Local Area Networks

Local Area Networks Content Chapter 14: Advanced Review (Part I)

Anatomy of a network • A set of interconnected resources • Hosts that run network applications software • Clients and servers • Set of peers • The network infrastructure that interconnects the hosts • The networking hardwareand software • Network node devices such as routers and switches • Links: cables, connectors, network interfaces

Transmission links • Convey bits, bytes, packets • Physical medium • Copper (or aluminium) • Optical fibre • Glass, plastic • Free-space optical • Infrared (IR): light-emitting diode or laser • Visible red or green: laser • Radio • Satellite, microwave link, mobile, wireless LAN, ‘Bluetooth’ • Mode • Point-to-point • Shared medium • Multidrop, multicast • Broadcast

Transmission & interconnection media & devices • The main types of interconnection media can be divided into: • Guided media: copper twisted pair, coaxial and optical fibre cables • Unguided media (wireless): • Infrared, microwave (point to point and satellite), radio and laser • Bits travel (propagate) at the speed of light (3 x 108 m/sec) in unguided media and roughly two-thirds this (5µs/km [what speed is this?]) In guided media • Rate at which connection operates (bits are put onto medium) know as transmission rate and depends on interface and properties of medium (measured in bit/sec) • The main types of interconnection device are: • Modems • For digital connection over analogue networks • Multiplexers , repeaters, hubs, switches, routers • For digital connection over digital networks

Network node devices • Router • Determines route packet takes through network & switches packet onto correct output link • Switch, bridge • Switches packet, byte, bit from one transmission link to anotherunder software control • Hub, repeater • Repeats digital data stream received on one input link to all output links • Patch panel • Physical, manually re-configurable wiring / cabling interconnect • Wiring closet • Cupboard where any of the above may be sited:distribution point for corridor, building, site network cabling

H H H H H H H H R R R R R R net net net net net net Concept of interconnection in a communication network • Shared medium • No hubs, switches, or routers • Examples: • Broadcast radio: wireless LAN, satellite • Shared links and / or hubs/repeaters: basic Ethernet, Token Ring • Interconnects hosts directly • Switches & hubs • Examples: • 10BaseT • Switched ethernet: fast ethernet, gigabit ethernet • Interconnect links • Routers • Interconnect networks • Hence internetwork (or just internet) Hub orSwitch Hub orSwitch H H

Types of network • Main types: • WAN - wide area network • LAN - local area network • Ownership can be either public or private • Clientele can be either public or private • Also • MAN - metropolitan area network • Global networks • Global network examples • Computer networks • The internet, companies’ private networks • The telephone network • Virtual & overlay networks • Subsets of participants • Is this a LAN or a WAN? • The Internet: • a set of interconnected overlay networks?

Structure and Infrastructure Overview Basic Techniques LAN Structure Circuit-switching and Packet-switching LAN Interconnection Services Course Structure

The first interconnection networks • Inside the computer room • Connect peripheral devices via device controllers to CPU • Controllers used simple hierarchical master/slave protocols • CPU polls controllers • Controllers poll devices • Devices respond to controller • Controller responds to CPU • Protocol simplicity arises from • Assumed reliability • Use of high-speed parallel links • Facilitates high-speed data transfer

local mux time- sharing computer system remote mux computer System remote card/printer Terminal networks • First WANs connected local and remote terminals to central mainframe • Often called multi-access or time-sharing systems • Developed out of early operating systems work at MIT • Used PSTN for remote links • Installed/owned local links • Communications need to be serial (bit-oriented) over these distances • Link error rate typically rather high • Typically between 10-3 and 10-4 • Required new protocols that could detect, and perhaps correct, transmission errors

