IT Hybrid Model
2016-11-09 19:03:55 0 举报AI智能生成
IT Hybrid Model是一种结合了公有云和私有云优势的云计算模型。在这种模型中，企业可以根据业务需求灵活地使用公有云和私有云资源。公有云提供了高度可扩展性和低成本的优势，而私有云则提供了更高的安全性和控制权。通过将这两种云环境结合起来，企业可以在需要时快速扩展资源，同时保持对数据和应用程序的安全控制。此外，IT Hybrid Model还可以帮助企业实现灾难恢复和业务连续性目标，因为它允许企业在两个不同的云环境中备份和恢复数据。总之，IT Hybrid Model为企业提供了一种灵活、安全且经济高效的云计算解决方案。
作者其他创作
大纲/内容
Hybrid
5. Application
1. Physical
Units
Speed of Light 3*10^8
Speed of electrical signal in copper wire: 2 * 10^8
Transmission Rate bit/sec or byte/sec
Propagation Delay = Length of Channel/Speed of Signals
Transmission Delay = Bits of per frame/Speed of Signals
Latency = P-Delay + T-Delay
NO NEED TO MULTIPLY BY 2 FOR P-Delay!!!!!
Medium
Wireless
Mechanism
Radio wave propagation
Naturally supports broadcasting over a region
Naturally supports mobility enabled network
Easy and inexpensive to deploy
Data rates vary greatly as signals are unstable
Wire
Fiber Optics
enormous bandwidth (THz) and tiny signal loss – hence high data rates over long distances
Optical fibre/fibre optics is fragile and requires more protection around the cable compared to copper.
Example: SONET
Copper Wire
Coaxial Cable
Better shielding than twisted pair = higher speeds over greater distances
Carry either analogue or digital signals(携带模拟或数字信号)
Still widely used for cable TV/Internet
Twisted Pairs
No specialist skills required
Easy to engineer a fixed data rate over point-to-point links
Doesn't readily support mobility or broadcast
Enormous bandwidth over long distances
Satellite (belongs to wireless)
Can rapidly set up anywhere/anytime communications (after satellites have been launched)
Can broadcast to large regions
Examples
high bandwidth but also high latency
Satellite links
Trans-continent optic fibres
low bandwidth and low latency
56-kbps modem/telephony modem
low-end personal area wireless network(e.g. Zigbee)
Link Model
Symbol Rate (Baud Rate)
Number of signal changes per second
Link Terminology
Full Duplex
Half Duplex
Simplex
Multiplexing
Multiple digital signals are combined into one signal over a shared medium.
Time/Space/Frequency
Data Communication Theory
Transform continuous signals into digital values: Sampling the amplitude values of the signal
Nyquist Theorem
Shannon Theorem
Digital Modulation
Modulation Types
Frequency Modulation
Interval of X axis between any two points
Amplitude Modulation
Interval of Y axis between any two points
Phase Modulation (Passband Transmission)
Distance of any point from itself to origin
QAM 16 and QAM 64
Shape is like Lattice
Time Modulation
Blocks of Band and Guard Band
Transmission Calculation
Week 2 Slides 8-11
2. Data Link
Week 3 Slides 5
Type
Connection-oriented/Connectionless
Acknowledged/Unacknowledged
Whether to notify the sender with msg of delivery
Example
Con-less/UnACK
LAN
It cannot ensure that bandwidth is available throughout the transmission.
Con-less/ACK
Con-Ori/ACK
It guarantees that each frame is received exactly once and that all frames are received in the right order.
Checksum
A checksum is a bit string that is computed from the contents of a frame and attached to the end of the frame as a parity
CRC
Cyclical Redundancy Checksum - polynomial code used to compute a checksum for a block and attached to block in transmission
Why there still needs error checking provided that there's already checksum in datalink layer?
Many data-link protocols do not have acknowledgments of successful frame reception and acceptance
Some data-link protocols might not even have any form of checksum to check for transmission errors.
Why put on trailer rather than header?
Hamming Distance
Framing Method
Character count
one !BYTE! (value ranges from 0 to 127) acts as counter placing at the head of frame
Flag bytes with byte stuffing
Start and end flags with bit stuffing
Each frame begins and ends with a special bit pattern e.g. 01111110
Physical layer coding violations
The encoding of bits as signals often includes redundancy to help the receiver.
Error Control
How can we ensure all frames are delivered to the network layer and delivered in canonical order ?
Control Frames
Positive and Negative acknowledgments sent by receiver
Timers
Timeout values independent of acknowledgments.
Sequencing
Sequence number helps to resolve sort orders in the case of multiple retransmissions.
Error Handling
Codeword/Hamming Code
TextBook page 205
Hamming Distance - the degree of similarity between two codewords
Include enough redundancy to allow receiver to determine an error occurred and request retransmission (error-detecting)
Include enough information in frames to allow reconstruction/deduction of original content (error-correcting)
Consider a code with only 4 valid codewords
Flow Control
Regulating the flow of data so as to resolve fast senders and slow receivers problem (slow receiver not swamped by fast sender).
