ms_dcn_nt03_3

ms_dcn_nt03_3 - INTERNATIONAL INSTITUTE OF INFORMATION...

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Unformatted text preview: INTERNATIONAL INSTITUTE OF INFORMATION INTERNATIONAL TECHNOLOGY Communication Networks Communication Networks 2 I2 IT NETWORKING & TELECOM Protocols: Used for communications between entities in different systems Must speak the same language Entities: – User applications: – file transfer/e-mail facilities/database management Systems: – Computer – Terminal – Remote sensor 2 2 I2 IT NETWORKING & TELECOM Key Elements of a Protocol: Syntax – Data formats – Signal levels Semantics – Control information for co-ordination – Error handling Timing – Speed matching – Sequencing 3 2 I2 IT NETWORKING & TELECOM Protocol Architecture: Task of communication broken up into modules For example file transfer could use three modules – File transfer application – Communication service module – Network access module 4 2 I2 IT NETWORKING & TELECOM Simplified File Transfer Architecture: 5 2 I2 IT NETWORKING & TELECOM A Three Layer Model: Network Access Layer Transport Layer Application Layer 6 2 I2 IT NETWORKING & TELECOM Network Access Layer: Exchange of data between the computer and the network Sending computer provides address of destination May invoke levels of service (e.g. priority) Dependent on type of network used (circuit switching/packet switching/LAN etc.) 7 2 I2 IT NETWORKING & TELECOM Transport Layer: Reliable data exchange Independent of network being used Independent of application 8 2 I2 IT NETWORKING & TELECOM Application Layer: Support for different user applications e.g. e-mail, file transfer 9 2 I2 IT NETWORKING & TELECOM Protocol Architectures and Networks: 10 2 I2 IT NETWORKING & TELECOM Protocols in Simplified Architecture: 11 2 I2 IT NETWORKING & TELECOM Protocol Data Units (PDU): At each layer, protocols are used to communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added – Destination SAP – Sequence number – Error detection code This gives a transport protocol data unit 12 2 I2 IT NETWORKING & TELECOM Network PDU: Adds network header – network address for destination computer – Facilities request (e.g. priority) 13 2 I2 IT NETWORKING & TELECOM Operation of a Protocol Architecture: DSAP: Destination Service Access Points 14 2 I2 IT NETWORKING & TELECOM Protocol Characteristics: Direct or indirect Monolithic or structured Symmetric or asymmetric Standard or nonstandard 15 2 I2 IT NETWORKING & TELECOM Direct or Indirect: Direct – Systems share a point to point link or – Systems share a multi-point link – Data can pass without intervening active agent Indirect – Switched networks or – Internetworks or internets – Data transfer depend on other entities 16 2 I2 IT NETWORKING & TELECOM Monolithic or Structured: Communications is a complex task Too complex for single unit In monolithic, whole task is considered as single Structured design breaks down the problem into smaller units Layered structure 17 2 I2 IT NETWORKING & TELECOM Symmetric or Asymmetric: Symmetric – Communication between peer entities(Terminal to Terminal) Asymmetric – Client/server 18 2 I2 IT NETWORKING & TELECOM Standard or Nonstandard: Nonstandard protocols built for specific computers and tasks 19 2 I2 IT NETWORKING & TELECOM Functions of Protocols: Encapsulation Segmentation and reassembly Connection control Ordered delivery Flow control Error control Addressing Multiplexing Transmission services 20 2 I2 IT NETWORKING & TELECOM Encapsulation: Addition of control information to data – Address information – Error-detecting code – Protocol control 21 2 I2 IT NETWORKING & TELECOM Segmentation (Fragmentation)/ Segmentation Reassembly: Reassembly: Data blocks are of bounded size Application layer messages may be large Network packets may be smaller Splitting larger blocks into smaller ones is called as segmentation in OSI model (or fragmentation in TCP/IP) – ATM blocks (cells) are 53 bytes long – Ethernet blocks (frames) are up to 1526 bytes long Reassembly is done at the receiving protocols at destination 22 2 I2 IT NETWORKING & TELECOM Why Fragment? 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 23 2 I2 IT NETWORKING & TELECOM Connection Control: Connection Establishment Data Transfer Connection Termination May be connection interruption and recovery Sequence numbers used for – Ordered delivery – Flow control – Error control 24 2 I2 IT NETWORKING & TELECOM Connection Oriented Data Transfer: 25 2 I2 IT NETWORKING & TELECOM Ordered Delivery: PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering 26 2 I2 IT NETWORKING & TELECOM Flow Control: Done by receiving entity Limit amount or rate of data Stop and wait HDLC Sliding Window Protocol 27 2 I2 IT NETWORKING & TELECOM Error Control: 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 28 2 I2 IT NETWORKING & TELECOM Addressing: Addressing level Addressing scope Connection identifiers Addressing mode 29 2 I2 IT NETWORKING & TELECOM Addressing level: Three levels of addressing required: – 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) – Each application on a (multi-tasking) computer needs a unique address within the computer • Port number (TCP/IP) • Service access point or SAP (OSI): – Physical Address: • Link to link communication • Each network must maintain a unique address for each device interface on a network (e.g. MAC address in LAN) 30 2 I2 IT NETWORKING & TELECOM Addressing Scope: Global non-ambiguity – Global address identifies unique system – There is only one system with address X Global applicability – Address X identifies that system from anywhere on the network 31 2 I2 IT NETWORKING & TELECOM Connection Identifiers: Used in Connection oriented data transfer (virtual circuits) Allocate a connection name during the transfer phase (initial phase) – Reduced overhead as connection identifiers are shorter than global addresses – Routing may be fixed and identified by connection name – Entities may want multiple connections - multiplexing – Use of State information: for flow/error control by using sequence numbers 32 2 I2 IT NETWORKING & TELECOM Addressing Mode: 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 33 2 I2 IT NETWORKING & TELECOM Multiplexing: 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 34 2 I2 IT NETWORKING & TELECOM Transmission Services: Priority – e.g. control messages Quality of service – Minimum acceptable throughput – Maximum acceptable delay Security – Access restrictions 35 ...
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