Link layer hardware and software • Most hosts attach to LAN using network interface card (NIC) • Ethernet NICs dominate • WLAN becoming popular • Other possibilities include Token Ring, cable modem, ADSL modem • Routers attach LANs to WAN • Include a mixture of interface types • For example, Ethernet, Frame Relay, X.25, ATM • Network interfaces include • Physical layer (layer 1) components • Timing, coding, and ‘line driver’ chips • Link layer components • Link protocol controller chip • Data memory for frame buffers • Link layer protocols provided as operating system ‘drivers’

Multiplexing: sharing the bandwidth • Historically, the term ‘bandwidth’ refers to the range of allowable frequencies on an analogue link • Today, we usually mean the available transmission capacity • LAN links tend to use the whole of the bandwidth for a single transmission • We sometimes see the term baseband in this context • WAN links tend to be shared in some way, for several reasons • Chief one is economics (service provider charges a lot for use of total capacity) • They tend to have greater capacity than is required by one transmission/customer • At the signal level (Layer 1) we call this sharing multiplexing • Two basic multiplexing techniques • Frequency division multiplexing (FDM) for use over analogue links • Time division multiplexing (TDM) for use on digital links • Wavelength division multiplexing is a variation of FDM used on optical fibre links

remote mux T 1 T 2 T 3 T 4 8 - 1 2 k H z 1 2 - 1 6 k H z 4 - 8 k H z 0 - 4 k H z FDM: analogue multiplexing • Bandwidth shared on basis of frequencies • Normally full-duplex transmission • Two-way simultaneous transmission • Uses four separate frequencies • For 0 & 1 in each direction of the transmission • Cables with a wide frequency range(e.g. coaxial cables) can have multiple channels, each with own sub-range • Each channel uses separate frequencyfor 0 & 1 in each direction • FDM used over unguided media • Wireless, microwave, satellite • Can also be used on guided media

WDM: Wave Division Multiplexing • Used on optical fibre • Comes in two main flavours • Colour multiplexing • Dense WDM • Colour multiplexing usually refers to multiplexing of a few wavelengths on to fibre • For example red and blue light in the visible spectrum • Dense WDM refers to multiplexing of many slightly different optical wavelengths onto a high-quality fibre • Current technology allows up to 120 wavelengths • Each separated by only a few (typically 4-6) nanometers • Each wavelength can carry a digital 40Gbit/s data stream • Equates to 4.8Tbit/s

remote mux labels TDM: digital multiplexing • Usually full-duplex • But if bandwidth is limited, can use • Half-duplex transmission • Asymmetric bit rates • Multiplexer allocates each end-system atransmission time-slot • Each device in turn gets all of the line capacity for a small fraction of time • For example, 3.90625 µsec for a 64 Kbit/s interfaceon a 32-way multiplex onto a 2.048 Mbit/s circuit • TDM used over guided media • Copper, fibre • Can also be used on unguided media

Statistical multiplexing • Multiplexing can be based on two techniques • Dedicated • Shared • Non-statistical multiplexing is dedicated • End-system gets part of total capacity (frequency or time slot) all the time • Whether or not the end-system is able to use it • Demultiplexing performed on basis of known frequency or timeslot • Also called positioned multiplexing • Statistical multiplexing is shared • End-system gets total capacity some of the time, as needed • Sometimes called “bandwidth on demand” • Requires some sort of channel label to identify end-system • Demultiplexing performed on basis on label • Also called labelled multiplexing • FDM and TDM systems can use both methods of multiplexing • WDM uses positioned multiplexing

Protocol stack A Protocol stack B Relays: Interconnectionat Different Layers • Router • Layer 3 relay • Lower layers can be LAN or WAN protocol stacks • e.g. Ethernet, PPP, X.25 • Bridge/switch • Layer 2 MAC sublayer relay • Layer 1 LAN protocol stack • e.g. Ethernet, wireless LAN • Repeater/hub • Layer 1 relay • Can interconnect different media • e.g. copper twisted pair, optical fibre ISO or IETF protocol stacks Layer nrelay Layer n Layer n Layer n-1 Layer n-1

