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

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

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

Transmission & interconnection media & devices n The main types of interconnection media can be divided into: – Guided media: copper twisted pair, coaxial and optical fibre cables – Unguided media (wireless): n Infrared, microwave (point to point and satellite), radio and laser n Bits travel (propagate) at the speed of light (3 x 10 8 m/sec) in unguided media and roughly two-thirds this (5µs/km [what speed is this?]) In guided media n 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) n The main types of interconnection device are: – Modems n For digital connection over analogue networks – Multiplexers, repeaters, hubs, switches, routers n For digital connection over digital networks

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

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

n Main types: – WAN - wide area network – LAN - local area network – Ownership can be either public or private – Clientele can be either public or private n Also – MAN - metropolitan area network – Global networks n Global network examples – Computer networks n The internet, companies’ private networks – The telephone network n Virtual & overlay networks – Subsets of participants Types of network 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 n Inside the computer room – Connect peripheral devices via device controllers to CPU n Controllers used simple hierarchical master/slave protocols – CPU polls controllers – Controllers poll devices – Devices respond to controller – Controller responds to CPU n Protocol simplicity arises from – Assumed reliability – Use of high-speed parallel links n Facilitates high-speed data transfer

remote mux computer System remote card/printer time- sharing computer system local mux Terminal networks n First WANs connected local and remote terminals to central mainframe n Often called multi-access or time-sharing systems n 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 n Typically between and n Required new protocols that could detect, and perhaps correct, transmission errors

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

Multiplexing: sharing the bandwidth n Historically, the term ‘bandwidth’ refers to the range of allowable frequencies on an analogue link – Today, we usually mean the available transmission capacity n LAN links tend to use the whole of the bandwidth for a single transmission – We sometimes see the term baseband in this context n 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 n At the signal level (Layer 1) we call this sharing multiplexing n 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

FDM: analogue multiplexing n Bandwidth shared on basis of frequencies n Normally full-duplex transmission – Two-way simultaneous transmission – Uses four separate frequencies n For 0 & 1 in each direction of the transmission n Cables with a wide frequency range (e.g. coaxial cables) can have multiple channels, each with own sub-range – Each channel uses separate frequency for 0 & 1 in each direction n FDM used over unguided media – Wireless, microwave, satellite n Can also be used on guided media T1T2T3T4 0-4kHz 4-8kHz 8-12kHz12-16kHz remote mux

WDM: Wave Division Multiplexing n Used on optical fibre n Comes in two main flavours – Colour multiplexing – Dense WDM n Colour multiplexing usually refers to multiplexing of a few wavelengths on to fibre – For example red and blue light in the visible spectrum n Dense WDM refers to multiplexing of many slightly different optical wavelengths onto a high-quality fibre – Current technology allows up to 120 wavelengths n 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

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

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

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

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

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

Me dia typ e Data rate (Mbit/ s) Max. cable length (metres ) Max. number of stations per cable Structured Cabling term Examples of Ethernet terminolog y Twi ste d Pai r cop per 10, 100, Two UTP-5, UTP-5e, UTP-6 10BASE-T, 100Base-T, 1000Base-T Opt ical Fib re 10, 100, 1000, Depend s on fibre type and data rate Two Multimode (MMF), Single mode (SMF) 10BASE-F, 100BASE- FX, 1000BASE- SX 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

Repeaters and Hubs n Operate at the Physical Layer – Physical Layer relays – Unit of transfer is the bit n Extend domain of MAC protocol – The collision domain – Repeat incoming bits to other ports n MAC frames seen by all systems n Systems contend for extended communication channel n Support a variety of media types – Allows old style shared coaxial segments to be connected to modern twisted pair segments n Most hubs are just multi-port repeaters relay logic Phys 1 Phys 2 Coaxial LAN segment hub Ph 1 relay logic

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

Shared vs. Switched Bandwidth n Example: Twelve users and four servers share 100Mbit/s LAN – All in same collision domain – Access time to shared channel increases as usage increases n Solution to increasing congestion: replace shared LAN with 10/100Mbit/s switch – Users divided into smaller collision domains n Each receives larger portion of bandwidth – Switch throughput at least port speed  ½ number of ports n Eight-port switch supports up to 400Mbit/s throughput

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

Evolving Technologies for Ethernet LAN Interconnection 1980 – 1984 Shared Ethernet (CVSMA/CD) deployed International LAN standards developed 1985 – 1989 Bridges used for LAN interconnection to limit size of collision domains, with Spanning Tree facilitating bridge redundancy; routers used for LAN-WAN interconnection High-speed, low-cost routers become alternatives to bridges ‘Backbone’ routers developed for site interconnections VLAN-capable switches replace bridges and LAN routers 100Mbit/s Ethernet becomes common, GbE developed Dedicated switched access and VLAN deployment become common 10GbE developed, Ethernet switches become QoS- enabled

The Rise and Fall of the LAN Router n In early 1990s, small routers introduced to limit size of broadcast domains – Became cheap, and fast, enough to use in LANs n But routers operate at Network Layer – Require configuration (are not plug-and-play) – Have higher per-port cost than equivalent bridge n 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 n For example with management and VLAN information n Routers still required for inter-site and inter-VLAN communication – Particularly suitable for interconnecting different technologies n For example, CSMA/CD & Frame Relay, CSMA/CD & Token Ring

