Telecommunication SoftwareLecture 2, September 24, 2002

Last time

• We defined networks as means for communication• We argued for monolithic vs modular communication

systems• We mentioned the networks state-of-the-art

– OSI reference model– Internet protocol suite

• We motivated our study of networks• We studied fundamentals of networks technology

– Transmission media– Transmission techniques– Medium access methods

Transmission media• Magnetic tape

– Cheap, but not online => delays

• Twisted pair– Used to be the commonest– Gets unfashioned

• Coaxial cable– Used for cable TV networks and some LANs

• Optical fiber– Very efficient; telephone companies prefer it – Fixed networks likely to use only fibers in the future

• Air– Enable wireless and mobile communication

Optical fibers vs. Copper wires• Advantages

– Much higher bandwidths than copper– Repeaters needed only every 30 km, compared to 5 km– Not affected by power surges, electromagnetic interference, power

failures, or corrosive chemicals– Thin and lightweight:

• 1000 twisted pairs, 1 km long weigh 8000 kg• 2 fibers have more capacity and weigh 100 kg

– Lower installation costs

• Disadvantages– Unfamiliar technology to common engineers– Optical transmission is unidirectional => two fibers or two frequency

bands needed for two way communication– Fiber interfaces cost more than electrical interfaces

Transmission techniques

• Baseband transmission– Use digital signals in form of discrete pulses of electricity or light

• Broadband transmission– Based on analog signals that are continuous and non-discrete

Medium access methods

• Ways to share a common transmission medium and to control medium accesses for transmission

• Transmission frame– Sequence of bits transmitted as a logical unit

• Transmission link– Physical channel between multiple stations– Able to transmit/receive information– Provides an abstraction on which transmission media and

topology is based– Types of links: point-to-point, multipoint, broadcast

Access contention methods

• Independent of the topology, all stations share a common link and transmission medium

• Different methods exists for solving access contentions (conflicts that may arise when using a broadcast link simultaneously)– ALOHA method– Carrier Sense Multiple Access– Token passing– Slotted ring– Register insertion

Today’s lecture

• We study various types of networks• We use the fundamental network technologies seen last

time• We give a map to networks• Next time we continue with more details

Network taxonomy• Criterion: transmission technology

– Broadcast links– Point-to-point links

• Criterion: scale (geographical extent)– Very local area networks (personal area network)– Local area networks LAN– Metropolitan area networks MAN– Wide area networks WAN– Internetworks

• Criterion: organization– Private networks– Public networks

• Other criterions– Wireless networks– Home networks

LANs

• Privately-owned networks within a single building/company/campus up to a few km in size

• Widely used to connect computers and workstations in company offices and factories to share resources and exchange information

• Restricted in size => the worst case transmission time is bounded and known in advance

• Transmission technology allows – speeds of 10 Mbps to 100 Mbps– low delays (microseconds, nanoseconds)– very few errors– Newer LANs up to 10 Gbps

LANs 2• IEEE specified most of today’s LANs (IEEE 802 committee)

LANs 3

• IEEE 802 reference model– Logical link control (LLC) layer– Medium access control (MAC) layer– Physical layer

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ent IEEE 802.2 LLC

IEEE 802.1 Bridging

802.3Ethernet

802.9VoiceData

802.6MAN

802.5Token

ring

802.4Token Bus

802.14Data overCable TV

802.12Demand Priority

802.11Wireless

LAN

LLC

MAC

Physical

-Lower: definitions of medium access control method and of physical transmission media-LLC: enhancement and unifying layer for all 802 network services-LLC enhancements can be seen as part of the communication system’s functionality

} Data link layer

IEEE 802.3 (CSMA/CD): Ethernet

• Medium access control method: CSMA/CD• All stations share the same transmission medium• 1970s: XEROX PARC -> Experimental Ethernet, 3 Mbps• Early 1980: DEC, Intel, XEROX (DIX) -> DIX Ethernet,

