Lecture 7 Network Property and ISO Reference Model

Lecture 7 Network Property and ISO Reference Model • • Network Ownership: Private and Public Connectionless Network and Connect-Oriented Network Performance Measures: Delay and Throughput Relationship between Delay and Throughput Network Protocols ISO Network Reference Model – 7 Layer Network Model Flow Control - Stop-And-Go Flow Control - Sliding Window Flow Control • Traffic Congestion and Congestion Control

Lecture 7 Network Ownership: Private and Public Private network: owned by individual, single organization or company - Restricted users in an organization or company - Complete, control, installation, management and mantainenance - Often LAN or Multiple LANs in different buildings, different sites linked by lease lines, and with external connects, called intranet. - Can be WAN like bank network Ichigaya To Internet T 2 need lease lines from phone company T 1 T 1 Koganei Tama Public network: owned by common carrier (e. g. , phone company like NTT or KDD) - Operated by common carrier, May be telephone company or other organization - Analogy telephone network, T-/OC- series, Frame Rely, ATM, CATV, etc. - One connection per subscriber * Typical for small corporation on individual * Communicate with another subscriber - Multiple connections per subscriber * Typical for large, multi-site connection * Communicate among multiple sites

Lecture 7 Connection-oriented Data Transmission

Lecture 7 Connect-Oriented Network & Connectless Network Connection-Oriented (CO) network - Sender and receiver establish and maintain a connection when they have data to exchange - Sender requests “connection” to receiver, waits for network to form connection, keeps the connection till no longer use, and terminates connection. - Network receives connection request, form path specified destination and inform sender, transfers data across connection, and removes connection when send requests. - The “connection” may not be physically a circuit path * The connection is a virtual path (VP) or virtual channel (VC) for packet switch network Connectionless (CL) network - Sender puts data to network without need to establish a connection similar to postal system - Sender forms packet, places address of intended recipient in packet, put packet to network - Network uses destination address to forward and deliver packet - Characteristics Header Payload Trailer * Packet contains identification of destination * Each packet handled independently * No setup requirement before transmitting data * No cleanup required after sending data Dest Addr Sour Addr

Lecture 7 Comparison and Examples of CO and CL CO - Connection setup overhead - More intelligence in network control - Can reserve bandwidth, reliability, etc. CL - Less overhead - Permits asynchronous use - Allows broadcast and multicast

Lecture 7 Network Performance Measures: Delay and Throughput Delay/Latency Network Performance - Measure of time for data (bit/packet) to transit network from source to destination - Propagation delay: time to travel across medium - Switching delay: time for network component (hub, bridge, packet switch) to forward data - Access delay: time to get control of medium (CSMA/CD, token) - Queuing delay: time enqueued in packet switches - The former two are nearly constant, the last two are variable Throughput - Measure of rate at which data can be transmitted in network - Capacity, speed and bandwidth of the underlying hardware e. g. , 34 Kbps modem, 1. 5 Mbps T 1, 6. 4 Mbps ADSL, 1 Gbps gigabit Ethernet - Effective throughput: the rate at which computer can send data less than network capacity because of frame header in each packet and delay Utilization - Ratio of sending data rate to network capacity U Relationship between delay and throughput D = D 0 /(1 - U) where D 0 is delay when the network is idle and D is total effective delay - Throughput and delay are not independent: T×D=C, data volume on the network - As traffic increase, delays increase. When U 1, D ∞ and T 0, congestion - Network should operates at U<75%, must not operate at U>90% Network Throughput & Utilization

Lecture 7 Traffic Congestion and Congestion Control Network congestion - When the input to some link/node reaches/exceeds maximum bandwidth - Packets will queue up at the link/node and some packets will be discarded - Ultimately, network will experience congestion collapse 1 2 6 30 M 50 M 20 M 4 Bandwidth=100 Mbps Congestion 5 3 7 Congestion control techniques - Rate control * Limit rate of data transmission * Performed by sending computer * Performed by network - Network Rate control * Monitor incoming traffic * Drop or reject packets over rate * Called traffic shaping Network Congestion Wikipedia, http: //en. wikipedia. org/wiki/Network_congestion

Lecture 7 Network Protocols Network Protocol - Agreement about communication * formats of messages (syntax) * meanings of messages (semantics) * rules for data exchange and connection establishment/termination * procedures for handling problems Set of Protocols - A network includes a set of protocols to work together, called protocol suite/family - Each protocol in a protocol suite solves part of communication problem Need for Protocols - Hardware is low level and can’t be directly interacted with computer - Need mechanisms to distinguish among * multiple computers on a network * multiple applications on a computer * multiple copies of a single application - Many problems can occur * bit error, packet lost, packet duplicated, packets in disorder, congestion, etc.

