Networking Protocols and Layers

Lecture Progression

• The layers include Physical, Link, Network, Transport, and Application
• Focus will go from the bottom up, starting with Chapter 2

Protocols and Layers

• Modularity is needed to manage complexity and maintain network functionality
• Protocols and layering divide up network functionality
• Each protocol instance communicates virtually with its peer
• Protocol instances use services of the lower layer
• A protocol stack refers to the set of protocols in use
• The wire connecting two nodes is the physical medium
• Example protocols: TCP, IP, Ethernet, and 802.11

Encapsulation

• Encapsulation is how protocol layering is achieved
• The lower layer takes higher layer content and adds information to send new messages
• Layers do not inspect the content inside
• Demultiplexing is the reverse of multiplexing
• Demultiplexing reconverts a signal with multiple streams
• In protocol layers demultiplexing keys are used in the headers

Advantages of Layering

• Helps information hiding and reuse
• Connects different systems through information hiding

Disadvantages of Layering

• Introduces overhead, reducing efficiency
• Hides information from apps which might need it

Reference Models

• Help organize protocols
• Determines which functions should be implemented in a protocol

OSI Reference Model

• Principled, international standard to connect systems
• A bottom-up model consisting of Physical, Data Link, Network, Transport, Session, Presentation, and Application Layers
• Physical Layer: Sends bits across a physical medium
• Data Link: Sends units of information
• Network Layer: Sends packets across multiple links, is broader than data link
• Transport Layer: Ensures reliable delivery
• Session Layer: Manages task dialogs, may use multiple networks
• Presentation Layer: Handles different data representations
• Application Layer: Provides functionalities for users

Internet Reference Model

• OSI-based but with some layers omitted, using IP as the network layer
• Link Layer: Sends frames over a link
• Internet: Sends packets over multiple networks, acts as the network layer from OSI
• Transport Layer: Ensures reliable delivery
• Application Layer: Provides user functionality
• IP is a standard reference so that diversity can exist in the Link and Transport/Application layers, creating the "narrow waist" of the Internet

Layer-Based Names

• Application: Message
• Transport: Segment
• Network: Packet
• Link: Frame
• Physical: Bit

Devices

• Hubs operate on the physical layer
• Switches operate on the link layer
• Routers operate on the network layer
• Proxy servers process requests and forward info between clients and servers
• Middleboxes transform, inspect, filter, or manipulate traffic
• Gateways connect two networks with different protocols, translating between them
• Host nodes are computers that serve pages to users
• Gateways connect the networks
• ISP computers and computers that control company network traffic

Physical Layer Overview

• Handles the transfer of message bits over a link
• Wired connections carry analog signals to send digital bits
• Topics includes properties of mediums, signal propagation, modulation and fundamental limits
• Focuses on rate, delay, and whether the connection is broadcast

Physical Layer Key Variables

• Rate in bits/second
• Delay in seconds, related to length
• Broadcast either wireless or wired

Message Latency

• Latency is the delay to send messages over a link
• Transmission delay is the time to put M-bit messages on the wire -> M(bits) / Rate (bits/sec) = M/R seconds
• Propagation delay is the time its take for bits to propagate across the wire -> P-Delay: Length / (⅔ the speed of light) = D(elay) (seconds)
• Formula: Latency = M/R + D

Prefixes

• Milli: 10^-3
• Micro: 10^-6
• Nano: 10^-9
• Kilo: 10^3
• Mega: 10^6
• Giga: 10^9
• Use powers of 10 for rates, and 2 for storage
• B is for bytes and b is for bits

Latency and Bandwidth Examples

• Dial-Up example with specified delay, rate, and message size showcases latency calculation
• Broadband cross-country example with specified delay, rate, and message size
• High latency results from either a long link or a slower rate where one delay component usually dominates

Bandwidth-Delay Product

• Messages take space on the wire
• The amount of data is the Bandwidth-delay product = R * D
• BD is measured in bits or in messages
• Small BD for LANs and big for long pipes

Bandwidth-Delay Product Example

• Using a Fiber line at home, from cross country
• Rate is 40 Mbps, Delay is 50 ms
• Bandwidth-Delay Product = 40 * 10^6 * 50 * 10^-3 = 2000 * 10^3 = 250 KB

Media Types

• Media such as wires, fiber optics, and wireless, propagate signals that carry bit information
• Twisted Pair cables are used in LANs and telephone lines
• Twists reduces signal radiation, a category 5 UTP cable has 4 twisted pairs
• Coaxial cables are used for video signals to home TVs, and offers better performance than twisted pairs
• Fiber cables are comprised of pure strands of glass, allowing for enormous bandwidth and high data speeds with minimal attenuation
• Wireless mediums radiate a wireless signal, with senders communicating over a region
• Multi-mode cables are for shorter links, and single mode cables are up to 100km