10 Mbps• 1985: IEEE 802.3 standard• 1995: IEEE 802.3u (Fast Ethernet), 100 Mbps• 1997: IEEE 802.3x -> full duplex mode doubling the

transmission speed (both directions)• 1998: IEEE 802.3z (Gigabit Ethernet), 1-2 Gbps• Newer LANs: up to 10 Gbps

Medium Access Method

• Half-duplex operation mode– CSMA/CD– Waiting a random time <--> truncated binary exponential backoff

algorithm– The algorithm is used by all collision-detecting stations to

calculate their individual retransmission delay (backoff delay)

• Full-duplex operation mode– Two stations share the physical medium– We assume the medium is capable of having simultaneous

bidirectional transmission w/o interference– No contention possible => no CSMA/CD needed

Transmission media

• 802.3 defines several transmission media and cabling topologies

• Medium specification identified by a 3-field type notation– Data rate– Medium type– Maximum segment length

IEEE 802.4 (Token Bus)

• Stations share the same transmission medium• Each station has a logical position within a virtual ring• Basic operation

– Stations know the addresses of their successors / predecessors– Stations arranged in a logical ring– A control frame (token) passes from station to station; it

regulates the right to transmit data frames– At any time there is only one token passed around enabling one

station to transmit frames– When station completed transmission, token passes to next

station

IEEE 802.5 (Token Ring)

• Stations are connected serially by a transmission medium• Information is transferred sequentially, bit-by-bit from one

station to its neighbor• Basic operation

– Access for data transmission in ring controlled by token frame– At any time there is only one token in ring and only the token holding

station can send data frames in sequence– A token holding timer controls the maximum period of time for which

a station may send data frames before passing the token

IEEE 802.11 (Wireless LAN)

• Standard specifying a LAN based on wireless technology• Specifies connectivity for portable and mobile stations

– Portable station: may be moved to different locations but used only while stationary

– Mobile station: may be used while in motion

• Different properties wrt wired LANs– Limited physical range– Vulnerability to security attacks– Significantly higher error rates– Dynamic topology

Wireless LAN 2

Backbonesegment

WSegWSeg

WS

WSWS

WS

APSAPS

Distribution Segment (DS)

- DS enables stations to move transparently between different wireless Segments (WSeg)- DS not specified by 802.11 (can be implemented based on many technologies including wired LAN)- each WSeg has one station connected to DS, functioning as Access Point station (APS)- APS enables wireless stations located in the respective WSeg areas to communicate with stations in different WSegs

Wireless LAN 3

• Stations of a wireless LAN possess functions to provide the following services– Association service– Disassociation service– Authentication service– Privacy service

• APS have additional functions to provide the following services– Distribution service– Integration service– Re-association service

Minimum wireless LAN topology

• Minimum LAN 802.11: one WSeg and two WSs– Requires only authentication, privacy, association and

disassociation services

• LAN 802.11 w/o backbone segment: AD-HOC NETWORK

Transmission media in Wireless LAN

• 2.4 GHz band frequency hopping spread spectrum (FHSS)– Frequency changed within a specified band in a pseudo-random

fashion, known only to transmitters and receivers– w/o knowing the frequency sequence and change interval,

eavesdropping is impossible

• 2.4 GHz band direct sequence spread spectrum (DSSS)– A spreading code is used to spread and despread the transferred

data– Each wireless station has its own spreading code

• Baseband infrared– Infrared technique (for remote controls of TV sets)– Infrared signals cannot penetrate walls, so cells in different rooms

are well isolated from each other

IEEE 802.12 (Demand-Priority)

• Specifies a 100 Mbps LAN controlled by a repeater• Its physical star topology can be enlarged by cascading

multiple repeaters => tree topology• Cascadable repeaters have a dedicated cascade port

reserved for connection to repeaters only• Ports used to connect stations are called local ports (>=2)• For interconnection with other LANs bridges may be

linked to local ports performing the required media and service adaptation

• Bridges are transparent to repeaters (treated as normal stations)

