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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