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CSC4430 Data Communication and Computer Networks 1

Lecture 10 (Data Link Layer) Outline

Data link layer protocols:

HDLC

PPP Ethernet

Wireless LAN

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10.1. Data Link Layer Protocols

Data link layer protocols

HDLC (High-level Data Link Control)

Base protocol for most data link layer protocols PPP (Point-to-Point Protocol)

Most popular dial-up networking protocol

EthernetMost popular LAN protocol

IEEE 802.11 WLAN

Most popular wireless LAN protocol

HDLCPPP

LANs EthernetWLAN

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10.1.1. High-level Data Link Control

High-level Data Link Control (HDLC):

Is a base protocol for all other bit-oriented data

link layer protocols. Supports both half-duplex and full-duplex mode.

Supports both point-to-point and multipoint.

Link layer services:Framing: uses bit-oriented framing.

Flow control: uses sliding window.

Error control: uses CRC with either go-back-n ARQor selective-reject ARQ.

Link access control: uses polling method.

MAC is not defined for multipoint configuration.

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HDLC frame:

Flag: 01111110 (6 ones). Address: Address of the secondarystation.

11111111 = broadcast address (to all stations).

Control: Differentiate 3 types of frames.Information, supervisory, and unnumbered frames.

FCS: Frame check sequence.

Uses CRC for error-detection.

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Bit stuffing:

Is the process of adding one extra 0 whenever

there are five consecutive 1s in the data so thatthe receiver does not mistake the data for a flag.

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Three frame types: Information (I-frame): transports user data.

Supervisory (S-frame): transports control information.

Unnumbered (U-frame): reserved for system management.

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Control field:

N(S) = N(R) = 3 bits means window size = 8

Uses piggybacking means combining data to besent with acknowledgement of the frame received.

Poll/final field:

Has meaning only when it is set (= 1).

Can mean poll or final depending the framesender.

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S-frame:

Used for link access control.

Also used for ACK/NACK when the receiver doesnot have data (I-frame) to send.

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Examples of S-frame usage in polling:

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10.1.2. Point-to-Point Protocol

Point-to-Point Protocol (PPP):

Is a protocol for configuring PPP connection and

managing data transfer in a dial-up line.Telephone line provides the physical link.

PPP provides the data link.

Uses a variation of HDLC. Link layer services:

Framing: uses bit-oriented framing.

Flow control: none.Error control: uses CRC for error detection, no errorcorrection.

Link access control: detect connection liveness.

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PPP frame:

Most fields are similar to HDLC.

Control: does nothing

11000000 show that the frame does not containany sequence numbers and no flow and errorcontrol

Protocol: defines what is being carried in the datafield.

RFC 1700 gives a list of protocol numbers

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Protocol field:

Allow to carry data of any network layer protocol

(not just IP) at the same time.LCP packets for link establishment.

CHAP packets for user authentication.

NCP, IPX, Appletalk packets other network layerprotocols.

This allows PPP to support:

User authentication.Dynamic assignment of IP address.

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Before exchanging network-layer data, datalink peers must

configure PPP link (max. frame length, userauthentication)

learn/configure network layer information

e.g. for IP, it carries IP Control Protocol (IPCP)messages to learn/configure IP address

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Typical PPP connection states:

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Typical PPP connection process:

Uses LCP

Uses PAP

Uses LCP

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LCP: Link Control Protocol

For establishing and terminating links.

For configuring links negotiating optionsbetween the two points.

All LCP packets are carried in the data field of

PPP frames.

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LCP packet:

Fields:

Code: defines the type of LCP packet.

ID: is used to match a request with a reply.

Length: length of the whole LCP packet.

Information: contains options that can be

negotiated between the two end points.

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PAP: Password Authentication Protocol

A simple protocol for authenticating users.

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PAP packet:

There are three types of PAP packets.

PAP packets are encapsulated in a PPP frame.

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10.1.3. Local Area Networks

A local area network:

Is a data communication system that allows a

number ofindependent devices to communicatedirectly with each other in a limited geographicarea.

Some LAN architectures:Ethernet

Token Bus

Token Ring, FDDIWireless LAN

The data link layer of the LAN protocols are all

based on HDLC.

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IEEE Project 802:

Specifies functions of the physical layer and data

link layer to interconnect various LAN protocols. Specifies the internetworking between the LAN

protocols.

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IEEE 802.2 logical link control (LLC).

Common to all LAN protocols

Contains the end-user portions of the HDLC frame. LLC protocol data unit (PDU):

DSAP and SSAP: identify the protocol stacks on

the receiving and sending machinesControl: same as HDLC control field.

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IEEE 802.3, 802.4, 802.5, 802.6, 802.11:

Contains the physical layer specifications.

Contains the data link layer specifications,including the framing, flow, error control to use,and physical address format.

Defines media access control to resolve thecontention for the shared media.

IEEE 802.3 (Ethernet) CSMA/CD

IEEE 802.4 (Token Bus) Token passingIEEE 802.5 (Token Ring) Token passing

IEEE 802.6 (FDDI) Token passing

IEEE 802.11 (Wireless LAN) CSMA/CA

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10.1.3.1. Ethernet

Ethernet:

Dominant LAN technology.

