Computer Networks Chapter 1 - Fundamentals CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1
Computer Networks • Need to share – Information – Resources • Communication vs. Storage Transmission across: – Space (communication) – Time (storage) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 2
Virtuality • Architecture – Layered – Hierarchical • Algorithms – Information hiding, ADTs, objects • Protocols – Distributed coordination algorithms • Programs – Modularity CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 3
Layered vs. Hierarchical • Both – Peer-to-peer communication – Encapsulation – Protocol = common language/behaviors • Layered – Layer i serves layer i+1 ONLY – Layer i gets service from layer i-1 ONLY • Hierarchical – Lower layers serve higher layers CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 4
Layered vs. Hierarchical • Issues – Flexibility – Efficiency – Modularity – Maintainability – Scalability/manageability – Future adaptability CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 5
Comparative Architectures User Application 7 - Application Network Functional 6 - Presentation Application Management Data Services 5 - Session Network Data Flow Control Services 4 - Transport Protocol Transmission Ctl 3 - Network Transport Path Control 2 – Data Link DDCMP Data Link Control 1 - Physical Physical 1 - Physical ISO - OSI DEC - DECnet IBM - SNA CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 6
OSI Reference Architecture • Physical (L1 = PHY) • Data Link (L2 = MAC/Link) • Network (L3) • Transport (L4) • Session (L5) • Presentation (L6) • Application (L7) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 7
PHY Layer • Hardware • Physical manipulation of medium (modulation) • Physical sensing of medium (detection) • Low level synchronization (bits/symbols/frames) • Forward error correction/error detection • Mechanical/electrical interconnect and medium CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 8
Link Layer • Firmware • Framing • Addressing • Medium access control (MAC) • Backward error detection/correction • Reliable delivery of frames from one STA to a directly connected STA • Pacing • Upward multiplexing CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 9
Network Layer • Software/firmware • Packets/cells • Routing • Packet fragmentation/reassembly • Backward error correction • Delivery of frames from source to an indirectly connected destination • Congestion control CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 10
Transport Layer • Software on end host: End-to-end layer • Reliable communication stream – Messages – Byte stream – Ordering – BEC • Upward Multiplexing • Delivery of messages/byte stream from source process to destination process • Congestion control CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 11
Session Layer • Software on end host: end-to-end layer • Stream management • Dialog control • Packet chaining (atomic delivery) • Downward Multiplexing • Authentication • Connection-oriented CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 12
Presentation Layer • Software on end host • Common utilities – Encryption – Compression – Uniform formatting (XML, ASN.1,…) • Standardized representations • Interfacing to local resources CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 13
Application Layer • Software on end host • Specific application programs – FTP – Remote terminal (rlogin, telnet, ssh,…) – Email – HTTP • May also be layered in distributed software system CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 14
Encapsulation • Receive SDU from higher layer • Hide uninterpreted SDU as payload of PDU source destination message M application M application transport M transport H t segment M H t network network M datagram H n H t M H n H t link link H n H t M H l frame M H l H n H t physical physical (thanks – Kurose & Ross) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 15
End-to-end Data Transport (thanks – Kurose & Ross) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 16
Service Models • Interface – Connectionless – Connection-oriented • Reliability – Best effort – Reliable • Combinations CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 17
Service Interface • Interface – Connectionless: memoryless • Send packet • Receive packet – Connection-oriented: stateful • Initialize (set up connection) • Use (send/receive) • Close (release state) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 18
Service Reliability • Reliability – Best effort • Lost packets • Duplicate packets • Delayed/reordered delivery • Damaged packets – Reliable • Undamaged packets • All packets sent delivered in timely fashion • Delivered in order sent CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 19
Message Conventions Request/Confirm Indication/Response 8 - CNF 5 - RSP 4 - IND 1-REQ 2,3 6,7 7 - Rx 6 - Tx 2 - Tx 3 - Rx CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 20
Combining Service Models Connectionless Connection-Oriented UDP, IP, IPX, CLNP, ATM DECnet, Appletalk, Best Effort CLNS,… TCP, X.25, CONS Reliable ??? CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 21
Network Service vs. Implementation Implementation Service Connectionless Connection-Oriented Connectionless UDP, CLNS ??? X.25, ATM, CONS, Connection-Oriented TCP, DNA SNA CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 22
Network Properties • Scope • Scalability • Robustness • Autoconfigurability • Tweakability • Determinism • Migration CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 23
Network Properties - Robustness • Types of Errors – Link/node failure – Data errors (esp. undetected!) – S/W errors – H/W errors – Human Errors • Features CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 24
Network Properties - Robustness • Types of Errors • Features – Safety Barriers – Self-stabilization – Fault detection – Byzantine robustness CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 25
Reliable Data Transfer • Models – Errors – Receiver capacity • Requirements – Duplexity – Timers – State CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 26
Data Transfer Model • Events – What can happen at node, channel • Frames – What do they hold • Duplexity – Simplex, half duplex, full duplex • Time costs – What does it take to complete transfer • Metrics – How do we measure the costs CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 27
Data Transfer - Events • Node – New frame to send from HLE – Frame/ACK arrival – good frame – Frame/ACK arrival – damaged frame – Timeout – Attempt to receive next frame by HLE • Channel – Error – damage frame – Error – lose frame CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 28
Data Transfer - Frames • Forward Control Info – Type – Sequence number – Timestamp Dst Src Type SN TS Len … Payload FCS – Length – Addressing – Error Detection (FCS) • Reverse Control Info – ACKs – Flow control/pacing – Piggybacking • Information – payload CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 29
Channel Model - Duplexity B A B B A A Simplex Half Duplex Full Duplex • Simplex – only one way • Half duplex – one way at a time • Full duplex – simultaneously both ways CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 30
Data Transfer - Delays Src Dest • Processing Tx REQ – Source Src Proc – Destination Transmission D a t a f r a m e • Transmission Propagation – Time to put bits on wire Dest Proc ACK Tx • Propagation ACK frame Propagation Rx REQ – Time for bit to traverse Src Proc channel Tx CNF Rx CNF CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 31
Data Transfer - Metrics • Utilization – Time sending info/total time • Storage requirements – At source – At destination • Channel type – duplexity • Timers – Retransmission – ACK transmission CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 32
Reliable Data Xfer - Utopia Src Dest • Infinitely fast receiver • Simplex channel • No errors • 100% utilization by protocol CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 33
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