Prasad Kularatne

Objectivey Understand the functions behind Physical layer and

Data Link layer of the OSI model with more in-depth treatment of the lattery Framing

& Synchronization, Error Control, Flow Control, Reliable Delivery

y Understand the popular data link layer protocol

implementationsy Ethernet y Wi-Fi y PPP

Physical Layery Standardizes transmission media y Cable types and specifications y Encoding and Decoding y Convert electrical pulses into signals that can be transmitted over the medium y Manchester codes, 8B/10B etc. y Common layer-1 communications y Ethernet, xDSL (ADSL, SDSL), Frame Relay, X.25, SONET, ATM, GSM, HSDPA, Wireless LAN y Note: We will not go into details of the Physical Layer

Encoding/Decodingy Determines how the binary data is represented on the

link or how a given bit stream should be converted into signals that can be transmitted over the mediay Allows clock synchronization y Reduces in DC content (for electrical signals) y Detects and correct errors

y Line codes and Block codes y Line codes: RZ, NRZ, NRZI, Manchester coding y Block Codes: 4B5B, 8B10By Q: Which line coding/block coding are used in Ethernet, PPP and WLAN?

Data Link Layery Responsible for transport of packets received from the network layer in a reliable manner to the immediate link destination

y Access control to physical mediay To allow many nodes to share a common transmission medium

y Framingy Synchronize the sender and receiver y Aid in devising error control mechanismsy Error control works on chunks of data rather than on a continuous steam of bits

y Efficient switching

Data Link Layer (cntd.)y Error Controly Detection of errors occurred in transity How many bit errors can be detected?

y Correction of errorsy Detect and errors have occurred y In which bit position has the errors occurred

y Flow Controly Address the problem of fast sender overl0ading a busy receiver y Uses a feedback mechanism to let sender know that he is sending too much

Data Link layer in context

Data Link Layer termination

Ethernet

PPP

Source: Computer Networks, A Tanenbaum

Media Access Controly When to listen , when to transmit and how to handle

contentiony Broadcast media vs. Point-to-Point y Typically original Ethernet LAN s are broadcast and WAN s are P2P y Broadcast media is shared y Mechanism to control access to media should be in place y Media Access Control (MAC)

MAC protocols: CSMAy CS: Carrier Sense -> Is there anyone transmitting? y MA: Multiple Access -> I can hear what others can hear y When to transmit depends on how you would want to

handle the contentiony CSMA/CD: Collisions are NOT prevented, detect them

and let others know of themy

Used in original Ethernet standard Used in Wireless LAN s Obviously more overheads, less effective throughput

y CSMA/CA: Collisions are prevented as much as possibley y

CSMA/CD vs. CSMA/CAy Detection of collisions are possible in wired, but virtually impossible in wireless mediaWireless medium is essentially half-duplex y Research is under way for full duplex wireless y Strength of the transmitted signal essentially masks detection of any other node transmission y If you cannot detect collisions you need to avoid occurring them as much as possibley

y In Wireless LAN s negotiate for media accessy

This negotiation tells other nodes not to transmit till the intended communication is complete

Framing: What is it?y At the Sender: Packets received has to be packaged into frames and sent reliably over the unreliable physical media y At the Receiver: Identify where the frame starts and where it endsy

Requires some special bit sequence to indicate the start and the end

y Synchronizing sender and receiver for frame transmissiony

Where does the frame starts and where does it end

Framing: Approachesy Fixed length y Send a special character at the beginning and end of

the frame y Send a special flag sequence at the beginning and end of the frame y What if the special character or flag sequence occurs in actual data?

