Data Communications

I N T R O T O C O U R S E A N D A R C H I T E C T U R E M O D E L S

Intro to TDC 460

2       Masters degree is now called Network Engineering and Management Prereqs: Java, TDC 311 or CSC 373, TDC 261, TDC 363 Foundation: TDC 460 (updated), TDC 463, TDC 464 (updated) Advanced: TDC 477 (security), TDC 511 (practicum), TDC 560, TDC 563 Electives: 5 courses Capstone : TDC 594

System Architectures

3  We have already been introduced to the various types of communications systems  Telephone      Internet Television Cable TV and modems LANs (wired and wireless) Wireless WAN

System Architectures

4     Let’s look at these system architectures in more detail What are the models that support each architecture?

What type of code conversions must be performed?

What protocols support these models?

 Where is the convergence?

What is a Protocol?

5  For two entities to communicate successfully, they must “speak the same language”.

  What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable conventions.

These conventions are referred to as a protocol.

Key Elements of a Protocol

6    Syntax   Data formats Signal levels Semantics   Control information for coordination Error handling Timing   Speed matching (between sender and receiver) Sequencing (the right commands in the right order – closely related to semantics)

Network Architecture

7   The task of communication is broken up into modules For example, a file transfer could use many modules:  The file transfer interface that the user runs (FTP)  The module that makes sure the file arrives at the destination exactly the same as when it left the source    The module that gets the packets from one router to another The module that get each packet from the user’s computer to the network The module that converts 1s and 0s to voltages

TCP/IP Protocol Suite

8     Dominant commercial protocol architecture Specified and extensively used before OSI Developed by research funded by U.S. Department of Defense Used by the Internet

TCP/IP Suite Architecture

9  No official model, but a working one.

 Application layer   Host to host or transport layer Internet layer   Network access layer Physical layer

TCP/IP Physical Layer

10      Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates etc.

TCP/IP Network Access Layer

11      Exchange of data between end system and network Frame created Destination address provided Error checking code provided Possible services like priority invoked

TCP/IP Internet Layer (IP)

12    Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers

TCP/IP Transport Layer (TCP)

13    Reliable delivery of data (error-free) Ordering of delivery Implemented in end systems only (not implemented in routers)

TCP/IP Application Layer

14   Support for user applications e.g. HTTP, SMTP, FTP, SNMP

OSI Model

15      Open Systems Interconnection Developed by the International Organization for Standardization (ISO) Seven layers A theoretical system delivered too late!

TCP/IP is the de facto standard

OSI - The Model

16      A layer model Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers

OSI as Framework for Standardization

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OSI Layers

18        Application Presentation Session Transport Network Data Link Physical  What is the function of each OSI layer?

The OSI Environment

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Figure 2.16

TCP/IP and OSI model

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Questions

21        What TCP/IP layer handles addressing?

What OSI layer handles voltage conversions?

What TCP/IP layer handles email?

What OSI layer handles routing?

What TCP/IP layer handles end-to-end connections?

What OSI layer handles session connections?

What TCP/IP layer handles synchronization?

SNA

22      IBM’s Systems Network Architecture Created in the 1970s Being replaced with TCP/IP but still out there a little bit Seven layers which map fairly closely to OSI Good website: http://www.cisco.com/univercd/home/home.ht

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Novell

23    Novell NetWare’s architecture used to rely heavily on IPX and SPX protocols Starting with NetWare version 5, IP became the default protocol replacing IPX NetWare protocol suite maps to the following OSI layers:

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Telephony Architecture

25        Subscribers Lines Central offices Trunks LATAs SS7 Switching centers

Standards

26  Required to allow for interoperability between equipment   Advantages  Ensures a large market for equipment and software  Allows products from different vendors to communicate Disadvantages  Freeze technology  May be multiple standards for the same thing

Standards Organizations

27      Internet Society ISO ITU-T (formally CCITT) IEEE ANSI

Functions of Standards

1.

2.

3.

4.

5.

6.

7.

8.

9.

28 Encapsulation Segmentation and reassembly Connection control Ordered delivery Flow control Error control Addressing Multiplexing Transmission services

Encapsulation

29  Addition of control information to data    Address information Error-detecting code Protocol control

Segmentation (Fragmentation)

30  Data blocks are of bounded size  Application layer messages may be large  Network packets may be smaller  Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP)  ATM blocks (cells) are 53 octets long  Ethernet blocks (frames) are up to 1526 octets long  Checkpoints and restart/recovery

Why Fragment?

31  Advantages  More efficient error control    More equitable access to network facilities Shorter delays Smaller buffers needed  Disadvantages  Overheads   Increased interrupts at receiver More processing time

Connection Control

32  Connection Establishment   Data transfer Connection termination   May be connection interruption and recovery Sequence numbers used for  Ordered delivery  Flow control  Error control

Connection Oriented Data Transfer

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Ordered Delivery

34    Packets may traverse different paths through network Packets may arrive out of order Sequentially number packets to allow for ordering

Flow Control

35      Done by receiving entity Limit amount or rate of data Stop and wait Credit systems  Sliding window Needed at application as well as network layers

Error Control

36     Guard against loss or damage Error detection     Sender inserts error detecting bits Receiver checks these bits If OK, acknowledge If error, discard packet Retransmission  If no acknowledge in given time, re-transmit Performed at various levels

Addressing Level

37     Level in architecture at which entity is named Unique address for each end system (computer) and router Network level address   IP or internet address (TCP/IP) Network service access point or NSAP (OSI) Process within the system  Port number (TCP/IP)  Service access point or SAP (OSI)

Figure 2.18

Relationship of layers and addresses in TCP/IP

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Figure 2.19

Physical addresses

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Example 2.3

Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

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Figure 2.20

IP addresses

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Example 2.4

Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer.

Note that although physical addresses change from hop to hop, logical and port addresses destination.

remain the same from the source to

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Figure 2.21

Port addresses

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Addressing Mode

44    Usually an address refers to a single system  Unicast address  Sent to one machine or person May address all entities within a domain   Broadcast Sent to all machines or users May address a subset of the entities in a domain  Multicast  Sent to some machines or a group of users

Multiplexing

45   Supporting multiple connections on one machine Mapping of multiple connections at one level to a single connection at another  Carrying a number of connections on one fiber optic cable  Aggregating or bonding ISDN lines to gain bandwidth

Transmission Services

46    Priority  e.g. control messages Quality of service  Minimum acceptable throughput  Maximum acceptable delay Security  Access restrictions

Review Questions

47     What are the layers of the TCP/IP protocol suite? The OSI model? What is meant by encapsulation?

Trace an FTP command as it moves down through the layers, across the medium, and up the layers on the receiving side.

What are the functions of standards?