Protocol stack A Protocol stack B Relays above Layer 3 • Called Application layer gateways • Or just “gateways” • Interconnect applications of same generic type, but which use different message formats • Possibly also different protocols • Examples • IETF-X.400 mail gateway • IP-PSTN gateway • Proxy server (firewall) ISO or IETF protocol stacks Layer nrelay Layer n Layer n Layer n-1 Layer n-1 PSTN = public switched telephone network

Notes: (1) UTP-5 is, more correctly, referred to as ‘Category 5 unshielded twisted pair’ cable (2) Coaxial cable very rarely found in modern LAN cabling

relay logic Phys1 Phys2 Coaxial LANsegment hub relay logic Ph1 Ph1 Ph1 Ph1 Ph1 Ph1 Ph1 Repeaters and Hubs • Operate at the Physical Layer • Physical Layer relays • Unit of transfer is the bit • Extend domain of MAC protocol • The collision domain • Repeat incoming bits to other ports • MAC frames seen by all systems • Systems contend for extended communication channel • Support a variety of media types • Allows old style shared coaxial segments to be connected to modern twisted pair segments • Most hubs are just multi-port repeaters

Coaxial LANsegment Ethernet Hubs • Have separate ports for each system • Enhances LAN resilience • Operate over structured cabling systems • Can be cascaded (to a limited degree) to interconnect multiple LAN segments • 10BASE-T: no more than four hubs • Same rules that applied to coax installations) • 100BaseT: no more than 2 with twisted pair cable • 1000BaseT: only one hub • And even that is very rare • Are becoming increasingly rare as switches get cheaper • Many users now connect to the LAN via a switch

Shared vs. Switched Bandwidth • Example: Twelve users and fourservers share 100Mbit/s LAN • All in same collision domain • Access time to shared channelincreases as usage increases • Solution to increasing congestion:replace shared LAN with 10/100Mbit/s switch • Users divided into smallercollision domains • Each receives larger portionof bandwidth • Switch throughput at leastport speed  ½ number of ports • Eight-port switch supports up to400Mbit/s throughput

Switched LANs • Switches commonly used for LAN-LAN interconnect • Usually interconnect same technologies • For example, Ethernet to Ethernet • Falling switch prices have killed off the hub market • Switches available for all versions of Ethernet • 10, 100, 1000 and 10000Mbit/ • But support for 10Mbit/s only is increasingly rare • Rapidly vanishing support for older technologies • Token Ring (16 & 100Mbit/s), FDDI and ATM (25, 155 & 622Mbit/s) • Ethernet switches have become widespread due to • Their versatility • Support of different bit rates and media types • Their lower per-port cost than alternative technologies

Evolving Technologies forEthernet LAN Interconnection

The Rise and Fallof the LAN Router • In early 1990s, small routers introduced to limit sizeof broadcast domains • Became cheap, and fast, enough to use in LANs • But routers operate at Network Layer • Require configuration (are not plug-and-play) • Have higher per-port cost than equivalent bridge • LAN switches began to replace bridges in mid-1990s • Still operate at Layer 2 • Have much lower per-port cost than routers • Can be operated in plug-and-play mode or configured • For example with management and VLAN information • Routers still required for inter-site and inter-VLAN communication • Particularly suitable for interconnecting different technologies • For example, CSMA/CD & Frame Relay, CSMA/CD & Token Ring

802.3,5,11, etc Physical: to matchData Link Protocol Bridges and Switches Bridge/LAN switch • Bridges and LAN switches • Used to interconnect LANs of same type • Are Layer 2 devices • Operate on MAC frames Layer 2 relay

router IP Layer 3 relay PPP, 802.3,5,11, etc Physical: to matchData Link Protocol Token Ring LAN CSMA/CD LAN WAN VLAN 2 VLAN 3 VLAN 3 Hub Routers • Routers • Used for LAN–WAN and VLAN interconnection • Layer 3 devices • Operate on packets

