William Stallings Data and Computer Communications Chapter 2
- Slides: 38
William Stallings Data and Computer Communications Chapter 2 Protocols and Architecture
Characteristics Ñ Direct or indirect Ñ Monolithic or structured Ñ Symmetric or asymmetric Ñ Standard or nonstandard
Direct or Indirect Ñ Direct Ñ Systems share a point to point link or Ñ Systems share a multi-point link Ñ Data can pass without intervening active agent Ñ Indirect Ñ Switched networks or Ñ Internetworks or internets Ñ Data transfer depend on other entities
Monolithic or Structured Ñ Communications is a complex task Ñ To complex for single unit Ñ Structured design breaks down problem into smaller units Ñ Layered structure
Symmetric or Asymmetric Ñ Symmetric Ñ Communication between peer entities Ñ Asymmetric Ñ Client/server
Standard or Nonstandard Ñ Nonstandard protocols built for specific computers and tasks Ñ K sources and L receivers leads to K*L protocols and 2*K*L implementations Ñ If common protocol used, K + L implementations needed
Use of Standard Protocols
Functions Ñ Encapsulation Ñ Segmentation and reassmebly Ñ Connection control Ñ Ordered delivery Ñ Flow control Ñ Error control Ñ Addressing Ñ Multiplexing Ñ Transmission services
Encapsulation Ñ Addition of control information to data Ñ Address information Ñ Error-detecting code Ñ Protocol control
Segmentation (Fragmentation) Ñ Data blocks are of bounded size Ñ Application layer messages may be large Ñ Network packets may be smaller Ñ Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP) Ñ ATM blocks (cells) are 53 octets long Ñ Ethernet blocks (frames) are up to 1526 octets long Ñ Checkpoints and restart/recovery
Why Fragment? Ñ Advantages Ñ More efficient error control Ñ More equitable access to network facilities Ñ Shorter delays Ñ Smaller buffers needed Ñ Disadvantages Ñ Overheads Ñ Increased interrupts at receiver Ñ More processing time
Connection Control Ñ Connection Establishment Ñ Data transfer Ñ Connection termination Ñ May be connection interruption and recovery Ñ Sequence numbers used for Ñ Ordered delivery Ñ Flow control Ñ Error control
Connection Oriented Data Transfer
Ordered Delivery Ñ PDUs may traverse different paths through network Ñ PDUs may arrive out of order Ñ Sequentially number PDUs to allow for ordering
Flow Control Ñ Done by receiving entity Ñ Limit amount or rate of data Ñ Stop and wait Ñ Credit systems Ñ Sliding window Ñ Needed at application as well as network layers
Error Control Ñ Guard against loss or damage Ñ Error detection Ñ Sender inserts error detecting bits Ñ Receiver checks these bits Ñ If OK, acknowledge Ñ If error, discard packet Ñ Retransmission Ñ If no acknowledge in given time, re-transmit Ñ Performed at various levels
Addressing Ñ Addressing level Ñ Addressing scope Ñ Connection identifiers Ñ Addressing mode
Addressing level Ñ Level in architecture at which entity is named Ñ Unique address for each end system (computer) and router Ñ Network level address Ñ IP or internet address (TCP/IP) Ñ Network service access point or NSAP (OSI) Ñ Process within the system Ñ Port number (TCP/IP) Ñ Service access point or SAP (OSI)
Address Concepts
Addressing Scope Ñ Global nonambiguity Ñ Global address identifies unique system Ñ There is only one system with address X Ñ Global applicability Ñ It is possible at any system (any address) to identify any other system (address) by the global address of the other system Ñ Address X identifies that system from anywhere on the network Ñ e. g. MAC address on IEEE 802 networks
Connection Identifiers Ñ Connection oriented data transfer (virtual circuits) Ñ Allocate a connection name during the transfer phase Ñ Reduced overhead as connection identifiers are shorter than global addresses Ñ Routing may be fixed and identified by connection name Ñ Entities may want multiple connections - multiplexing Ñ State information
Addressing Mode Ñ Usually an address refers to a single system Ñ Unicast address Ñ Sent to one machine or person Ñ May address all entities within a domain Ñ Broadcast Ñ Sent to all machines or users Ñ May address a subset of the entities in a domain Ñ Multicast Ñ Sent to some machines or a group of users
Multiplexing Ñ Supporting multiple connections on one machine Ñ Mapping of multiple connections at one level to a single connection at another Ñ Carrying a number of connections on one fiber optic cable Ñ Aggregating or bonding ISDN lines to gain bandwidth
Transmission Services Ñ Priority Ñ e. g. control messages Ñ Quality of service Ñ Minimum acceptable throughput Ñ Maximum acceptable delay Ñ Security Ñ Access restrictions
OSI - The Model Ñ A layer model Ñ Each layer performs a subset of the required communication functions Ñ Each layer relies on the next lower layer to perform more primitive functions Ñ Each layer provides services to the next higher layer Ñ Changes in one layer should not require changes in other layers
The OSI Environment
OSI as Framework for Standardization
Layer Specific Standards
Elements of Standardization Ñ Protocol specification Ñ Operates between the same layer on two systems Ñ May involve different operating system Ñ Protocol specification must be precise Ñ Format of data units Ñ Semantics of all fields Ñ allowable sequence of PCUs Ñ Service definition Ñ Functional description of what is provided Ñ Addressing Ñ Referenced by SAPs
OSI Layers (1) Ñ Physical interface between devices Ñ Mechanical Ñ Electrical Ñ Functional Ñ Procedural Ñ Data Link Ñ Means of activating, maintaining and deactivating a reliable link Ñ Error detection and control Ñ Higher layers may assume error free transmission
OSI Layers (2) Ñ Network Ñ Transport of information Ñ Higher layers do not need to know about underlying technology Ñ Not needed on direct links Ñ Transport Ñ Exchange of data between end systems Ñ Error free Ñ In sequence Ñ No losses Ñ No duplicates Ñ Quality of service
OSI Layers (3) Ñ Session Ñ Control of dialogues between applications Ñ Dialogue discipline Ñ Grouping Ñ Recovery Ñ Presentation Ñ Data formats and coding Ñ Data compression Ñ Encryption Ñ Application Ñ Means for applications to access OSI environment
Use of a Relay
TCP/IP Protocol Suite Ñ Dominant commercial protocol architecture Ñ Specified and extensively used before OSI Ñ Developed by research funded US Department of Defense Ñ Used by the Internet
TCP/IP Protocol Architecture(1) Ñ Application Layer Ñ Communication between processes or applications Ñ End to end or transport layer (TCP/UDP/� ) Ñ End to end transfer of data Ñ May include reliability mechanism (TCP) Ñ Hides detail of underlying network Ñ Internet Layer (IP) Ñ Routing of data
TCP/IP Protocol Architecture(2) Ñ Network Layer Ñ Logical interface between end system and network Ñ Physical Layer Ñ Transmission medium Ñ Signal rate and encoding
PDUs in TCP/IP
Some Protocols in TCP/IP Suite
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