Transcript Week 1 - DePaul University

Data Communications
INTRO TO COURSE
AND
ARCHITECTURE MODELS
Intro to TDC 460
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 Masters degree is now called Network Engineering
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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
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 We have already been introduced to the various
types of communications systems
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Telephone
Internet
Television
Cable TV and modems
LANs (wired and wireless)
Wireless WAN
System Architectures
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 Let’s look at these system architectures in more
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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?
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 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
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 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
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 The task of communication is broken up into
modules
 For example, a file transfer could use many modules:
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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
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 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
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 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
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 Physical interface between data transmission device
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(e.g. computer) and transmission medium or
network
Characteristics of transmission medium
Signal levels
Data rates
etc.
TCP/IP Network Access Layer
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 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)
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 Systems may be attached to different networks
 Routing functions across multiple networks
 Implemented in end systems and routers
TCP/IP Transport Layer (TCP)
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 Reliable delivery of data (error-free)
 Ordering of delivery
 Implemented in end systems only (not implemented
in routers)
TCP/IP Application Layer
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 Support for user applications
 e.g. HTTP, SMTP, FTP, SNMP
OSI Model
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 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
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 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
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 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
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 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
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 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
m
Novell
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 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
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 Subscribers
 Lines
 Central offices
 Trunks
 LATAs
 SS7
 Switching centers
Standards
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 Required to allow for interoperability between
equipment
 Advantages
Ensures a large market for equipment and software
 Allows products from different vendors to communicate
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 Disadvantages
 Freeze technology
 May be multiple standards for the same thing
Standards Organizations
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 Internet Society
 ISO
 ITU-T (formally CCITT)
 IEEE
 ANSI
Functions of Standards
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1.
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Encapsulation
Segmentation and reassembly
Connection control
Ordered delivery
Flow control
Error control
Addressing
Multiplexing
Transmission services
Encapsulation
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 Addition of control information to data
 Address information
 Error-detecting code
 Protocol control
Segmentation (Fragmentation)
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 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
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 Checkpoints and restart/recovery
Why Fragment?
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 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
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 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
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 Packets may traverse different paths through
network
 Packets may arrive out of order
 Sequentially number packets to allow for ordering
Flow Control
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 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
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 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
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 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)
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 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 remain the same from the source to
destination.
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Figure 2.21 Port addresses
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Addressing Mode
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 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
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 Supporting multiple connections on one machine
 Mapping of multiple connections at one level to a
single connection at another
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Carrying a number of connections on one fiber optic cable
Aggregating or bonding ISDN lines to gain bandwidth
Transmission Services
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 Priority
 e.g. control messages
 Quality of service
 Minimum acceptable throughput
 Maximum acceptable delay
 Security
 Access restrictions
Review Questions
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 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?