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Lecture 10 (Data Link Layer) Outline „

Data link layer protocols: z z z z

HDLC PPP Ethernet Wireless LAN

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10.1. Data Link Layer Protocols „

Data link layer protocols HDLC

PPP LANs

z

Ethernet WLAN

HDLC (High-level Data Link Control) Base protocol for most data link layer protocols

z

PPP (Point-to-Point Protocol) Most popular dial-up networking protocol

z

Ethernet Most popular LAN protocol

z

IEEE 802.11 WLAN Most popular wireless LAN protocol

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10.1.1. High-level Data Link Control „

High-level Data Link Control (HDLC): z

z z z

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 ARQ or selective-reject ARQ. Link access control: uses polling method. MAC is not defined for multipoint configuration.

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10.1.1. High-level Data Link Control „

HDLC frame:

z z

Flag: 01111110 (6 ones). Address: Address of the secondary station. 11111111 = broadcast address (to all stations).

z

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

z

FCS: Frame check sequence. Uses CRC for error-detection.

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10.1.1. High-level Data Link Control „

Bit stuffing: z

Is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.

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10.1.1. High-level Data Link Control „

Three frame types: z z z

Information (I-frame): transports user data. Supervisory (S-frame): transports control information. Unnumbered (U-frame): reserved for system management.

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10.1.1. High-level Data Link Control „

Control field: z z

„

N(S) = N(R) = 3 bits – means window size = 8 Uses piggybacking – means combining data to be sent with acknowledgement of the frame received.

Poll/final field: z z

Has meaning only when it is set (= 1). Can mean poll or final depending the frame sender.

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10.1.1. High-level Data Link Control „

S-frame: z z

Used for link access control. Also used for ACK/NACK when the receiver does not have data (I-frame) to send.

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10.1.1. High-level Data Link Control „

Examples of S-frame usage in polling:

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10.1.2. Point-to-Point Protocol „

Point-to-Point Protocol (PPP): z

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.

z z

Uses a variation of HDLC. Link layer services: Framing: uses bit-oriented framing. Flow control: none. Error control: uses CRC for error detection, no error correction. Link access control: detect connection liveness.

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10.1.2. Point-to-Point Protocol „

PPP frame:

z z

Most fields are similar to HDLC. Control: does nothing 11000000 show that the frame does not contain any sequence numbers and no flow and error control

z

Protocol: defines what is being carried in the data field. RFC 1700 gives a list of protocol numbers

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10.1.2. Point-to-Point Protocol „

Protocol field: z

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 layer protocols.

z

This allows PPP to support: User authentication. Dynamic assignment of IP address.

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10.1.2. Point-to-Point Protocol „

Before exchanging network-layer data, data link peers must z

z

configure PPP link (max. frame length, user authentication) 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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10.1.2. Point-to-Point Protocol „

Typical PPP connection states:

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10.1.2. Point-to-Point Protocol „

Typical PPP connection process:

Uses LCP

Uses PAP

Uses LCP CSC4430 – Data Communication and Computer Networks

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10.1.2. Point-to-Point Protocol „

LCP: Link Control Protocol z z

z

For establishing and terminating links. For configuring links – negotiating options between the two points. All LCP packets are carried in the data field of PPP frames.

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10.1.2. Point-to-Point Protocol „

LCP packet: z

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. CSC4430 – Data Communication and Computer Networks

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10.1.2. Point-to-Point Protocol „

PAP: Password Authentication Protocol z

A simple protocol for authenticating users.

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10.1.2. Point-to-Point Protocol „

PAP packet: z z

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: z

z

Is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area. Some LAN architectures: Ethernet Token Bus Token Ring, FDDI Wireless LAN

z

The data link layer of the LAN protocols are all based on HDLC.

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10.1.3. Local Area Networks „

IEEE Project 802: z

z

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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10.1.3. Local Area Networks „

IEEE 802.2 logical link control (LLC). z z z

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 machines Control: same as HDLC control field.

