Telecommunication Software

Telecommunication Software

Lecture 3 , Octo ber 8, 2002

Map of last and today lectures

• Last time:

– We overviewed various types of networks – We gave a map to networks

• Today’s plan:

– First ½ we dig into details for some special networks – Second ½ we start reviewing network applications

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)

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

Ethernet frame format

• Preamble field

– 7 bytes used by the receiving station to establish bit synchronization

• Start Frame Delimiter (SFD) field

– bit sequence 10101011, enables the receiving station to detect the beginning of a frame and locate its first bit

• Destination and Source address fields

– identify the producing station and the station(s) for which the frame is intended

– 2 or 6 bytes long, chosen a priori for a particular network

Ethernet frame format 2

• Length field

– the current number of bytes transported in the Data field

• Pad field

– Used to fill up the frame with extra bytes for obtaining a frame with minimum length of 64 bytes, w/o preamble and SFD

• Checksum field

– The Frame Check Sequence (FCS) is used to convey a 4-byte Cycle Redundancy Check (CRC) value for detecting bit

transmission errors

– Polynomial (international standard for CRC):

X³²+X²⁶+X²³+X²²+X¹⁶+X¹²+X¹¹+X¹⁰+X⁸+X⁷+X⁵+X⁴+X²+X+1

– CRC produced at transmitting station as function of destination, source, length, data, and pad fields

– Receiving station computes a CRC sum of the same fields and compares it with the checksum value; if a transmission error occurred, it is highly detectable

(Truncated) binary exponential backoff algorithm

• CSMA/CD

• used by all collision-detecting stations to calculate their individual retransmission delay (backoff delay)

– after 1^st collision each station waits 0 or 1 slot times before trying again; if each station picks the same random number, they will collide again

– after 2^nd collision each station picks 0,1,2, or 3 slot times before trying again, and waits that number of slot times

– if a 3^rd collision occurs, the next time number of slots to wait is chosen randomly by each station from the interval 0 to 2³-1

– after i collisions, a random number between 0 to 2ⁱ-1 is chosen, and that number of slots is skipped

– after 10 collisions the randomization interval is frozen at a maximum of 1023 slots

– after 16 collisions a failure is reported and recovery is up to higher layers

(Truncated) binary exponential backoff algorithm 2

• The algorithm dynamically adapts to the number of stations trying to send

• If randomization interval for all collisions was 1023

– the chance for two stations to collide for a second time: negligible – the average wait after a collision: hundreds of slot times => delay

• If each station always delayed for 0 or 1 slots

– stations will collide again and again

• The algorithm ensures a low delay when only few stations collide

• Also ensures collision is resolved in reasonable interval when many stations collide

• Truncating the backoff at 1023 keeps the bound from growing

too large

Ethernet retrospective

• Used for over 2 decades, enters its 3

: rare phenomenon

• Simple and flexible => reliable, cheap, easy to maintain

• Interworks easily with TCP/IP, the dominant network protocol (both connectionless)

• Able to evolve in certain essential ways

– Speeds have gone up by several orders of magnitude – Hubs and switches have been introduced

– The above changes did not require changing the software

IEEE 802.11 (Wireless LAN)

• Ethernet is getting competition

• 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 in a network only while stationary

– Mobile station: may be used in a network while in motion

• Different properties wrt wired LANs

– Limited physical range

– Vulnerability to security attacks – Significantly higher error rates – Dynamic topology

Wireless LAN 2

Backbone segment

WSeg WSeg WS

WS WS WS

APS APS

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

Frequency Hopping Spread Spectrum

• Uses 79 channels, each 1 MHz wide, starting at the low end of the 2.4 GHz band

• Uses a pseudorandom number generator to produce the sequence of frequencies hopped to

• If all stations use same seed to the generator and stay synchronized in time => they will hop to same

frequencies simultaneously

• The time interval spent at each frequency: dwell time

– Adjustable parameter, but < 400 ms

• Intruder not knowing the hopping sequence & dwell time

cannot eavesdrop on transmissions

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)

Possible IEEE 802.12 LAN Topology

Repeater

Repeater Station

Station Station Station

Station Station

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

Station and Repeater architecture

• Stations and repeaters possess the same physical layer functionality, divided into 2 sublayers:

– physical medium independent sublayer (PMI): provides basic frame transmission by applying certain data scrambling and encoding techniques

– physical medium dependent sublayer (PDI): provides signal generation and recognition, and clock recovery

• MII: medium-independent interface

– between PMI and PDI

– specifies signaling time and electrical interface characteristics

• MDI: medium-dependent interface

– between PDI and medium

– specifies mechanical, electrical, optical and transmitted signal interface requirements

• Both interfaces may be implemented physically or logically

Station and Repeater architecture 2

• MAC layer is different for stations and repeaters

• Stations sublayer supports 2 well defined MAC formats and interfaces (IEEE 802.3 and IEEE 802.5)