Multilayer Switches • Multilayer switches have both switching and routing modules • Operate at Layer 2 and Layer 3 • Often very high-speed and rather expensive devices • Typically equipped with hardware acceleration • Used in backbone (or ‘distribution’) networks Multilayer switch

Wiring closet 10/100 switch Workgroup servers Fiber links Hub Hub Hub Multilayer switch Site backbone Gigabit Ethernet/ATM Modern LAN Structure • Workgroups connected to small switches • Workgroup servers get dedicated ports • 10 and 100Mbit/s connections • Workgroup switches interconnected by multilayer switches • The backbone or distribution network • 100 and 1000Mbit/s connections used

Interconnection overview • LANs on same site typically linked by higher-speed LANs • For example, Ethernet LANs can be linked by higher-speed Ethernet links • Offsite connections usually provided by a service provider • Often referred to as a public network operator (PNO) • For example, telephone company, cable TV operator, satellite communications company • Type of service provided divides into two main categories • Dedicated inter-site capacity • Shared public network • It is rare for company to provide own inter-site links • Due mainly to installation cost

Dedicated capacity services • Can be “always on” and charged one basis of permanent availability * • Physical inter-site point-to-point links • Often referred to as leased lines • Can be implemented over cables (copper, fibre), satellite, microwave, high-speed wireless • Or can be provided on demand, charged for duration of connection • Also called a “switched service” or a “dial-up” service • Special call control (signalling) protocols used to set-up and clear down connection • Examples are PSTN, ISDN • Essentially a Physical Layer service • Attached systems run own Link layer protocols end-to-end across service PSTN = public switched telephone network ISDN = integrated services digital network * Means permanently available; also referred to as “24/7”, meaning 24 hours a day, 7 days a week

Shared capacity services • Can also be “always on” • Virtual point-to-point links over a shared public network • Referred to as virtual private networks (VPNs) • ADSL and cable modem for domestic and small offices • Connection over public frame relay *, ATM, SMDS or IP network • Again, charged on basis of permanent connection • Or can be provided as a switched service • Again, signalling protocols used to set-up and clear down connection • Main examples is X.25 • Provided either as Layer 2 service: FR, ATM, SMDS • Or Layer 3 service: IP, X.25 ADSL = asymmetric digital subscriber line ATM = asynchronous transfer mode SMDS = switched multimegabit data service IP = internet protocol * Frame Relay and ATM standards define a demand service, but it is rarely, if ever, used for site-interconnection

Dedicated vs. shared • Dedicated Layer 1 service appears to be point-to-point circuit • Could be non-switched – i.e. a real circuit • But is often a dedicated part of larger circuit belonging to the PNO • Usually provided as a feed into a public switched network • Hence the term circuit switched network • Circuit switched means • Dedicated capacity • Fixed path through network • Fixed inter-site communications delay • Shared Layer 2 or 3 services referred to as packet switched services • Always cheaper than comparative circuit switched service • Packet switched means • Shared capacity • Fixed or varying path through network • Variable inter-site communications delay

Packet switched services • Term “packet switching” has historical significance • Shared, as opposed to dedicated service • End-systems chop up (‘segment’) messages before transmission • Network interleaves packets from different users on an as-needed basis • Connection oriented or virtual circuit services use fixed network path • Guarantee delivery sequentiality • Often include built-in safety checks • Connections can be permanently set up (PVC) or demand (SVC) • Examples include X.25, Frame Relay, ATM • Connectionless services use any available path • Do not offer delivery sequentiality • Have no built-in safety checks • Examples include IP, SMDS

Some service comparisons

Interconnection topologies • The topology (interconnection shape) is implicit in many network types • Growth and the need for redundancy blurs the topology somewhat • Here are some common topologies • You should be able to name them, and the basic network types to which they usually apply

2 2 2 1 1 1 3 3 3 5 5 5 4 4 4 ? Public network Connecting Multiple Sites:Circuit or Packet-switched Solution? Circuit-based solution Packet-based solution Not shared, gives predictable end-to-end delay, but needs n(n-1)/2 long-distance, leased lines. Expensive! Shared, so end-to-end delay is less predictable, but needs only n local access lines. Cheaper than comparable circuit-based service.