Error Controly Error Detection & Correction and a mechanism to deal with detected errors if correction is not possible y Can use eithery Error-Correcting Codes (ECC)y Not all errors can be corrected y Usage: Many wireless networks

y Error-Detecting Codes (EDC)y Just detect errors and deal with them (may be reject them) y Usage: Most wired networks (low BER and fast transmission speeds)

Error Detectiony Introduces some additional bits/bytes into the frame header y MechanismsParity checking (Even or Odd parity) y Checksum y Cyclic Redundancy Check (CRC)y

y Not all errors can be detectedResidual errors may remain y A good mechanism must minimize the residual errors y How to deal with residual errors?y

Error Correctiony Mechanisms y FEC: Forward Error Correction y ARQ: Automatic Repeat Request y FEC is used when it is important to mitigate latency

than to avoid errorsy Voice, video transmissiony

It is OK to lose one or two frames that incurring latency if it has to retransmit!

y E.g. Hamming Codes

Reliable Deliveryy Mechanism: ARQ y Acknowledge the received frame y Timer to stop waiting indefinitely for an ACK y Sequence Numberingy y

Identify what is acknowledged Distinguish between transmissions and retransmissions

y Protocols y Idle ARQ: Stop and Wait y Continuous ARQ: Sliding Window protocolsy y y

n-bit sliding window Go-back-n Selective Repeat

Go-Back-Ny Receiver Windows Size = 1; Sender Windows Size = N

y The sender keeps transmitting until the number of

unacknowledged frames equals its window size y Still if it doesn t receive what it expects it starts resending everything in the windowSource: Computer Networks, A Tanenbaum

Go-Back-N issuesy A pipeline (continuing to send frames when the past

frames have not yet been acknowledged) is a good idea

y Fairly simple and fast mechanism and Frames are

always received in order, no need to re-order

y When the pipeline breaks, a lot of retransmission is

needed

y Can we avoid unnecessarily sending frames?

Selective Repeat ARQy Receiver Windows Size = 1; Sender Windows Size = N

y The sender keeps transmitting frames till a NACK is received, then it starts resending everything not acknowledged. y The receiver can buffer (and reorder) frames after sending a NACK. y No need for sender to re-send buffered framesSource: Computer Networks, A Tanenbaum

Selective Repeat ARQ issuesy Assumptionsy A pipeline is a good idea y Unique sequence numbers can be generated and sent

y Very fast and when pipeline breaks less

retransmissions are required

y But frames are received out of order when pipeline

breaks, DLL has to reassemble

Flow Controly Idea: Sender does not flood the receiver, but maximizes

throughput y Sender throttles until receiver grants permissionSame Sliding Window protocols are used with the receiver window tuned based on the size of receive buffer space

y Mechanism y To advertise Receiver window size (use ACK frame) y To block the Sender if Receiver Windows size is zero

Flow Control (cntd.)y Receiver Window size y Increases when network layer takes control y Decreases when packets are received from the sender y Sender window size y Increases when ACK s are received y Packets in sender window must be buffered at sourcey

Why? May be needed for retransmissions

Flow Control ExampleSenderApplication Does a 2K Write

Receiver

Receiver Buffer

0K Empty2K

4K

Application Does a 3K Write

FullSender is blocked Application Reads 2K 2K Sender may send up to 2K

1K

2K

Individual Assignmenty Examine what Media Access Control, Error control, flow control

and framing techniques are used in the following DLL protocols y Ethernet y Wi-Fiy Ethernet Virtual LAN s are a layer-2 technology that allows multiple

logical networks to be carved out from a single physical network. With Wireless LAN s becoming commonplace in todays enterprise network access layer, it is important to extend Ethernet VLAN s to Wireless LAN environment as well. Briefly describe how Wireless LAN s can accommodate this requirement

DLL Protocolsy Most popular Data Link Layer protocols y Ethernet (most popular Wired-LAN protocol) y Wi-Fi (Wireless LAN protocol) y PPP (a popular WAN protocol) y We will discuss Ethernet protocol and Wi-Fi in detail

Ethernety The most dominant DLL protocol in the networking

world y Demonstrated the fastest growth in last 5-7 yearsy Speed: 100Mbps -> 10Gbps (three orders of magnitude) y Range: LAN -> MAN -> WAN

y Part of Physical layer and Data Link Layer y Standardized under IEEE 802.3 y 802.3u: Fast Ethernet y 802.3z: 1Gbps Ethernet y 802.3ae: 10Gbps EthernetSameFrame format Addressing Ease of deployment Cost effectiveness