Basic operation

• Stations first send a transmission request to associated repeater

• Repeater returns a transmission grant• Requests: normal priority (NP) or higher priority (HP)• HP: real time voice, video, data transmission• Two queues (for NP and HP) maintained to store incoming

requests• While HP queue has items to process, it serves them

before serving requests from NP queue (FIFO queues)• Timers monitor the pending time of NP requests to avoid

their starvations– At about 250 ms a NP request is upgraded to a HP request

Processing a request

• Repeater (R) sends a transmission grant to relevant station

• R sends an idle-down signal to remaining stations• R issues an incoming signal to remaining stations

– To be prepared to receive the frame

• Frame sent by transmitting station is forwarded to addressed receiving stations

• During frame transmission, new requests can be accepted

• These new requests will never interrupt ongoing transmissions, even if they are HP

MANs• Network optimized for a geographical area ranging from buildings to

entire cities • Best known example: cable television networks• Based on broadcast transmission link• Most common: DQDB and FDDI

DQDB

• Distributed-Queue Dual-Bus• Broadcast link consists of 2 unidirectional buses A and B• Each station connected to both buses• A and B arranged in opposite directions enabling full-

duplex communications• Assumption: each station knows which bus has to be

used to communicate with a particular station• Both buses operate independently while using the same

access method/medium for data transmission

DQDB 2

• Stations at the head-end of each bus generate continuously either– Empty, fixed-size data frames (slots) for transferring user data– Management frames to maintain DQDB operational integrity

• Stations have read and write access to each bus• For data transmission 2 types of frames are used

– Queued arbitrated (QA) frames– Pre-arbitrated (PA) frames– They contain two control fields: BUSY and REQUEST

• Used in early MANs

FDDI• Fiber Distributed Digital Interface• High performance optic fiber token ring MAN• Runs at 100 Mbps over distance s of 200 km with up to

1000 stations connected (‘90-’94)• Designed by X3T9.5 (subcommittee of ANSI) starting back

in 1982• It can be used as any of the 802 LANs• Also usable as backbone to connect copper LANs

– Due to its high bandwidth

• Extensions– Single mode optical fiber enabling optical links between stations of

up to 60 km– FDDI-II -> hybrid ring control

FDDI Transmission media

• Multimode fiber– Maximum distances between stations: 2 km

• Single mode fiber– Maximum distances between stations extended to up to 60 km– Higher costs for fiber components

• Twisted pair– Shielded or unshielded twisted pairs for low cost FDDI networks– Maximum distances between stations: 100 m

Internetworks• There are many networks, with different HW and SW• People connected to one often want to communicate

with people attached to another one• For this, different and frequently incompatible networks

have to connect – By means of machines called gateways– These provide the necessary translations, both for SW and HW

• Internetwork or internet = collection of interconnected networks

• Worldwide INTERNET -> one specific internet• Common form of internet: collections of LANs connected

by a WAN

Network vs Distributed System

• Computer Network: collection of autonomous computers interconnected by a single technology

• Internet and World Wide Web are NOT networks– Internet: collection of interconnected networks– WWW: distributed system running on top of Internet

• Distributed System: collection of autonomous computers that appears to its users as a single coherent system– Typically it has a single model or paradigm that it presents to its

users– WWW: everything looks like a document (web page)– Software system built on top of a network

Home Networks

• At the horizon• In the future most homes will be set up for networking• Every device in the home

– will be capable of communicating with every other device– will be accessible over the Internet

• Devices capable of being networked:– Computers (desktop and notebook PC, PDA, shared peripherals)– Entertainment (TV, VCR, DVD, camcorder, camera, stereo)– Telecommunications (fixed and mobile phone, fax)– Appliances (microwave, refrigerator, clock, furnace, airco, lights)– Telemetry (utility meter, smoke/burglar alarm, thermostat, babycam)

Properties for home networks

• Fundamentally different than other network types– Easy to install– Foolproof in operation– Low price– Sufficient capacity– No format wars– Security and reliability extremely relevant

Home networks 3

• Wired or wireless?• Most homes have already 6 networks installed:

– Electricity– Telephone– Cable television– Water– Gas– Sewer

• Cost favors wireless• Security favors wired