Link Layer&

Physical Layer

CPE 400 / 600Computer Communication Networks

Lecture 24

2

Lecture 24: Outline 5.1 Introduction and Services

5.2 Error-detection and Error-correction

5.3 Multiple Access Protocols

5.4 Link-layer Addressing

5.5 Ethernet

5.6 Link-layer Switches

5.7 Point to Point Protocol

5.8 Link Virtualization ATM , MPLS

Physical Layer Data and Signals

DataLink Layer 3

Point to Point Data Link Control one sender, one receiver, one link: easier than

broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line

popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link

used to be considered “high layer” in protocol stack!)

DataLink Layer 4

PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer

datagram in data link frame ability to demultiplex upwards

bit transparency: must carry any bit pattern in the data field

error detection (no correction)

connection liveness: detect, signal link failure to network layer

network layer address negotiation: endpoint can learn/configure each other’s network address

Error recovery, flow control, data re-ordering all relegated to higher layers!

DataLink Layer 5

PPP Data Frame Flag: delimiter (framing)

Address: does nothing (only one option)

Control: does nothing; in the future possible multiple control fields

Protocol: upper layer protocol to which frame delivered (eg, IP, PPP-LCP, IPCP, etc)

info: upper layer data being carried

check: cyclic redundancy check for error detection

DataLink Layer 6

PPP Data Control ProtocolBefore exchanging network-

layer data, data link peers must

configure PPP link (max. frame length, authentication)

learn/configure network layer information for IP: carry IP Control

Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

DataLink Layer 7

Virtualization of networks

Virtualization of resources: powerful abstraction in systems engineering:

computing examples: virtual memory, virtual devices Virtual machines: e.g., java IBM VM os from 1960’s/70’s

layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

DataLink Layer 8

The Internet: virtualizing networks

ARPAnet satellite net

gateway

Internetwork layer (IP): addressing: internetwork

appears as single, uniform entity, despite underlying local network heterogeneity

network of networks

Gateway: “embed internetwork

packets in local packet format or extract them”

route (at internetwork level) to next gateway

DataLink Layer 9

Cerf & Kahn’s Internetwork ArchitectureWhat is virtualized? two layers of addressing: internetwork and local

network new layer (IP) makes everything homogeneous at

internetwork layer underlying local network technology

cable satellite telephone modem today: ATM, MPLS

… “invisible” at internetwork layer. Looks like a link layer technology to IP!

DataLink Layer 10

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)

DataLink Layer 11

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

DataLink Layer 12

ATM architecture

adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer

ATM layer: “network” layer cell switching, routing

physical layer

physical

ATM

AAL

physical

ATM

AAL

physical

ATM

physical

ATM

end system end systemswitch switch

DataLink Layer 13

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)

AAL PDU

ATM cell

User data

small payload -> short cell-creation delay for digitized

voice

DataLink Layer 14

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

DataLink Layer 15

ATM VCs

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

DataLink Layer 16

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

DataLink Layer 17

IP-Over-ATM

AALATMphyphy

Eth

IP

ATMphy

ATMphy

apptransport

IPAALATMphy

apptransport

IPEthphy

IP datagrams into ATM AAL5

PDUs

IP addresses to ATM addresses

DataLink Layer 18

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!

PPP or Ethernet header

IP header remainder of link-layer frameMPLS header

label Exp S TTL

20 3 1 5

DataLink Layer 19

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 RSVP-TE 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

DataLink Layer 20

R1R2

D

R3R4

R50

1

00

A

R6

in out outlabel label dest interface 6 - A 0

in out outlabel label dest interface10 6 A 1

12 9 D 0

1

in out outlabel label dest interface 8 6 A 0

0

in out outlabel label dest interface 10 A 0

12 D 0 8 A 1

MPLS forwarding tables

DataLink Layer 21

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 Ethernet switched LANs PPP virtualized networks as a link layer: ATM, MPLS

Physical Layer

Slides are modified from Behrouz A. Forouzan22

TCP/IP and OSI model

23

Source-to-destination delivery

24

Physical layer

25Physical 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

26

3-1 ANALOG AND DIGITAL3-1 ANALOG AND DIGITAL

Data can be Data can be analoganalog or or digitaldigital

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 useperiodic analog signals and nonperiodic digital signals.

27Physical Layer

Comparison of analog and digital signals

28Physical Layer

3-2 PERIODIC ANALOG SIGNALS3-2 PERIODIC ANALOG SIGNALS

Periodic analog signals can be classified as Periodic analog signals can be classified as simplesimple or or compositecomposite..

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.

29Physical Layer

Signal amplitude

30Physical 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

31Physical Layer

Frequency and period are the inverse of each other.

Units of period and frequency

Frequency and Period

32Physical Layer

Two signals with the same amplitude,but different frequencies

33Physical 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?

34Physical Layer

Phase describes the position of the waveformrelative to time 0

Phase

Three sine waves with the same amplitude and frequency,but different phases

35Physical Layer

A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?

Example

SolutionWe know that 1 complete cycle is 360°. Therefore, 1/6 cycle is

36Physical Layer

Wavelength and period

37Physical Layer

Wavelength = Propagation speed x Period = Propagation speed / Frequency

Time-domain and frequency-domain plots of a sine wave

38Physical Layer

A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

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

o We need to send a composite signal, a signal made of many simple sine waves. 39Physical Layer

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.

40Physical Layer

A composite periodic signal

Decomposition of the composite periodic signal in the time and frequency domains

41Physical Layer

Time and frequency domains of a nonperiodic signal

A nonperiodic composite signal o It can be a signal created by a microphone or a telephone set when a word or two is pronounced. o 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.

42Physical Layer

The bandwidth of a composite signal is the difference between the highest and the lowest

frequencies contained in that signal.

Bandwidth

43Physical 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.

SolutionThe lowest frequency must be at 40 kHz and the highest at 240 kHz.

Example

44Physical Layer