Services to network layery Connectionless unacknowledged service to the network

layery Provide unreliable communication interface y Considering the reliability of frame transmissions in

LAN environments frame losses are less y Remember Concepts:y y

Connection oriented vs. Connectionless Acknowledged vs. unacknowledged

MAC layery CSMA/CD y Addresses issues with transmission over shared mediay y y

Ethernet Bus topology Ethernet Hub Half duplex operation

y Not relevant today y Ethernet networks are full duplex, switched y 10Gbps Ethernet does not even talk about this

Ethernet FramingFCS

y Preamble allows receiver to obtain clock synchronization y Addressing y 6-byte world unique y Unicast, multicast, broadcast y Length of data field has to be between 46 Bytes

1500 Bytes

(pad if less than 46 B) y Error control: 32-bit CRC [FCS]Source: Computer Networks, A Tanenbaum

Ethernet Flow Controly Standardized under IEEE 802.3x y Paces a high speed transmitter y Can be a any network element: switch or a host y Receiving station upon overwhelming traffic ingestion

sends a PAUSE MAC control frame to a multicast addressy Control frame carries how long to wait before sending

the next message

y Enhanced and used in Data Center Ethernet (DCE) y Discussed later

Ethernet Management Infrastructure

y IEEE 802.1: LAN/MAN Bridging and Management y We will discuss those applicable to the management of

Ethernet based LAN sIEEE 802.1Q: VLAN s y IEEE 802.1P: LAN based QoS y IEEE 802.1D: Spanning Tree ProtocolySource: www.10gea.org

IEEE 802.1Q: VLAN

y VLAN: Gives a logical topology to LAN s y Segments network load (traffic isolation) y Segments broadcast domains y Facilitates MAC s (Moves, Adds, Changes) y Improves security y VLAN-aware switches vs. VLAN-aware hostsSource: Computer Networks, A Tanenbaum

VLAN: Applicationsy Use of VLAN tagging in y Virtualized server environments y Blade Servers y Design LAN s based on a topology aligned with

business organization rather than physical location of network elementsy Separate out LAN-based backup traffic, management

traffic to separate VLAN s

IEEE 802.1P: Prioritizationy Prioritization of LAN traffic y Level 7: Network-critical traffic (Routing) y Level 5 & 6: Delay-sensitive applications (interactive video and voice) y Level 1 through 4: Controlled-load applications streaming multimedia and business-critical traffic carrying SAP data- down to "loss eligible" traffic y Enhanced and used in offering flow control per

priority in Data Center Ethernet (discussed later)

IEEE 802.1D: Spanning Tree Protocoly Prevent loops from occurring in a LAN y Automatically activates redundant links in failure scenarios y Idea y Go through each path in the LAN and figure out a topology that is loop-free (Tree) y Determine the ISL ports that can be used to maintain just enough connectivity among all segments (The tree is spanning all segments) y Block all other ports

Spanning Tree Algorithmy All switches are assigned a Bridge ID y Select Root Bridge as the node with the lowest bridge ID and

MAC addressy Mark Root Port in all other switches such that it has the least

cost to the Root Bridgey For each LAN segment, select the bridge with least cost to the

root bridge and mark the corresponding port as the Designated Porty Forward frames only over Root & Designated ports and block

all others

Spanning Tree illustrationBridge ID: 1 Bridge ID: 2

Root Port Designated Port Blocked Port

Bridge ID: 3

Bridge ID:

Note on Breaking the ties

Equal cost paths through different bridges -> Select the path to the bridge with lowest Bridge ID Equal cost paths through the same bridge -> Select the path that connect to the lowest Port ID