Design Principles
1. Provide Service to network layer
3.1. Regulating the flow of data so that slow receivers are not swampedby fast senders.(Data flow regulation)
Principles to control when sender can send next frame (Fast sender vs low receiver problem resolved)
When the sender sends too much data the receiver will respond a msg to tell it send less next time()
3.2. ARQ (Automatic Repeat reQuest) Protocols (Flow Control)
Concept of ARQ
Receiver acks frames that are correctly delivered
Sender sets timer and resends frame if no ack
Stop and Wait Protocol
Uses ACK and timeout to achieve a reliable transmission over an unreliable service
Application
In Noisy Channel
Frames can be lost either entirely or partially
Requires timeout function to determine arrival or non-arrival of complete frames
In Error-free Channel
SEND - ACK - SEND - ACK
Link Utilization = Tf / (Tf + 2Tp) = (L/B ) / (L/B + 2Tp) = L/ (L+ 2Tp B).
Sliding Window Protocol
Overview
Senders don't need to wait for acknowledgement for each frame before sending next frame
Data is commonly transmitted in both directions simultaneously
Send acknowledgments by piggy-backing
Utilisation usually 100%.
One-bit Sliding Window Protocol
Problems can arise if both sides send an initial packet-synchronization issues
Week 3 Slides 34
Mechanism is almost same as stop and wait protocol
Selective Repeat
NAK (negative ack) causes sender retransmission of a missing frame before a timeout re-sends window
Go-Back-N
PiggyBagging In This Context
Now consider building duplex link using two simplex links
We can intermix \"forward\" (for data between A and B) and \"reverse\" (acks between A and B) using thesame channel.
piggybacked data when returning acks
Sub-Layers
MAC (lower layer) - Medium Access Control
1. Framing/De-Framing
2. Handle transmission errors of frames
3. Decide when medium is used
Protocols
Point to Point Protocol (PPP)
PPP (Point-to-Point Protocol) is a general method for delivering packets across links
Framing uses a flag (0x7E) and byte stuffing
Connectionless un-acknowledged service
Errors are detected with a checksum
Week 3 Slides 43
Differentiation
Medium Access Control (MAC) sub-layer is used to assist in resolving transmission conflicts
ADSL
Week 3 Slides 46
Channel Allocation
Static Channel Allocation
Frequency Division Multiplexing (FDM)
Used in TV and Radios
Arbitrary division of a channel into segments and each user allocated a dedicated segment for transmission
Dynamic Channel Allocation
Time Division Multiplexing (TDM)
2G network
Contention
ALOHA
Users transmit frames whenever they have data; users retry after a random period of time for any collisions came across
Slotted ALOHA
Allows the users to start sending only at the beginning of defined slots. Increase efficiency of pure ALOHA by reducing possibility of collisions
CSMA/CD
1-persistent
Wait (polling) until channel idle; transmit one frame and check collisions
non-persistent
p-persistent
with a probability p it chooses to either 1-persistent or non-persistent
(Less likely to be examined) Binary Exponential Backoff Algorithm (typically used in Ethernet)
Each station randomly pick a time slot within the range of its own window size every time.
Collision Free
BitMap
1 bit per station overhead
Binary Countdown
By this way the overhead is reduced
Limited Contention
Adaptive Tree Walk
Wireless LAN
MACA
Sender asks receiver to transmit short control frame
Stations near receiver hear control frame
Sender can then transmit data to receiver
MACAW
Multiple Access with Collision Avoidance for Wireless
Hidden Terminal and Exposed Terminal Problems
Classic Ethernet (A typical implementation of LAN)
Week 4 Slides 31
MAC Address
A 48-bit number encoded in the frame
eg 00:02:2D:66:7C:2C
LLC (upper layer)
1. Handle packets from Internet Layer/Pack frames into packets(fragmentation)
3. Regulate Flow
3. Internet
Differences between Transport layer and Network layer
2. Services of transport layer should be independent of router technologies (running on hosts) while network layer code runs almost entirely on router
What is inter-networking?
1. Multiple LAN connected through routers (if there's no router then it is not an inter-network)
Vitual Circuit & Datagram
Internetworking
This address allows a routing decision to be made each time the packet is forwarded.
The packets being sent over TCP/IP are simply put on the wire and routed.
Allows flexibility in routing around obstructions.
But network performance can vary a great deal depending on conditions at the routers and on the degree of network traffic at any given time.
IP Packet
IPv4 Frame Structure
Header
Version: IPv4 or IPv6
IHL(Header Length)
Total Length: maximum length 65535 bytes
Identification
DF: Don't Fragment byte
MF: More Fragment byte - are there more or is this the last one ?