– Only one format and interface supported at the same time within an IEEE 802.12 network

• Repeaters sublayer provides arbitration and control

of frame forwarding between different ports of a

repeater

Wide Area Networks (WANs)

• Span a large geographical area (country, continent)

• Contain a collection of machines (hosts) intended for running user (application) programs

• Hosts are connected by a (communication) subnet

• Hosts are owned by customers, subnets are typically

owned and operated by a phone company or an Internet Service Provider

• Subnet carries messages from host to host

• Separation of pure communication aspects of the

network (subnet) from application aspects (hosts) greatly

simplifies the complete network design

WAN Subnet

• Subnet consists of transmission lines and switching elements (routers)

– Transmission lines move bits between machines; made of copper wire, optical fiber, radio links

– Routers are specialized computers connecting >= 3 transmission lines

Packet-switched WANs

Satellite WANs

• Each router has an antenna through which it can send and receive

• All routers can hear output from the satellite

• In some cases they can also hear upward transmissions of their fellow routers to the satellite

• In some cases routers are connected to a substantial point-to-point subnet with only some of them having a satellite antenna

• Satellite networks are inherently broadcast => most

useful when broadcast property important

Example WANs

• Telex networks

• Telephone networks

• Packet-switched data networks

– Intended for computer data transmission

• Television distribution networks

• Radio distribution networks

• Integration of these networks and their services into a single Integrated Services Digital Network (ISDN) is studied by the International Telecommunication Union Standardization

Sector (ITU-T)

Addressing

• Common to all networks: they require means to address connected stations

• Different addressing techniques and formats exist:

– IEEE address format – ITU-T address format – OSI address format

Applications

• Applications in a communication network provide a variety of services

• We overview

– Classical applications – Distributed applications – Multimedia applications – Real-time applications

• Some of these applications may belong to several categories above

• We study their various perspectives

Classical applications

• Designed for and applied in today’s

telecommunications and data communication networks

• We review

– File transfer – Virtual terminal – Electronic mail – Remote job entry – Telephone

– Telefax

– Telex and Teletex

File transfer

• Computing nodes in a network may maintain their own file (storage) system

• A networked application providing file transfer services:

enables remote users to copy files between storage devices located at different nodes

• FTP (File Transfer Protocol): one of the 1

and still widely used file transfer application

– Specified in RFC959 (1985)

– Based on client-server paradigm

– Requires control connection and data connection

– For a reliable end-to-end file transmission TCP is used

FTP

• Data transfer processes (DTP)

– Server and client used to read from and write to the local file systems and control data connections while transferring files

• Protocol interpreters (PI)

– Server and client used to process and exchange FTP commands and replies via the control connection

• Client interface

– May additionally exist at the client side to enable human users to interact with the client PI in a user-friendly way

– May have a local definable language

• FTP allows clients to set up control connections to 2

different servers to arrange file transfer between them,

remotely

FTP Options

• While transferring files, representation transformations may be performed

– 3 parameters that are adjustable by clients at each transfer job individually: data type, data structure, transmission mode

• Data type ->

• Data structure -> defines whether file is organized as a

– File structure (sequence of bits)

– Record structure (list of data records)

– Page structure (set of independent indexed data pages)

• Transmission mode -> denotes the functions performed on the file while transferred

– Stream mode (data transferred uninterpreted as flow of bytes)

– Block mode (file transferred as series of data blocks enabling error recovery and restart at application level)

– Compressed mode (data amount reduced) to get higher throughput

Virtual terminal

• Human users typically interact with a computing system via a terminal: keyboard and display

• In networks is desirable to use a terminal of any local node to interface with any other remote node

• Variety of terminals with different capabilities exists

– Their heterogeneous interfaces must be supported by each computing system

• To avoid this: VIRTUAL TERMINAL (VT)

– abstract and universal terminal covering a wide range of existing terminals

– The remote node can only provide one terminal type: VT – The node with the actual terminal performs the required

adaptations between VT and its physical terminal characteristics

TELNET

• Widely used application providing a VT service

– Defined in RFC854 (1983)

– Requires bidirectional and reliable connection between local and remote nodes

– On both sides a Network VT (NVT) exists, providing a 8-bit oriented protocol to provide the VT service

– NVT can be used independently to interact with local terminal device or local application process

– Interface between NVT and terminal/process: display+keyboard – NVT operates generally in line-buffered mode

– TELNET provides the concept of option negotiation (enables sophisticated terminals/processes with new service capabilities)

X.400 electronic mail

• ITU-T’s X.400 series of recommendations specifies a Message

Handling System (MHS) enabling users to exchange messages in a store-and-forward fashion

• Seen as electronic mail (e-mail)

• Around since 80’s; before 90’s only in academia; after that everybody

• Very informal

– Own jargon: CUL8R, ASAP, BTW, ROTFL, IMHO

MHS

• MHS messages (msg) consist of an envelope in which information is carried as msg content between users