Lecture 7 ISO 7 -Layer Network Reference Model - Network protocols are very complex Protocols are divided into layers Each layer devoted to one sub-problem Ease protocols design and implementation ISO 7 -layers reference model Open System Interaction (OSI) Model “Layer N” means the Nth layer in ISO model Not completely same in practical networks Layer Layer 7: 6: 5: 4: 3: 2: 1: Application layer - Application-specific protocols such as FTP and SMTP (email) Presentation layer - Common formats for representation of data Session layer - Management of sessions such as login to a remote computer Transport layer - Reliable delivery of data between computers Network layer - Address assignment and data delivery across a physical network Link layer - Format of data in frame and delivery of frame through network interface Physical layer - Basic network hardware - such as RS-232 or Ethernet encoding OSI model – Wikipedia, http: //en. wikipedia. org/wiki/OSI_model

Lecture 7 Layers, Protocol Software and Protocol Stack - One software module per layer; Modules cooperate - Incoming or outgoing data passes from one module to another - Entire modules called protocol stack

Lecture 7 Protocol Layering Principle Two constraints: - The software for each layer depends only on the services of the software provided by lower layers - The software at layer n at the destination receives exactly the same protocol message sent by layer n at the sender These constraints mean that protocols can be tested independently and can be replaced within a protocol stack

Lecture 7 Layers and Packet Headers Each layer - Adds its header to outgoing packet and - Removes header from incoming packet Headers are nested at the front as the message traverses the network Header, here, may include trailer

Lecture 7 Layers, Headers and Data Encapsulation Anim

Lecture 7 Protocol Techniques - For For For bit corruption: parity, LRC, checksum, CRC out-of-order delivery: adding sequence numbers to packets duplicated packets: using the sequence numbers lost packets: sending acknowledge (ACK) information and retransmission reply delay (excessive delay): unique message ID data overrun: flow control * Stop-and-go flow control * Sliding window flow control P 5 P 4 P 3 P 2 P 1 Sender Rate Ds in sending P 5 P 2 P 3 P 1 network Data overrun if Ds > Dr Network throughput Dn (> Ds) out-of-order: P 2 P 3 duplicated: P 5 P 5 lost packet: P 4 Receiver Rate Dr receiver can process

Lecture 7 Connection-oriented Data Transmission

Lecture 7 Stop-And-Go Flow Control - Allow sender sends one packet each time and waits -Sending side * Transmits one packet * Waits for acknowledgement from receiver - Receiving side * Receives and consumes packet * Transmits acknowledgement to sender -Inefficient * No data sending in waiting time Anim Flow Control -Wikipedia, http: //en. wikipedia. org/wiki/Flow_control_(data) time

Lecture 7 Sliding Window Flow Control - Allows sender to transmit multiple packets before receiving an acknowledgment Number of packets that can be sent is defined by the protocol and called the window Window size is determined by the empty buffer in receiver Receiver tells how many packets can be sent Sender transmits a number of packets, specified by available window (buffer) size Receiver sends acknowledgements as packets arrive Anim

Lecture 7 Comparison of Flow Control - Sliding widow can send data much fast than stop-and-go L L - For stop-and-go, each packet takes 2 L time to deliver where L is the latency, or network delivery time. - Sliding window can improve by number of packets in window: T w = Tg * W Tw is sliding window throughput Tg is stop-and-go throughput - Transmission time also limited by network transmission rate: Tw = min(B, Tg * W) B is maximum network bandwidth Routing table of switch 2

Exercise 7 1. Explain what are the connection-oriented communication and the connectionless communication. Give some examples for each of the above two types of communications. 2. Describe concepts of delay, throughput, bit corruption (error), disordered packet, duplicate packet, lost packet, flow control and network congestion, respectively. 3. Professionals sometimes refer to a “knee” in the delay curve. To understand what they mean, plot the effective delay for values of utilization between 0 and 0. 95. Explain why utilization of a network should not approach to 1. 4. Match the following to one of the ISO 7 layers: a. Route determination. b. Flow control. c. Provides reliable end-to-end transmission of entire message. d. Reassembly of data packets into a message. e. Provides format and code conversion services. f. Provides user services such as electronic mail and file transfer. g. Provides verification of login and logout. h. Mechanical, electrical, and functional interface. 5. Why does a stop-and-go protocol have especially low throughput over a satellite channel that operates at two megabits per second. How to improve throughput using different flow control method?