layering of abstractions : don’t sweat the details of the lower layer, only deal with lower layers abstractly
The Internet: virtualizing networks
“ embed internetwork packets in local packet format or extract them”
route (at internetwork level) to next gateway
DataLink Layer ARPAnet satellite net gateway
Internetwork layer (IP):
addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity
network of networks
Cerf & Kahn’s Internetwork Architecture
What is virtualized?
two layers of addressing : internetwork and local network
new layer (IP) makes everything homogeneous at internetwork layer
underlying local network technology
today: ATM, MPLS
… “ invisible” at internetwork layer. Looks like a link layer technology to IP!
ATM and MPLS
ATM, MPLS separate networks in their own right
different service models, addressing, routing from Internet
viewed by Internet as logical link connecting IP routers
just like dialup link is really part of separate network (telephone network)
Asynchronous Transfer Mode: ATM
1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture
Goal: integrated, end-end transport of carry voice, video, data
meeting timing/QoS requirements of voice, video (versus Internet best-effort model)
“ next generation” telephony: technical roots in telephone world
packet-switching (fixed length packets, called “cells”) using virtual circuits
adaptation layer: only at edge of ATM network
roughly analagous to Internet transport layer
ATM layer: “network” layer
cell switching, routing
DataLink Layer physical ATM AAL physical ATM AAL physical ATM physical ATM end system end system switch switch
ATM Adaptation Layer (AAL)
Different versions of AAL layers, depending on ATM service class:
AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation
AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video
AAL5: for data (eg, IP datagrams)
DataLink Layer AAL PDU ATM cell User data small payload -> short cell-creation delay for digitized voice
ATM Layer: Virtual Circuits
VC transport: cells carried on VC from source to dest
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination ID)
every switch on source-dest path maintain “state” for each passing connection
link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf.
Permanent VCs (PVCs)
long lasting connections
typically: “permanent” route between to IP routers
Switched VCs (SVC):
dynamically set up on per-call basis
Advantages of ATM VC approach:
QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter)
Drawbacks of ATM VC approach:
Inefficient support of datagram traffic
one PVC between each source/dest pair) does not scale (N*2 connections needed)
SVC introduces call setup latency, processing overhead for short lived connections
ATM cell header
5-byte ATM cell header
VCI: virtual channel ID
will change from link to link thru net
PT: Payload type (e.g. RM cell versus data cell)
CLP: Cell Loss Priority bit
CLP = 1 implies low priority cell, can be discarded if congestion
HEC: Header Error Checksum
cyclic redundancy check
IP-Over-ATM DataLink Layer IP datagrams into ATM AAL5 PDUs IP addresses to ATM addresses AAL ATM phy phy Eth IP ATM phy ATM phy app transport IP AAL ATM phy app transport IP Eth phy
Multiprotocol label switching (MPLS)
initial goal : speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address!
DataLink Layer PPP or Ethernet header IP header remainder of link-layer frame MPLS header label Exp S TTL 20 3 1 5
MPLS capable routers
a.k.a. label-switched router
forwards packets to outgoing interface based only on label value (don’t inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwarding
use MPLS for traffic engineering
forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !!
must co-exist with IP-only routers
MPLS forwarding tables DataLink Layer R1 R2 D R3 R4 R5 0 1 0 0 A R6 1 0 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A 1 12 9 D 0 in out out label label dest interface 8 6 A 0 in out out label label dest interface 10 A 0 12 D 0 8 A 1
Chapter 5: Summary
principles behind data link layer services:
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
instantiation and implementation of various link layer technologies
virtualized networks as a link layer: ATM, MPLS
Slides are modified from Behrouz A. Forouzan
TCP/IP and OSI model
Physical layer Physical Layer To be transmitted, data must be transformed to electromagnetic signals.
Physical Layer Chapter 3: Data and Signals Chapter 4: Digital Transmission Chapter 5: Analog Transmission
3-1 ANALOG AND DIGITAL
Data can be analog or digital
Analog data refers to information that is continuous
Analog data take on continuous values
Analog signals can have an infinite number of values in a range
Digital data refers to information that has discrete states
Digital data take on discrete values
Digital signals can have only a limited number of values
In data communications, we commonly use periodic analog signals and nonperiodic digital signals . Physical Layer
Comparison of analog and digital signals Physical Layer
3-2 PERIODIC ANALOG SIGNALS
Periodic analog signals can be classified as simple or composite .
A simple periodic analog signal, a sine wave , cannot be decomposed into simpler signals.
A composite periodic analog signal is composed of multiple sine waves.
Signal amplitude Physical Layer
Frequency is the rate of change with respect to time.
Change in a short span of time means high frequency.
Change over a long span of time means low frequency.
If a signal does not change at all, its frequency is zero
If a signal changes instantaneously, its frequency is infinite.
Frequency Physical Layer
Frequency and period are the inverse of each other. Units of period and frequency Frequency and Period Physical Layer
Two signals with the same amplitude, but different frequencies Physical Layer
The power we use at home has a frequency of 60 Hz . What is the period of this sine wave ? Examples The period of a signal is 100 ms. What is its frequency in kilohertz? Physical Layer
Phase describes the position of the waveform relative to time 0 Phase Three sine waves with the same amplitude and frequency, but different phases Physical Layer
A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians? Example Solution We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is Physical Layer
Wavelength and period Physical Layer Wavelength = Propagation speed x Period = Propagation speed / Frequency
Time-domain and frequency-domain plots of a sine wave Physical Layer A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
The frequency domain is more compact and useful when we are dealing with more than one sine wave.
A single-frequency sine wave is not useful in data communication
We need to send a composite signal , a signal made of many simple sine waves.
According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. Fourier analysis
If the composite signal is periodic , the decomposition gives a series of signals with discrete frequencies;
If the composite signal is nonperiodic , the decomposition gives a combination of sine waves with continuous frequencies.
A composite periodic signal Decomposition of the composite periodic signal in the time and frequency domains Physical Layer
Time and frequency domains of a nonperiodic signal
A nonperiodic composite signal
It can be a signal created by a microphone or a telephone set when a word or two is pronounced.
In this case, the composite signal cannot be periodic
because that implies that we are repeating the same word or words with exactly the same tone.
The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Bandwidth Physical Layer
A nonperiodic composite signal has a bandwidth of 200 kHz, with a middle frequency of 140 kHz and peak amplitude of 20 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal. Solution The lowest frequency must be at 40 kHz and the highest at 240 kHz. Example Physical Layer