NationalNet LocalRegional Net Internal routers or switches only connect to other routers or switchesin same net Edge routers or switches connect to other nets, or to subscriber access Hierarchical Network Architecture • Hierarchy typically local region & national • Highest level of hierarchy often referred to as backbone • Repeated at international level • International backbones connect to national backbones

Services and Interfaces* • Public data networks offered for public use by • PNO: Public Network Operator • Often telecommunications service provider • ISP: Internet Service Provider • Typically IP only • Usually standardized as interface specifications • Specify the service(s) offered • The access protocol(s) for connecting to the service • But not the internal operation of network providing the service • Common to offer multiple services over a single infrastructure • Level of Service depends protocol structure of interface specification • Layer 1 : Physical connection only • Layer 2: Frame or cell-based • Layer 3: Packet-based Circuit-based services Packet-switched services Two things tend to get standardized: (i) the service offered by the network, and (ii) how customers interface to the network

2005 1975 1985 1995 Circuit-Based Services Dial-up digital connections ISDN Dial-up telephone connections using modems Fixed leased lines

Leased Lines • Non-switched end-to-end digital connections provided by PNOs • No features added • Really just a connection, not strictly a service • Early offerings were analogue, requiring connection by modem • Digital services in use since early 1980s • Digital bit-rates based on 64kbit/s voice channel • Higher bit rates are multiples of these • Customer access based on Plesiochronous Digital Hierarchy (PDH) • Core network uses Synchronous Digital Hierarchy (SDH) • Replacement of PDH • Overcomes many problems of older PDH • Optimized for use on optical fibre infrastructure • American equivalent called Synchronous Optical Network (SONET) • 24/7 operation is expensive because • It is charged on basis of bit-rate and distance • Service provider must perform per-customer bandwidth reservation

Digital Hierarchies(Outside US – Reference Only) PDH SDH

ADSL VPN ATM SMDS Frame Relay X.25 2005 1975 1985 1995 Packet-Switched Services

DSL Overview • Always-on, dedicated broadband service • Operates over ‘subscriber lines’ (lines to local telephone exchange) • DSL – one term, many variations • ADSL – Asymmetric DSL – 8 Mbps down, 640 kbps up • EDSL – Enhanced DSL – up to 1Mbps total (2-wire) • G.Lite – Slower version of ADSL that is easier to install • Also called UDSL - Universal DSL – 1.5Mbps down, 512kbps up • HDSL – High bit-rate DSL – up to 2Mbps total (4-wire or 2-wire) • IDSL – Integrated DSL – ISDN 2B+0D access (128/144 kbps) • SDSL – Symmetric DSL – (= HDSL2) • VDSL – Very high speed DSL – 52 Mbps down, 2 Mbps up • Maximum data rates and range depend on individual installations • Quality and thickness of copper and line installation quality • Range generally 3km or more (less for VDSL)

Full ADSL G.lite ADSL ISDN 56K 28.8K 14.4K 0 1000 2000 3000 4000 5000 6000 7000 8000 Kbit/s ADSL vs. Other Modems Maximum Speed - Actual speed will vary Source: ADSL Life www.adsllife.com

1. Networking devices: hubs, switches and routers 7. Operating at theNetwork Layer 4. Switched Ethernet LANs 9. Routers androuting protocols 3. Communications protocols 6. Ethernet bridging,STP and VLANs 10 Queuing systems 5. Using sharedmedia networks 8. Transmissionand coding 11. Switches, routersand interconnection networks Structure of rest of course 2. The IP Suite

Local Area Networks