Signals

• Analog signals encode digital bits and propagate over a medium
• A signal over time is represented by its frequency components
• Fewer frequencies degrade signals with less bandwidth, but high fidelity is not necessary for clear 1s and 0s

Wire Signals

• Signals are delayed and attenuated when passing over a wire
• Frequencies above a cutoff are highly attenuated
• Noise is added to signals
• Electrical engineering defines bandwidth as the width of a frequency band
• Computer science bandwidth is the information carrying capacity

Fiber Signals

• Light propagates with minimal loss across wide frequency bands using a carrier to send information

Wireless Signals

• Signals transmit on a carrier frequency
• Wireless travels at the speed of light but tends to attenuate
• Multiple signals on the same frequency interfere at the receiver
• Wireless is complex and dependent on the environment
• Wireless is frequency dependent, with multipath at microwave band

2-4 Modulation

• How to represent bits as signals?
• Baseband modulation involves directly applying signals on the wire
• Using Non-Return-to-Zero (NRZ): +V = 1, -V represents 0
• After a while, if timing can be lost so clock recovery is needed
• Two clock recovery options exist in manchester coding, and scrambling via XOR
• Higher frequencies are going to pass through the media
• In passband modulation changes in Amplitude, frequency and phase happen

Fundamental Limits

• Nyquist states (1924), and Shannon (1948) define practical systems
• B signal bandwidth limits the rate of transitions
• S Signal strength limit how many signal levels can be distinguished
• N is the Noise strength

Data Link Layer

• Involves the actual implementation of the protocols
• Concerns transferring messages over links with limited size
• It combines physical with frames

Framing Methods

• Including byte count with length fields included
• Bute stuffing with special flag values for start/end
• Bit stuffing with a flag sets for 1s

Error Coding Overview

• Error coding addresses the issue of bits received in error due to noise
• Redundancy is added through error detection codes to add check bits
• Error codes provide more check bits to correct errors
• Detection testing will require analysing how many bits are used in the code

Hamming Distance

• Hamming distance defines the number of bits flips needed to change D + R1 to D + R2
• The minimum distance between any pair data is used
• A code of certain distance (d + 1) detects up to d errors
• A code of distance (2d+1) corrects up to d errors

Error Detection

• Addresses the issue of bits received in error due to noise
• Fixed through parity, checksums and CRCs
• Retransmission is required if errors are detected

Error Detection - Internet Checksum

• Parity defines that if bits have certain sum the system is correct
• More complex options like error checksums with algorithms are stronger
• TCP IP uses 16 bit checksum

Error detection - CRC

• Even more secure
• cyclic redundancy checks are better
• it extends the data with zeros
• And adjusts check bits

Error Correction

• Techniques are used to address bits received in error due to noise
• Check bit values are used to narrow down the error
• Errors should be correct even if in check bits

Error Correction - The Hamming Perspective

• Coding allows to fix error
• Decoding Hamming codes involves recomputing check bits
• Arranging values tells error
• Otherwise flip bit correct

Retransmission ARQ

• Checking and errors should happen everwhere for stabillity
• Error correction happens in the stack
• Layers are responsible for overall reliability
• Most used where errors are common

###ARQ

• Receiver automatically acknowledges
• Timeout until ACK is received

Timeout Issues

• Short timeouts lead to needless resends
• Long timeouts link can go idle

Multiple ARQS and stop and wait

• Lost ACKs lead to loss so sequencing is import
• Stop and Waite to prevent
• A short single bit is sufficient

Multiplexing

• Sharing A link over different users
• Uses scheduling and bandwidth allocations
• TDMA Takes turns on a fixed schedule
• FDM Divides frequencies for data rate
• Multiple access schemes multiplex network traffic
• Nodes randomize resource attempts

Randomised Multiple Access Control for Sharing links (Such As WIFI)

• Classic ethernet is base but is bursty
• Nodes send when have traffic
• Wait if collision
• More efficient if traffic not high
• CSMA improves
• If listening
• Not eliminate because listen may heard nothing
• BEB eliminates need queue the senders
• Doubles collision increases efficiency
• Collision increase time for work

Wireless protocol

• Complicated because nodes use different areas
• Nodes dont hear while sending

Terminals

• Hidden - collisions can occur even though senders thought it was free
• Exposed interferes, and prevent

MACA Protocol

• MACA uses small handshake A . request sender B. Clear to send Collissons possbile

802.11 is popular Wireless

• Clients connect
• Multi Access Issues
• Has become faster
• CSTA and CMAC can fix
• Needs short handhsake