STP Enhancementsy Why? - To address the shortcomings of the conventional STPTakes a long time to converge upon a bridge/link failure y Poor link utilizationy

y IEEE 802.1W: Rapid Spanning Tree Protocol (RSTP)y

Improve the convergence time

y IEEE 802.1S: Multiple Spanning Tree Protocol (MSTP)Use redundant links for load balancing y Traffic of different VLAN s spans through different redundant links between the same set of switchesy

Multiple Spanning Tree ProtocolBridge ID: 1Bridge ID: 2 Bridge ID: 1 Bridge ID: 2

Root for STPi1

Root for STPi2

Bridge ID: 3

Bridge ID: 4

Bridge ID: 3

Bridge ID: 4

Res lting traffic flo (all lin s sed)

VLAN to MSTP instance Mappings VLANs 10,20,30 STPi1 VLANs 15,25,35 STPi2

Gigabit Ethernety Why? - Proliferation of bandwidth hungry appsy

Content networking, file serving, increase in concurrent server connections

y Compatible to Ethernet and Fast Ethernet (UTP Cat-5) y Same MAC layer as Fast Ethernet y Burrowed Physical Layer from Fibre Channel y Support for MM and SM fiber y 8B/10B encoding y Support for jumbo frames y Ideal for high throughput applicationsy

NFS, CIFS, iSCSI, backup traffic

Gigabit Ethernet overview

Source: Broadband Network Technologies, Technical University of Berlin

Gigabit Ethernet (Protocol Stack)

Source: Broadband Network Technologies, Technical University of Berlin

GigE Distance specifications

Source: Broadband Network Technologies, Technical University of Berlin

10Gbps Ethernety Objective I: Promote Ethernet s LAN experience to

WAN s - Ethernet Broadbandy IEEE 802.1P and 802.1Q enables services provider

capabilities y Defines two PHY options LAN PHY and WAN PHYy

Separate WAN PHY allows Ethernet over existing telco networks

y Objective II: Data Center Ethernet y Increased load on LAN s due toy y

Server virtualization I/O consolidation

Ethernet Broadbandy IEEE 802.1P and 802.1Q standards allowService differentiation (802.1P) y Traffic separation and security (VLAN)y

Source: http://www.10gea.org/ethernet-wan.htm

Data Center Ethernet (DCE)y Architectural extensions to address increased traffic

load & new traffic types in Data Center caused by Server virtualization and I/O consolidationy Also known as Converged Enhanced Ethernet (CEE) y IEEE 802.1Qbb: Priority Flow Control y IEEE 802.1Qau: Congestion Notification y IEEE 802.1Qaz: Bandwidth Management y IEEE 802.1Aq: Shortest Path Bridging

IEEE 802.1Qbb: Priority Flow Controly Combined use of Flow Control (IEEE 802.3x) and Prioritization (IEEE 802.1p) y Send PAUSE frame on a per-user-priority basis y Loss sensitive protocols (Fibre Channel) to get a lossless lane

Ethernet Enhancements supporting I/O consolidation, Nuova Systems Inc.

IEEE 802.1Qaz: BW Managementy Re-allocate the unused bandwidth in one traffic class to another in need of more bandwidth y Allow for bursty LAN traffic while guaranteeing bandwidth for Fibre Channel traffic

Ethernet Enhancements supporting I/O consolidation, Nuova Systems Inc.

What is Wi-Fi?y Its not a protocol, but a set of standards that

defines the compliance of devices to IEEE 802.11 y IEEE 802.11 defines a set of protocols that defines the physical and data link layers of Wireless LANy We will not discuss MAC, framing, flow control

and error control here y They are covered via the Assignment

Unlicensed Frequency BandsShort-Wave Radio AM Broadcast Audio FM Broadcast Television Cellular (840MHz) NPCS (1.9GHz) Infrared Wireless LAN

Extremely Very Low Medium High Very Ultra Super Infrared Visible Ultra- X-Rays Low Low High High High Light violet

