Lectures 5-6, 20.9.2000
Read: Tanenbaum to the end of chapter 1 1st lecture 12:15-13:00
• Lecture 4 cont.
• Some fundamental concepts of telecom
• Telecom architectures
• Layered models - OSI and Internet Break
2nd lecture 13:15-14:00
• Examples of real networks
Lecture 5: Telecom architectures
• Protocol basics
• Error control
• Flow control
• Link management
• Some fundamental concepts
• Hardware/Software
• Telecom architectures
• Open Systems Interconnection (OSI)
• Internet (TCP/IP)
Protocol basics
• (Data link) protocol is a set of functional components that include
• Error control
• Flow control
• Connection management
Error control
• Automatic Repeat Request (ARQ)
• Idle RQ (half-duplex)
• Continuous RQ (full-duplex)
• Go-back-N
• Selective reject
• ARQ type used affects the efficiency of utilization of the
available link capacity: needed window size N ≥≥≥≥ (T+t+2D)/T, where:
• T = transmission time of a data frame
• t = transmission time of an acknowledgement frame
• D = one-way transmission delay
• Point-to-point connections use mainly Go-Back-N, Selective reject is not implemented
Flow control
• Flow control is needed that transmitter won’t transmit data (eg. a large file) faster than the receiver is able to receive.
• Simplest mechanism: X-on/X-off (async-terminals, ctrl-S/ctrl-Q)
• More advanced: sliding window (eg. HDLC)
Link management
• Data link protocol can be connectionless (eg. local area networks)
• Connection oriented protocol has the following steps:
• Link set-up
• Data transfer
• Link disconnect
Some fundamental concepts
• Network
• Terminal
• Data Terminal Equipment (DTE)
• Data Circuit Terminating Equipment (DCE)
• Transmission media
• Baud # bit/s
• Bandwidth # transmission speed # capacity
• Nyqvist: ideal channel C=2Blog2M bps
C=maximum data transfer rate B=bandwith
M=levels per signalling element
• Shannon: noisy channel C=Blog2(1+S/N) bps
S/N is signal-to-noice ratio in dB
Some fundamental concepts (cont.)
• Nature of information, stemming from real-life applications and their requirements:
• Stream vs. bursty
• Real-time vs. not real-time
• High capacity vs. low capacity
• Reliability requirements
=> Quality of Service (QoS) is a complex issue
• The purpose of telecom is to fill the gap between the needs of the applications and the realities of the physical world
Hardware/Software
• The division between hardware and software is not very clear and is constantly changing
• At the same time, hardware is developing very fast and more and more functionality is being implemented with hardware (Application Specific Integrated Circuit, ASIC)
• The tools for defining ASICs are getting to be more and more like software tools
• There will not be a clear division between hardware and software but the main challenges of large, highly parallel systems are the same
• Creating telecommunication software and large networked applications are the great challenges of today
Telecommunication architectures
• The architecture of a large system means its division into smaller elements and the relationships between these
elements
• Large and complex structures are best understood and handled by humans when divided into smaller pieces
• Telecommunication systems are among the largest and most complex systems ever designed and implemented by man, so they are best understood as structures consisting of various architectural elements
• In this course we will start by briefly reviewing layered reference models, especially OSI and Internet
• Telecommunication is then discussed within this framework
• The purpose is to help the student see the forest for the trees
OSI reference model
• The Open Systems Interconnection (OSI) reference model of the International Standardization Organization (ISO)
• Divides telecommunications into seven layers
• Defines the layers, their functions and basic concepts
• Does not define the actual telecommunications protocols
• Does not take a stand to implementational issues
• The purpose of the OSI model was to:
• offer a generic structure for telecommunications systems
• act as a framework for standards and requirements
• facilitate the interconnection of various types of equipment
• ease the deployment of new technologies
• OSI is analogous to the System Networks Architecture (SNA) of the IBM world (whose significance has rapidly decreased)
• OSI still is a useful and generally accepted reference model
Basic principles of the OSI model
• OSI-model is based on the concept of peer-to-peer
communications, where two layer (N) protocol entities communicate using a (N) protocol
• Layer (N) is based on the service provided by layer (N-1), it adds value to the (N-1) service and provides more refined (N) service to layer (N+1)
• The layers are independent and only ”see” each other through the service interfaces between the layers
• (N+1) entity uses (N) service through (N) service primitives
• (N) entity communicates with its peer entity by exchanging (N) Protocol Data Units (or (N) PDUs)
OSI model
media Application
Presentation Session Transport
Network Data Link
Physical application
process
Application Presentation
Session Transport
Network Data Link
Physical application
process
Network DL
Phys.
