org$ traceroute www.irtf.orgtraceroute to www.irtf.org (192.150.187.18), 30 hops max, 38 byte packets1 eebu4s1.uwn.unsw.EDU.AU.92.171.149.in-addr.arpa (149.171.92.2) 14.624ms 0.775ms 1.040ms2 129.94.255.181 (129.94.255.181) 0.436ms 0.409ms 0.384ms3 gig2-2.nswrnosbb.nswrno.net.au (138.44.1.37) 0.582ms 0.563ms 0.527ms4 vlan948.gbe3-0.sccn1.broadway.aarnet.net.au(192.231.212.49) 1.450ms 0.805ms 0.758ms5 pos1-0.sccn1.seattle.aarnet.net.au (192.231.212.34) 157ms 156ms 157ms6 Abilene-PWAVE-1.peer.pnw-gigapop.net (198.32.170.43) 166ms 165ms 166ms7 snvang-sttlng.abilene.ucaid.edu (198.32.8.10) 174ms 173ms 173ms8 losang-snvang.abilene.ucaid.edu (198.32.8.94) 180ms 180ms 180ms9 hpr-lax-gsr1--abilene-LA-10ge.cenic.net (137.164.25.2) 190ms 190ms 190ms

10 dc-lax-dc1--lax-hpr1-ge.cenic.net (137.164.22.12) 181ms 181ms 181ms11 dc-sac-dc1--lax-dc1-pos.cenic.net (137.164.22.127) 190ms 190ms 189ms12 dc-oak-dc2--csac-dc1-ge.cenic.net (137.164.22.110) 201ms 201ms 201ms13 dc-oak-dc1--oak-dc2-ge.cenic.net (137.164.22.124) 192ms 193ms 192ms14 dc-svl-dc1--oak-dc1-10ge.cenic.net (137.164.22.30) 192ms 193ms 193ms15 ucb--svl-dc1-egm.cenic.net (137.164.23.66) 194ms 194ms 193ms16 fast4-0-0.inr-667-eva.Berkeley.EDU (128.32.0.99) 203ms 203ms 204ms17 router2-fast0-0-0.ICSI.Berkeley.EDU (169.229.0.30) 195ms 195ms 195ms18 www.irtf.org (192.150.187.18) 195ms 195ms 194ms

common phrases:gig, ge: Gigabit Ethernetpos: Packet Over SONET

3 delay measurements for each hopDelays vary with link congestion

Large increase in delay as packetspass over the Pacific Ocean

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Switch classification 2:By location in hierarchical network

moving towards network core

Desktop switch(may merely be a

shared-media LAN)

Workgroup /LAN switches

Campusswitch

Enterpriseswitches

Accessnetworks

Distribution /“transport” networks

Private networks Public networks

DLinkDES-1250G

Cisco Catalyst

4006

Cisco12000router

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1. Availability becomes increasingly importantHigh-reliability componentsRedundancy in power supplies, even redundant fabricsHot swapping of line interfaces & power suppliesMay employ “protection switches” to bypass severed links (low

switching rate, high throughput)

2. Throughput becomes increasingly important(though load may be less variable)

3. Reduced functionality, e.g. NAT, DHCP servers, firewalls, QOS tend to be implemented in workgroup switches but not core switches.

How do switches change as you move into the network core?:

Switch trends as location in hierarchy changes

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4. Fewer interfaces with higher capacity (& cost)e.g. fiber (not twisted pair), single mode (not multi-mode) fibreMay also offer public-network interfaces, e.g. ISDN – low-speed,

pay-per-use

5. More heterogeneous interfaces (although workgroup switch often has fast interface to connect to backbone/servers)

6. More symmetrical data flow

7. “Transit switching” rather than “line switching” (see next slide)

Switch trends as location in hierarchy changes(continued)

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Transit and line switchingLine switches: specific input to specific outputTransit switches: specific input to one of several outputs, e.g. several lines connecting this switch to another.

Transitswitching

S

Lineswitching

DOften discussed in the context of hierarchical networks, where a low-level network may connect to multiple higher-level networks for fault tolerance.Transit vs line switching is analogous to anycast vs unicast:= line/unicast must go in a particular direction, transit/anycast may have a choice≠ switching methods refer to switch outputs, whereas anycast/unicast refers to

final destinations

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