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

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

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

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

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

Interconnection overview n LANs on same site typically linked by higher-speed LANs – For example, Ethernet LANs can be linked by higher-speed Ethernet links n 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 n Type of service provided divides into two main categories – Dedicated inter-site capacity – Shared public network n It is rare for company to provide own inter-site links – Due mainly to installation cost

Dedicated capacity services n Can be “always on” and charged one basis of permanent availability * – Physical inter-site point-to-point links n Often referred to as leased lines n Can be implemented over cables (copper, fibre), satellite, microwave, high-speed wireless n 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 n Essentially a Physical Layer service – Attached systems run own Link layer protocols end-to-end across service * Means permanently available; also referred to as “24/7”, meaning 24 hours a day, 7 days a week PSTN = public switched telephone network ISDN = integrated services digital network

Shared capacity services n Can also be “always on” – Virtual point-to-point links over a shared public network n 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 n Or can be provided as a switched service – Again, signalling protocols used to set-up and clear down connection – Main examples is X.25 n 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 n 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 n Usually provided as a feed into a public switched network – Hence the term circuit switched network n Circuit switched means – Dedicated capacity – Fixed path through network – Fixed inter-site communications delay n Shared Layer 2 or 3 services referred to as packet switched services – Always cheaper than comparative circuit switched service n Packet switched means – Shared capacity – Fixed or varying path through network – Variable inter-site communications delay

Packet switched services n 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 n 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 n Connectionless services use any available path – Do not offer delivery sequentiality – Have no built-in safety checks – Examples include IP, SMDS

Some service comparisons Circuit switched Connection oriented packet switched Connectionless packet switched Layer 1 service only: Layer 2 (FR, ATM) or Layer 3 (X.25) service Layer 2 (SMDS) or Layer 3 (IP) service Fixed network path means service lost if path fails Variable network path means tolerance to failure Message blocks associated with fixed circuit/virtual circuit identifier Message blocks need to carry full end-system addresses No buffer storage required in network switches Buffer storage required in switches Network capacity set aside for circuit Network capacity shared between service subscribers

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

Connecting Multiple Sites: Circuit or Packet-switched 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 Public network ? Circuit-based solutionPacket-based solution

Hierarchical Network Architecture n Hierarchy typically local region & national – Highest level of hierarchy often referred to as backbone n Repeated at international level – International backbones connect to national backbones National Net Local Regional Net Internal routers or switches only connect to other routers or switches in same net Edge routers or switches connect to other nets, or to subscriber access

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

Services and Interfaces * n Public data networks offered for public use by – PNO: Public Network Operator n Often telecommunications service provider – ISP: Internet Service Provider n Typically IP only n 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 n Common to offer multiple services over a single infrastructure n 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

Circuit-Based Services Dial-up digital connections ISDN Dial-up telephone connections using modems Fixed leased lines Service Non- Switche d Circuit- Switche d Leased lines PSTN ISDN

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

Multiplexi ng Level Name Bit Rate (Mbit/s) 0STM STM STM STM STM Digital Hierarchies (Outside US – Reference Only) Multiplex ing Level Name Bit Rate (Mbit/s) 0 Basic channel 64kbit/s 1E E E E PDH SDH

Servic e Laye r 2 Laye r 3 X.25 FRFrame Relay SMDS Switched Multimegabit Data Service ATM Asynchronou s Transfer Mode VPN Virtual Private Network ADSL Asymmetric Digital Subscriber Line Packet-Switched Services SMDS Frame Relay X.25 ATM VPN ADSL

DSL Overview n Always-on, dedicated broadband service – Operates over ‘subscriber lines’ (lines to local telephone exchange) n 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 n 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 n 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)

ADSL vs. Other Modems Kbit/s 14.4K 28.8K 56K ISDN G.lite ADSL Full ADSL Source: ADSL Life Speed - Actual speed will vary

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

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

Summary n We've discussed the following practical issues – How networks evolved – Data link interfaces and protocols – Interconnection media and devices – Multiplexing – LAN structure – Circuit-switching and packet switching – Interconnection topologies – Interconnection services

Tutorial Questions  Approximately, what is the speed of propagation in a telephone line?  Give two examples which illustrate the idea of the Internet being a set of interconnected overlay networks.  When bits are to be transmitted onto the network medium, two of the choices that must be made are: (i) W hether to use digital or analogue communication, and (ii) How to represent 0 and 1 (in other words, the line coding technique). a) Which type of transmission does a modem use: analogue or digital? What is the main difference between the two? How might it represent 0’s and 1’s? b) Is it possible for a modem to transmit more than one bit at a time? c) Which type of transmission does ISDN use? Analogue or digital? How many channels are multiplexed together on a ‘Basic Rate Interface’ and what multiplexing scheme is used: TDM or FDM?  Verify the arithmetic for the example of 32 x 64 kbit/s channels multiplexed onto Mbit/s channel given on slide 34 of week 1 lecture.

Tutorial Questions (continued)  Give an example of a standard that defines how: a) You connect a computer to a modem? b) You connect a computer to a LAN?  Give an example of: a) An FDM technique used on guided media; b) A TDM technique used on unguided media.  List the network topologies and, for each topology, say what it is called and whether it is exclusive to a particular LAN or WAN technology or, if not, to which LAN and WAN technologies it applies.  Many operating systems provide some sort of terminal emulator program that turns your PC into a dumb terminal so that you can connect to a remote computer system. What is the protocol that provides this facility on an IP network?