2.42.4835 GHz 83.5 MHz (IEEE 802.11b,g)IEEE 802.11n

5 GHz (IEEE 802.11a)IEEE 802.11nSource: Wireless Protocols, Todd Lammle

WLAN design goalsy Easy to use and plug-and-play setup y Enables operation globally over wirelessy Uses license-free ISM band of frequencies

y Low power operationy Extend the use to cover battery operated devices

y Easily inter-operate with the existing wired

infrastructure protecting the committed investments y Support for mobility through roaming

Wi-Fi in IEEE 802Broadband MobilityIEEE 802.16e

WMANIEEE 802.16 / ETSI HiperMAN WiMAX (256-FFT OFDM) 48+ km.

WLANIEEE 802.11x Wi-Fi 100m.

WPANIEEE 802.15 Bluetooth 10m.

Source: Wireless Broadband Mobility, Shawn Taylor

WLAN terminologyy Access Point (AP): Provides co-ordination of

communication within a BSS and provides services to integrate with the Distribution Systemy Basic Service Set (BSS): One or more wireless nodes

that share a single Access Point (co-ordination function)y Extended Service Set (ESS): One or more BSS s

connected via Access Points

WLAN Terminology (Cntd.)y Distribution System (DS): Network that

interconnects several BSS s to form an ESSy Portal: Function that allows BSS to integrate with

the non IEEE 802.11 network

WLAN terminology illustrated802.11 LANBSS1STA1 Access Point Portal STA5

BSS3Access Point

STA4

Portal

Access Point

Distribution System (DS)Portal

STA2

BSS2 ESS

STA3

Portal function802.11 LANBSS1STA1 Access Point STA5

BSS3Access Point

STA4

WirelessAccess Point

Distribution System (DS)Portal

802.3 LAN

STA2

BSS2 ESS

STA3

Services offered by a DSAssociation, Re-association & de-association Distribution service and Integration service

Source: CWAP Certified Wireless Analysis Professional Official Study Guide, Devin Akin and Jim Geier

WLAN topologiesy Infrastructure WLAN y A set of wireless nodes whose communication is coordinated via an access point y Ad-hoc WLAN y A set of wireless nodes that established communication between them without a central coordinator

IEEE 802.11 standardsCriteria 802.11a 802.11b 802.11g 802.11n

Maximum Data Rate

54 Mbps

11 Mbps

54 Mbps DSSS or CCK or OFDM 2.4 GHz 1 20 MHz

600 Mbps DSSS or CCK or OFDM 2.4 GHz or 5 GHz 1, 2, 3 or 4 20 MHz or 40 MHz

Modulation Technique

OFDM

DSSS or CCK

RF-Band Number of Spatial Streams Channel Width

5 GHz 1 20 MHz

2.4 GHz 1 20 MHz

IEEE 802.11ny Need for higher data rates & longer rangey Increased adoption of bandwidth hungry LAN applicationsy

Voice, Video, Gaming & multimedia, file serving etc. Mainly increase in the range

y Need for more simplified and flexible WLAN deploymentsy

y Features introduced in IEEE 802.11n to address the

above requirementsy Re-engineer the Physical layer y Enhance the MAC layer

IEEE 802.1n: Re-engineering PHYy Key technology - MIMO

y Option for the use of an increased frequency band y Use of dual-band antennas 40 MHz channel BW

Source: http://www.computerlanguage.com/ydict.html

MIMO

y Uses a technique called Spatial Multiplexing y Input data stream is split into multiple streams and fed into multiple transmit antennas y Each antenna transmit them in parallel y At the received parallel data streams are merged to reconstruct the original data stream

IEEE 802.11n: Enhanced MAC layery Block Acknowledgementsy Send only one ACK for a number of frames y ACK overhead is minimized, hence the effective

throughput

y Frame Aggregationy Increases the Maximum Frame Size at the MAC layer

Service Frames: 2304 Bytes -> 8 kB y Data Frames: 2304 Bytes -> 64 kB y Benefits especially voice and video trafficy