Data/Link Layer Issues Protocol & Services Topology Error Detection & Recovery

Topology vs Geography Logical Layout "How devices talk to each other" -or- "How devices hear each other" Physical Layout How the signal actually travels

Topologies

BUS Every node hears every other node's transmission directly.

Ring Series of unidirectional point-to-point links without "store & forward", usually with a bypass ability.

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 and control information (layer 2), and data (info from layer 3 and up) Point-to-Point vs Broadcast - Key idea is that not all packet formats are alike. One needs to look at particluar technologies to 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 operation and 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 information so 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 taken to 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 Control MAC = Media Access Control PHY = Physical

IEEE Fields Destination address Source address Length field IEEE field CRC DSAP address SSAP address Control Information 1 byte 1 or 2 bytes Length of the Information field is access method dependent I/G D D D D D D D DC/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 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 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 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 OUI Type field 3 bytes2 bytes Length field DSAP SSAPControl SNAP header DataPadCRC-32 Source address Destination address AA 03 Protocol discriminator

Verification Finite State Machines Estelle & Other Languages Petri Nets Blind Faith (or, code it in C...)

Naming Conventions {and Confusion}

Naming Conventions {cont}