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10.1.3. Local Area Networks „

IEEE 802.3, 802.4, 802.5, 802.6, 802.11: z z

z

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 the contention for the shared media. IEEE 802.3 (Ethernet) – CSMA/CD IEEE 802.4 (Token Bus) – Token passing IEEE 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: z

“Dominant” LAN technology. Cheap HK$100 for 100 Mbps. Simpler, cheaper than token passing LANs. Kept up with speed race: 10, 100, 1000 Mbps

z

Link layer services: Framing: Bit-oriented framing. Flow control: no flow control. Error control: CRC for error detection, no error correction – provide unreliable services. Link access control: uses CSMA/CD for MAC protocol.

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10.1.3.1. Ethernet „

Ethernet frame:

z

Preamble: 7 bytes with pattern 10101010. used to synchronize receiver, sender clock rates

z

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

z

z

Destination/source address: contains the physical address of the frame destination/source Data: LLC PDU, 46-1500 bytes.

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10.1.3.1. Ethernet „

Physical or Ethernet address: z

z z

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.

z

On your PC, try to find out the Ethernet address using winipcfg or ipconfig -all command.

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10.1.3.1. Ethernet „

Access method: CSMA/CD A: sense channel, if idle

then { transmit and monitor the channel; If detect 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} CSC4430 – Data Communication and Computer Networks

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10.1.3.1. Ethernet „

Access method: CSMA/CD z z

Jam signal: make sure all other transmitters are aware of collision = 48 bits. Exponential backoff: Goal: adapt retransmission attempts to estimated current load. heavy load: random wait will be longer. First collision: choose K from { 0,1 } wait K × 512 bit transmission times After 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: z

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. CSC4430 – Data Communication and Computer Networks

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10.1.3.1. Ethernet „

Example: 10Base5 (thick Ethernet or Thicknet) z

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 its other interfaces: physical layer device only! To reduce collisions, total length of bus should not exceed 2500 meters (5 segments).

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10.1.3.1. Ethernet „

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

Star topology using unshielded twisted pair (UTP). Most popular standard.

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10.1.3.1. Ethernet „

Newer Ethernet standards: z

Fast Ethernet Operates at 100 Mbps instead of 10 Mbps

z

Gigabit Ethernet Operates at 1 Gbps

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10.1.3.2. Wireless LAN „

IEEE 802.11: z z

Defines physical layer and MAC functionalities. Physical: Uses unlicensed RF frequency 2.4Ghz and infrared. Allows bit rate 1–2 Mbps.

z

Link layer services: Framing: Bit-oriented framing. Flow control: no flow control. Error control: CRC for error detection, no error correction – provide unreliable services. Link access control: uses CSMA/CA for MAC protocol.

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10.1.3.2. Wireless LAN „

IEEE 802.11 architecture: z

Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station

z

Multiple BSS’s form the distribution system (DS).

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10.1.3.2. Wireless LAN 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 busy then binary backoff „

Receiver: - if received OK then return ACK after SIFS

„

Others z

defer access for NAV time units

DIFS = Distribution Inter Frame Space SIFS = Short Inter Frame Space NAV = Network Allocation Vector

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10.1.3.2. Wireless LAN „

Hidden terminal effect: z

Due to decreasing signal strength (obstacles, signal attenuation), A and C cannot hear each other. If they transmit at the same time to B, collisions will occur at B.

z

Goal: to avoid collisions at B. Use CSMA/CA: CSMA with Collision Avoidance

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10.1.3.2. Wireless LAN IEEE 802.11 CSMA/CA: „ Explicit channel reservation z z z

„

„

sender: send short RTS receiver: reply with short CTS CTS reserves channel for sender, notifying (possibly hidden) stations

Avoid hidden station collisions RTS and CTS short: z z

collisions less likely end result similar to collision detection

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RTS = Request to Send CTS = Clear to Send

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10.1.4. Summary „

Four popular data link protocols: z z z z

z

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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10.1.5. Further Readings „

Data link layer protocols: z z z z

HDLC : Section 10.4. PPP : Chapter 15. Ethernet : Section 12.1 – 12.3. Wireless LAN Ross Section 5.7 http://www.palowireless.com/i802_11/tutorials.asp

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Next Lecture

Internetworking Devices (Forouzan Chapter 21)

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