Cheap HK$100 for 100 Mbps.Simpler, cheaper than token passing LANs.

Kept up with speed race: 10, 100, 1000 Mbps

Link layer services:Framing: Bit-oriented framing.

Flow control: no flow control.

Error control: CRC for error detection, no errorcorrection provide unreliable services.

Link access control: uses CSMA/CD for MACprotocol.

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10.1.3.1. Ethernet

Ethernet frame:

Preamble: 7 bytes with pattern 10101010.

used to synchronize receiver, sender clock rates

SFD: signals the beginning of the frame.Preamble+SFD = Flag in HDLC.

Destination/source address: contains the physicaladdress of the frame destination/source

Data: LLC PDU, 46-1500 bytes.

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10.1.3.1. Ethernet

Physical or Ethernet address:

Is a unique address encoded on its network

interface card (NIC). Its length is six bytes.

Commonly written as six pairs of hexadecimal

digits with dashes or colons.e.g. 00-B0-D0-3C-D4-A4.

On your PC, try to find out the Ethernet address

using winipcfg oripconfig -all command.

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10.1.3.1. Ethernet

Access method: CSMA/CDA: sense channel, if idle

then {

transmit and monitor the channel;

Ifdetect another transmission

then {

abort and send jam signal;update # collisions;

delay as required by exponential backoff algorithm;

goto A

}

else {done with the frame; set collisions to zero}

}

else {wait until ongoing transmission is over and goto A}

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10.1.3.1. Ethernet

Access method: CSMA/CD

Jam signal: make sure all other transmitters areaware of collision = 48 bits.

Exponential backoff:Goal: adapt retransmission attempts to estimated currentload.

heavy load: random wait will be longer.

First collision:

choose K from { 0,1 }

wait K 512 bit transmission timesAfter second collision: choose K from { 0,1, 2, 3 }

After ten or more collisions, choose K from { 0,1, 2, 3, 4,, 1023 }

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10.1.3.1. Ethernet

Physical specifications:

Two categories:

First number indicates the data rate in Mbps.

Support data rates between 1 to 100 Mbps.

Last number indicates the maximum cable length or

the type of cable.

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10.1.3.1. Ethernet

Example: 10Base5 (thick Ethernet or Thicknet)

Bus topology using a thick coaxial cable.

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10.1.3.1. Ethernet

Repeaters used to connect up to multiple segments.

Repeater repeats bits it hears on one interface to itsother interfaces: physical layer device only!

To reduce collisions, total length of bus should notexceed 2500 meters (5 segments).

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10.1.3.1. Ethernet

Example: 10Base-T (twisted-pair Ethernet).

Star topology using unshielded twisted pair (UTP).

Most popular standard.

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10.1.3.1. Ethernet

Newer Ethernet standards:

Fast Ethernet

Operates at 100 Mbps instead of 10 Mbps Gigabit Ethernet

Operates at 1 Gbps

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10.1.3.2. Wireless LAN

IEEE 802.11:

Defines physical layer and MAC functionalities.

Physical:Uses unlicensed RF frequency 2.4Ghz and infrared.

Allows bit rate 12 Mbps.

Link layer services:Framing: Bit-oriented framing.

Flow control: no flow control.

Error control: CRC for error detection, no errorcorrection provide unreliable services.

Link access control: uses CSMA/CA for MAC

protocol.

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IEEE 802.11 architecture:

Basic Service Set (BSS) (a.k.a. cell) contains:

wireless hostsaccess point (AP): base station

Multiple BSSs form the distribution system (DS).

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IEEE 802.11 CSMA:

Sender- if sense channel idle for

longer than DIFS time unit

then transmit entire frame(no collision detection)

- if sense channel busythen binary backoff

Receiver:- if received OK

then return ACK afterSIFS

Others defer access forNAV time

units

DIFS = Distribution Inter Frame Space

SIFS = Short Inter Frame Space

NAV = Network Allocation Vector

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Hidden terminal effect:

Due to decreasing signal strength (obstacles, signalattenuation), A and C cannot hear each other.

If they transmit at the same time to B, collisions will occur at B. Goal: to avoid collisions at B.

Use CSMA/CA: CSMA with Collision Avoidance

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IEEE 802.11 CSMA/CA:

Explicit channel reservation sender: send short RTS

receiver: reply with short CTS

CTS reserves channel forsender, notifying (possibly

hidden) stations Avoid hidden station

collisions

RTS and CTS short: collisions less likely

end result similar to collisiondetection

RTS = Request to Send

CTS = Clear to Send

10 1 4 S

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10.1.4. Summary

Four popular data link protocols:

HDLC, PPP, Ethernet, IEEE 802.10.

All uses bit-oriented framing. HDLC provides reliable services.

All others derive from HDLC but provide no flow

control and unreliable services. Popular MAC method: CSMA.

Carrier sensing allow high throughput even at high

load.

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