Local Area Networks
ContentChapter 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
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)
H
H
H
HH
H
HH Hub orSwitch
Hub orSwitch
net
netnet
net
net
netR
R
R
R
R
R
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
Types of network
Is this a LAN or a WAN?
The Internet:a set of interconnected overlay networks?
Structure and Infrastructure
OverviewOverview
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
remotemux
computerSystem
remote card/printer
time-sharing
computersystem
localmux
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
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
T1 T2 T3 T4
0-4kHz 4-8kHz 8-12kHz 12-16kHz
remotemux
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
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
labels
remotemux
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
Structure and Infrastructure
OverviewOverview
Basic Techniques
LAN Structure
Circuit-switching and Packet-
switching
LAN Interconnection Services
Course Structure
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
Protocol stack A Protocol stack B
Layer nLayer n
Layer n-1 Layer n-1
Layer nrelay
ISO or IETF protocol stacks
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)
Protocol stack A Protocol stack B
Layer nLayer n
Layer n-1 Layer n-1
Layer nrelay
ISO or IETF protocol stacks
PSTN = public switchedtelephone network
Media type
Data rate
(Mbit/s)
Max. cable length
(metres)
Max. number
of stations
per cable
StructuredCabling term
Examples of Ethernet terminolog
y
Twisted
Pair
copper
10, 100, 1000
100 TwoUTP-5,
UTP-5e, UTP-6
10BASE-T,100Base-T,1000Base-T
Optical
Fibre
10, 100, 1000, 10000
Depends on fibre type and data rate
TwoMultimode
(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
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
relay logicPhys
1
Phys
2
Coaxial LANsegment
hub
Ph1 Ph1 Ph1 Ph1 Ph1 Ph1 Ph1
relay logic
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
Coaxial LANsegment
Shared vs. Switched Bandwidth
Example: Twelve users and fourservers share 100Mbit/s LAN
– All in same collision domain– Access time to shared channel
increases 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
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
Switched LANs
Evolving Technologies forEthernet LAN Interconnection
1980 – 1984
Shared Ethernet (CVSMA/CD) deployedInternational 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
1990 - 1994 High-speed, low-cost routers become alternatives to bridges‘Backbone’ routers developed for site interconnections
1995 - 1999 VLAN-capable switches replace bridges and LAN routers100Mbit/s Ethernet becomes common, GbE developed
2000 - Dedicated switched access and VLAN deployment become common10GbE developed, Ethernet switches become QoS-enabled
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
Bridges and Switches
Bridges and LAN switches– Used to interconnect LANs of same
type– Are Layer 2 devices
Operate on MAC frames
802.3,5,11, etc
Physical: to matchData Link Protocol
Bridge/LAN switch
Layer 2relay
Routers
Routers– Used for LAN–WAN and VLAN
interconnection– Layer 3 devices
Operate on packets
WAN
TokenRing LAN
CSMA/CD LAN
Hub
IP
PPP, 802.3,5,11, etc
Physical: to matchData Link Protocol
router
Layer 3relay
VLAN 2
VLAN 3
VLAN 3
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
Modern LAN Structure
Wiringcloset
Workgroupservers
Fiberlinks
10/100switch
Multilayerswitch
Site backboneGigabit Ethernet/ATM
Hub
Hub
Hub
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
Structure and Infrastructure
OverviewOverview
Basic Techniques
LAN Structure
Circuit-switching and Packet-switching
LAN Interconnection Services
Course Structure
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
* Means permanently available; also referred to as “24/7”, meaning 24 hours a day, 7 days a week
PSTN = public switched telephone networkISDN = integrated services digital network
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 lineATM = asynchronous transfer modeSMDS = switched multimegabit data serviceIP = 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
Circuit switchedConnection
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 switchesBuffer storage required in switches
Network capacity set aside for circuit
Network capacity shared between service subscribers
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
Connecting Multiple Sites:Circuit or Packet-switched Solution?
1
54
3
2
1
54
3
2
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.
1
54
3
2
Public network
?Circuit-based solution Packet-based solution
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
NationalNetLocal
Regional Net
Internal routers or switches only connect to other routers or switches
in same net
Edge routers or switches connect toother nets, or to subscriber access
Structure and Infrastructure
OverviewOverview
Basic Techniques
LAN Structure
Circuit-switching and Packet-
switching
LAN Interconnection Services
Course Structure
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
Circuit-Based Services
Dial-up digital connections ISDN
Dial-up telephone connections using modems
Fixed leased lines
1975 1985 1995 2005
ServiceNon-
Switched
Circuit-Switche
d
Leasedlines
PSTN ISDN
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
Multiplexing Level
NameBit Rate(Mbit/s)
0 STM-1 155.52
1 STM-4 622.08
2 STM-16 2488.32
3 STM-64 9953.28
4 STM-256 39893.12
Digital Hierarchies(Outside US – Reference Only)
Multiplexing Level
NameBit Rate(Mbit/s)
0Basic
channel64kbit/s
1 E-1 2.048
2 E-2 8.448
3 E-3 34.368
4 E-4 139.264
PDH
SDH
Service
Layer 2
Layer 3
X.25 FR Frame Relay
SMDS
Switched Multimegabi
t Data Service
ATMAsynchronou
s Transfer Mode
VPNVirtual Private
Network
ADSL
Asymmetric Digital
Subscriber Line
Packet-Switched Services
SMDS
Frame Relay
X.25
ATM
VPN
ADSL
1975 1985 1995 2005
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)
ADSL vs. Other Modems
0 1000 2000 3000 4000 5000 6000 7000 8000 Kbit/s
14.4K
28.8K
56K
ISDN
G.lite ADSL
Full ADSL
Source: ADSL Life www.adsllife.com Maximum Speed - Actual speed will vary
Structure and Infrastructure
OverviewOverview
Basic Techniques
LAN Structure
Circuit-switching and Packet-
switching
LAN Interconnection Services
Course Structure
Structure of rest of course
5. Using sharedmedia networks
9. Routers androuting protocols
1. Networking devices:hubs, switches and
routers
3. Communicationsprotocols
6. Ethernet bridging,STP and VLANs
2. The IP Suite7. Operating at the
Network Layer
10 Queuing systems
11. Switches, routersand interconnection
networks
4. SwitchedEthernet LANs
8. Transmissionand coding
Summary
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
1. Approximately, what is the speed of propagation in a telephone line?
2. Give two examples which illustrate the idea of the Internet being a set of interconnected overlay networks.
3. When bits are to be transmitted onto the network medium, two of the choices that must be made are: (i) Whether 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?
d) Verify the arithmetic for the example of 32 x 64 kbit/s channels multiplexed onto 2.048 Mbit/s channel given on slide 34 of week 1 lecture.
Tutorial Questions(continued)
4. 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?
5. Give an example of:a) An FDM technique used on guided media;b) A TDM technique used on unguided media.
6. 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.
7. 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?