Fragment offset: where in the datagram the current fragment belongs
TTL: limits packet lifetimes - hops or seconds
Header Checksum: verifies the header only
The checksum ensures that the information in a received header is accurate.
Source Address: IP - host/network
Destination Address: IP - host/network
Fragmentation and Reassembly
Large packets need to be routed through a network whose maximum packet size is too small.
Solution
Transparent
1. when your packet travels to one network the gateway will fragment it2. reassemble at another gateway3. send to the next network until destination is reached
packets fragmented / reassembled in each network
Non-Transparent
1. when your packet travels to one network the gateway will fragment it2. send THROUGH another gateway to the other end3. resesemble at destination
fragments are reassembled at destination
The main advantage of nontransparent fragmentation is that it requires routers to do less work. IP works this way. A complete design requires that the fragments be numbered in such a way that the original data stream can be reconstructed.
IP Address
Mask of Subnet
Subnetting allows networks to be split into several parts for internal uses whilst acting like a single network for external use
Routing Table
Class
Private IP Range
Tunneling
Tunneling is used when the source and destination have different networks in between. Source Packets are encapsulated over the packets in the connecting network. e.g. Tunneling IPv6 through IPv4.
Routing Algorithm
Non-Adaptive Algorithm(static method)
Dijkstra
Flooding
Adaptive Algorithm(change according to the state)
Distance Vector Routing (Bellman-Ford Algorithm)
Link State Routing
More computation but simpler dynamics
Widely used in practice
3. done
Hierarchical Routing
Each router knows everything about other routers in its region but nothing about routers in other regions
Routers which connect to two regions act as exchange points for routing decisions
Broadcast Routing
Multicast Routing
Internetwork Routing
ICMP (Internet Control Messaging Protocol)
ICMP is a companion to IP that returns error info
ARP (Address Resolution Protocol)
finds MAC address of a local IP address (pls see Extension Diagram for more info)
Glue that is needed to send any IP packets
Host queries an address and the owner replies
DHCP (Dynamic Host Configuration Protocol)
Assigns a local IP address to a host
Gets host started by automatically configuring it
4. Transport
Purpose
Integrate data and distribute them into processes
Reliable Connection Establishment
Connection Release
Asymmetric Disconnection
may result in data loss
Symmetric Disconnection
Strategies
Again use Three way handshake
Finite retry
you try N times if there is no response then the host will end this process
Timeouts
Addressing
Addressing in transport layer is typically doneusing Transport Service Access Points (TSAPs)
Usually this is done via port 80
Transport Primitives
SOCKET
BIND
LISTEN
ACCEPT
CONNECT
SEND
RECEIVE
CLOSE
User Datagram Protocol(UDP)
ADV
Appropriate for multicasting to recipients without having to keep states at senders.
DSV
Main advantage of using UDP over raw IP packets is the ability to specify ports for source and destination pairs.
Usage
RPC (Remote Procedure Control)
Transmission Control Protocol(TCP)
Features
Full duplex - data in both directions simultaneously
Point to point - exact pairs of senders and receivers
Buffer capable - TCP entity can choose to buffer or not prior to sending depending on the context
Allows segments to be disassembled and reassembled during transmission
Requirements needed to establish reliable connection
Packets in the network have a maximum life time
Use 3-way handshake to exchange initial sequence numbers between both ends of connection
Policy choices used that can affect network congestion
Retransmission policy
Out-of-order caching policy
Acknowledgment policy
Negative ack can be used to avoid timeouts
Flow control policy
Timeout determination
Approach
Three Way Handshake
Congestion Window
Slow Start
Address fast link to slow link problem (the limit of different networks may be different)
Refer to Extension
General
Interface (between two layers)
Datagram network (packet switching)
connection-less/properties
packets injected into subnet independently
packets individually routed to destination
each packet contains full source and destination address
get routed through variable paths
routers do not hold state information about connections
difficult to implement QoS
probably faster than virtual circuit
post office model
Circuit-switched network (circuit switching)
connection-oriented
packet routed through fixed path
telephony model
Virtual-circuit network (packet switching)
connection-oriented/properties
require additional setup phase
each packet contains a VC identifier
each VC requires router table space per connection
fixed path
less overhead for storing full addresses of source and destination after connection has been set up.
Data-link layer and network layer virtual circuit protocols are based on connection-oriented packet switching.
Connection-oriented transport layer datalink protocols such as TCP may rely on a connectionless packet switching network layer protocol such as IP.
IP is connection-less but it is able to support TCP as virtual circuit because segments have numbering to identify their order.
Connection-oriented 理解而非记忆例子
Reliable message stream
Sequence of pages
Reliable byte stream
Remote login
Unreliable connection
Digitized voice
Connectionless
Unreliable datagram
Electronic junk mail
Acknowledged datagram
Registered mail
Request-reply
Database query
Structrue
Frame-Packet-Segment
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