• Users: persons or computer processes

• Msg is submitted by its originator and conveyed by MHS to one/more recipients

• Each user is attached a dedicated computer process, the User Agent (UA) that interacts on the user’s behalf with the Message Transfer System (MTS)

• MTS -> multiple Message Transfer Agents (MTAs) that cooperate in a store-and-forward fashion to transport and delivery msg

• Basic MTS capabilities

– Submission, Transfer, Delivery of msg between UAs

– Additionally: (non-)delivery notifications to msg originators

MHS as a basis

• Users connected to different MHSs can interact with a given MHS via Access Units (AUs)

– Located between MTS and the external MHS – Perform the required adaptations between both

• F.410: recommended time targets for

– Msg delivery, transfer, delivery notification

• MHS as foundation on which other application specific msg systems are built

– InterPersonal Messaging (IPM) – Electronic Data Interchange (EDI) – Voice messaging system

Internet email

• Internet community has its own email system since 1982

• Simple Mail Transfer Protocol (SMTP)

– Used to reliably transfer mail between Mail Transfer Agents (MTAs)

– Used also by user (agents) to post mail

• To retrieve mail: protocols used between UAs and MTAs

– Post Office Protocol (POP)

– Internet Message Access Protocol (IMAP)

• Format of mail msg: RFC822, ’82: ASCII

– Msg envelope (header)

– Unstructured msg body conveying the msg content

Internet email 2

• MIME (Multipurpose Internet Mail Extensions)

– Allows textual msg headers and text in character sets richer than ASCII

– Provides an extensible set of different formats for non-textual msg bodies (e.g. audio, video, ps)

– Allows multipart msg bodies

• SMTP and POP can convey only RFC822 msg

– MIME msg need to be adequately encoded and decoded before and after transfer

• IMAP supports MIME

SMTP, ‘82

• Used to reliably transfer RFC822 mail between MTAs

• Used also by UAs to post mail

• If receiving MTA is not the final destination, it relays the msg to the next relevant MTA until final destination

• If multiple recipients defined, sending and receiving agents

negotiate which recipients can be reached through respective MTA

• If some recipients cannot be reached through selected MTA, the sending agent tries another relay agent

• One copy only of the mgs is forwarded to all identified recipients

• Expects a reliable, connection-oriented transport service (such as TCP)

• Transaction-oriented protocol, exchanging requests and responses in ASCII plaintext.

POP-version 3, ‘96

• Simple protocol enabling UAs to retrieve msg stored at MTA

• POP connection established between one UA and one MTA

• Requires a reliable connection-oriented transport service (such as TCP)

• Connection-oriented protocol using ASCII for requests/replies

• All mail retrieved must agree with RFC822 (

)

• Reason to have UA and MTA with different protocols

– MTA is part of MTS which requires that every node is continuously running to receive/relay mail

– To release terminals from this requirement, they make use of an UA process to retrieve mail on demand

IMAP-version 4, revision 1,’96

• Allows users to retrieve/manipulate electronic mail msg stored at an MTA’s site

• Permits users to create, delete, control multiple remote msg folders (mailboxes)

• Permits parsing, searching, and selective fetching of msg attributes, texts, and portions

• A UA may work offline and resynchronize with its related MTA later on

• Requires reliable connection-oriented transport service

• Transaction-oriented protocol interacting by requests and responses in ASCII plaintext

• Handles and interprets MIME msg

• Able to process multiple requests in parallel

A comparison of POP3 and IMAP

MIME, ‘98

• Problem: RFC 822 allows only ASCII msg to be sent, the mail lines must be <= 1000 characters (SMTP)

• Solution: Internet community defined a MIME standard compliant and compatible with RFC822 and SMTP

• Adds

– New header fields to the original RFC822msg header to indicate MIME version (1.0)

– The content media type conveyed in the msg body – The content transfer encoding applied

– 2 additional fields carrying the content identifier and content description

• Content media types are defined in RFC2046, currently

divided into 7 categories

MIME Content Media types

Remote job entry

• Problem: in a network environment it may be worthwhile migrating particular jobs to remote nodes for processing

• Solution: a general concept to transfer, control and

manipulate jobs in a network specified in ISO8831, ’92

• The users of a Job Transfer and Manipulation (JTM) service have different roles

• Initiation agency

– User that initiates the execution of a job description to a JTM provider

– Upon the issued job description, JTM compiles a work specification containing instructions such as to

• Which files are requires

• Where are they stored

• To which node they must be moved for further processing

• Where the new generated files should be stored finally

Remote job entry 2

• Source agency

– User storing files that can be requested by the JTM provider depending on the work specification

• Sink agency

– User at which the JTM provider may dispose files (printers/file storage)

• Execution agency

– User that performs functions on input files given by the JTM provider and returns output files to the JTM provider

• It is transparent to JTM provider how source, sink, or

execution agencies process given files or manage

retrievable files