DL Phys.
7.
6.
5.
4.
3.
2.
1.
Layers of the OSI model and their functions
• Application processes reside outside the OSI system and use the communication services provided by it
7. Application layer acts as an interface between the application process and the telecommunications world
6. Presentation layer negotiates a transfer syntax and performs transformations between the local and transfer syntax
5. Session layer provides organized and synchronized data transfer to the presentation layer
4. Transport layer raises the service provided by the network layer to the level required by the session layer providing reliable end-to-end transport service
3. Network layer is responsible for routing messages through the network 2. Data Link layer transfers data between two neighboring nodes forming
frames out of bits, it may also perform flow and error control
1. Physical layer transfers bits (and possibly non-data symbols) over the physical media from one network node to another
• Physical media (such as copper cable, optical fiber or the ”ether”) act as
OSI terminology
• System
• (N) Layer
• (N) Protocol
• (N) Service
• (N) Connection
• (N) Association
• (N) Protocol Data Unit, (N)PDU
• (N) Primitive
• (N) Interface Data Unit, (N)-IDU
• Interface Control Information, ICI
• (N) Service Data Unit, (N)SDU
• (N) Service Access Point, (N)SAP
• Connection End-Point, CEP
• CEP Identifier, CEPI
Critique of the OSI model
• Too heavy - too many layers with overlapping functionality
• Too connection oriented
• Overly heavy and slow standardization process
• The standards produced tend to be rather theoretical and rarely provide solution to real-life problems
• The standardization of OSI protocols (such as X.400 and
FTAM) and OSI profiles (such as GOSIP) has been a complete flop
• The main function of the OSI model today is to server as a generic framework and terminology, not as a protocol family
• The TCP/IP protocol suite has fulfilled all the promises made by OSI when it was conceived
Internet layer model
The Internet model only has four layers:
higher-level protocols TCP / UDP
Internet Protocol
communication networks
Internet and OSI layer structures
• The figure below shows the layers of Internet and those of OSI
• In some cases (such as X.25) the ”physical network” of Internet reaches up to the network layer of OSI
• The Internet Protocol (IP) can be run on virtually any network, e.g. LAN, leased line, ATM, Frame Relay, X.25 or packet radio
1. Physical layer 2. Data link layer 3. Network layer 4. Transport layer 5. Session layer 6. Presentation layer 7. Application layer
IP
TCP UDP
Application level protocols:
etc.
IEEE 802.3, FDDI, E1, ATM, Frame Relay etc.
application processes
OSI Internet
Telnet FTP SMTP HTTP DNS SNMP ISAKMP
Critique of the Internet model
• No well defined terminology (inconsistencies and ambiguities)
• No clear distinction between protocols, services and interfaces
• Protocol definitions are often closely tied up with implementational issues
• Cannot describe all telecommunications systems
• No distinction between physical, data link and lower part of the network layer (just one "physical network")
• Nevertheless, Internet has proved its power and continues to be the leading network architecture
• The OSI model with layers 5 and 6 combined into layer 7 can be used to discuss Internet and other protocols
• In this course we will use the OSI reference model as a
generic framework for both telecom and datacom systems
How layering works in practice
• Each protocol entity sends its PDUs using the service provided by the layer immediately below
• (N)PDU (including its header) is packed into the data field of an (N-1)PDU
• At the receiving end, the headers (and possible trailers) are removed and decoded in reverse order
• Example: sending a UDP message over IP and Ethernet
headerUDP- UDP data
IP-
header IP data
E-net-
