Data/Link Layer Issues

• Protocol & Services

• Topology

• Error Detection & Recovery

Topology vs Geography

Logical Layout

"How devices talk toeach other" -or-"How devices heareach other"

Physical Layout

How the signal actually travels

TopologiesBus

Star

Ring

Mesh

BUS

• Every node hears every other node's transmissiondirectly.

Ring

• Series of unidirectional point-to-point linkswithout "store & forward", usually with a bypassability.

Star

• Switching functions all in central node

Mesh

• Each node independently routes over (bi-directional) point-to-point links.

Data Link Layer

Uses 'bit pipe' Physical Layer to send packets

Packet Formats - Generic: Framing (Layer 1), Addresses andcontrol information (layer 2), and data (info from layer 3 and up)

Point-to-Point vs Broadcast - Key idea is that not allpacket formats are alike. One needs to look at particluar technologiesto see what is needed.

Data Link Services

• Unacknowledged Connectionless Service– Most LANs– Upper layers handle error recovery

• Acknowledged Connectionless Service– Odd duck. Example?

• Connection-oriented Service– Reliable Delivery ...

Link Protocols

Used to provide reliability. Basic idea can be used at any layer

ABP

SRP

GoBack N

Windowing & Flow Control

Don't need to know details at this time, but know general operationand that they provide assured delivery.

Performance

• Overhead vs Frame Length

• Error rate (bit error vs block error)

• Physical Layer– distance– propagation delay

Error Control

Error Detection - Methods: Parity, Checksum, CRC --generically Frame Check Sequences

Error Correction - The basic idea is to add redundant informationso that the receiver can deocde the message even if some (specified)number of bits are damaged (e.g., Hamming codes)

Error Recovery includes error correction but also includes actions takento get a message retransmitted

Connection Oriented Services

• Two modes of operation:

–Operational–Non-operational

• Operational mode incorporates three functions:

–Link establishment.• A source station sends a frame to a destination station requesting a connection.• The destination station may accept or reject the connection request.

– Information transfer.• Allows information to be transferred after a connection is set up and the required

handshaking has taken place.• Reliable information is transferred between the two stations.

–Link termination.• Either side of the connection may terminate the connection at any time.

IEEE & OSI {again}

PHY

MAC

LLC 2

1

LLC = Logical Link ControlMAC = Media Access ControlPHY = Physical

IEEE 802.2 Fields

Destinationaddress

Source address

Lengthfield

IEEE 802.2 field CRC

DSAPaddress

SSAPaddress

Control Information

1 byte 1 byte 1 or 2 bytes

Length of the Information fieldis access method dependent

I/G D D D D D D D D C/R S S S S S S S

Bit 0

SAP Types• E0 - Novell NetWare

• F0 - NetBIOS

• 06 - TCP/IP

• 42 - Spanning Tree BPDU

• FF - Global SAP

• F4 - IBM Network Management

• 7F - ISO 802.2

• 00 - NULL LSAP

• F8, FC - Remote Program Load

• 04, 05, 08, 0C - SNA

• AA - SNAP

• 80 - XNS

• FE - OSI

SubNetwork Access Protocol (SNAP)• Most common implementation of LLC1 is from a subsection of the IEEE 802.2 standard known

as SNAP.

• At the time of IEEE 802.2’s introduction, most network protocols were designed to use the Ethernet packet format.

• SNAP allows for the migration of the standard network protocols to the IEEE 802.2 format.

• Supported by TCP/IP, NetWare, OSI, AppleTalk, and many other protocols.

• The second purpose for the SNAP protocol is to allow those protocols that do not support the IEEE 802 standard to be able to traverse IEEE 802 LANs.

• SNAP uses a reserved SAP: AA (for both the DSAP and SSAP).

– It uses the unnumbered frame format: control field equal to 03.– Actual SNAP header consumes 5 bytes:

• Three bytes for the Organizationally Unique Identifier (OUI) field, and• Two bytes for an Ethernet Type field.

Protocol Discriminator

OUIType field

3 bytes 2 bytes

Length field DSAP SSAP Control SNAP

header Data Pad CRC-32Sourceaddress

Destinationaddress

AA AA 03

Protocol discriminator

00-00-00 08-00

Verification

• Finite State Machines

• Estelle & Other Languages

• Petri Nets

• Blind Faith (or, code it in C...)

Naming Conventions{and Confusion}

Segment

Segment

Segment

Segment

Bridge

Repeater

Router

Link Layer Subnet

Link Layer NetworkNetwork Layer Subnet

Network Layer Network

Naming Conventions {cont}

Application

Presentation

Session

Transport

Network

Data/Link

Physical

Application

Presentation

Session

Transport

Network

Data/Link

PhysicalRepeater

Bridge

Router