Local Area Network Lesson 7 NETS2150/2850

Lesson Outline Common LAN topologies Logical Link Control sublayer Medium Access Control sublayer ARP protocol for IP  MAC map LAN interconnection devices

Topologies LAN topology refers to the ways end systems are interconnected Common topologies: Tree Bus Special case of tree Ring Star

LAN Topologies

Bus and Tree Transmission propagates throughout medium Heard by all stations Need to identify target station Each station has unique address Full duplex connection between station and tap Allows for simultaneous transmission and reception Need to regulate transmission To avoid collisions To avoid hogging Data in small frames (fragmentation!) Terminator absorbs frames at end of medium Prevent from being reflected into the channel

Frame Transmission on Bus LAN

Ring Topology Repeaters joined by point to point links in closed loop Receive data on one link and retransmit on another Links unidirectional Stations attach to repeaters Data in frames Circulate past all stations Destination recognizes address and copies frame Frame circulates back to source where it is removed MAC protocol determines when station can insert frame

Frame Transmission Ring LAN

Star Topology Each station connected directly to central node Usually via two point to point links Central node can broadcast Only one station can transmit at a time Or central node can act as frame switch More stations can transmit at a time

IEEE 802 v OSI RM

802 Layers - Physical Encoding/decoding Preamble generation/removal 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronise receiver, sender clock rates Bit transmission/reception Transmission medium and topology

802 Layers - Logical Link Control Based on HDLC Provides interface to higher levels Transmission of LLC PDU between two stations Flow and error control Must support multiaccess, shared LAN media Link access handled by MAC layer

LLC Services Unacknowledged connectionless service No handshake and no ack (unreliable) Connection mode service Use handshake and ack Acknowledged connectionless service No handshake but uses ack

Media Access Control Assembly of data into frame with address and error detection fields Disassembly of frame Address recognition Error detection Govern access to transmission medium

MAC Frame Format MAC layer receives data from LLC layer and adds: MAC control Destination MAC address (6-octet or 48-bit) Source MAC address CRC MAC layer detects errors and discards frames MAC broadcast address: FF FF FF FF FF FF16 LLC optionally retransmits unsuccessful frames

IEEE 802.3 MAC Frame Format Addresses: 6 octets Length Addresses: 6 octets if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocol otherwise, adapter discards frame Length: length of data field in octets, max frame size is 1518 octets (excluding preamble & SFD) CRC: checked at receiver, if error is detected, the frame is simply dropped (32-bit CRC)

MAC protocols Assume single shared broadcast channel Two or more simultaneous transmissions by nodes will cause interference only one node can send successfully at a time MAC protocol: distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit

MAC Protocols: A taxonomy Three broad classes: Channel Partitioning or Reservation divide channel into smaller “pieces” (time slots, frequency, code) allocate a piece to node for exclusive use Random Access or Contention channel not divided, thus can’t avoid collisions Need to “recover” from collisions “Taking turns” or Round Robin tightly coordinate shared access to avoid collisions

Address Resolution Protocol (ARP) Even if you have the IP address of your destination, you need its MAC to get your data across a physical network So, we need a way to do this mapping ARP performs dynamic mapping between IP and MAC Any resolved mapping is stored in a host’s ARP cache

ARP operation McGraw-Hill The McGraw-Hill Companies, Inc., 2004

An ARP request is broadcast; an ARP reply is unicast. Note: An ARP request is broadcast; an ARP reply is unicast. An ARP reply is only generated by the destined node.

ARP Packet Format McGraw-Hill The McGraw-Hill Companies, Inc., 2004

Encapsulation of ARP Packet Length McGraw-Hill The McGraw-Hill Companies, Inc., 2004

Interconnecting LAN segments Hubs Bridges Switches

Hubs Hub acts as a repeater (physical layer device) When single station transmits, hub repeats signal on outgoing line to each station Limited to about 100 m Optical fibre may be used Max about 500 m Physically star, logically bus Transmission from any station received by all other stations Forms a single collision domain Two stations transmit at the same time  collision!!

Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between stations But individual segments’ collision domain become one large collision domain when a node in CS and a node in EE transmit at same time  collision!! Can’t interconnect 10BaseT & 100BaseT

Bridges Link layer device (layer-2 device) stores and forwards Ethernet frames examines frame header and selectively forwards frame based on MAC dest address transparent stations are unaware of presence of bridges plug-and-play, self-learning bridges do not need to be configured

Bridges: traffic isolation Bridge installation breaks LAN into LAN segments bridges filter packets: same-LAN-segment frames not usually forwarded onto other LAN segments segments become separate collision domains bridge collision domain = hub = station LAN segment LAN segment LAN

Forwarding How to determine to which LAN segment to forward frame? Looks like a routing problem...

Self learning A bridge has a bridge table entry in bridge table: (Station MAC Address, Bridge Interface, Timestamp) stale entries in table dropped (TTL can be ~ 60 min) bridges learn which hosts can be reached through which interfaces when frame received, bridge “learns” location of sender: incoming LAN segment records sender/location pair in bridge table

Bridge example Suppose C sends frame to D and D replies back with frame to C. Bridge receives frame from from C updates bridge table, C is on interface/port 1 because D is not in table, bridge sends frame into interfaces 2 and 3 frame received by D

Bridge Learning: example C 1 D generates frame for C, and sends it bridge receives frame notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so selectively forwards frame to interface 1

Interconnection without backbone Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

Backbone configuration Recommended ! Note: A bridge does not change the physical (MAC) addresses in a frame.

Loop of Bridges

Spanning Tree Algorithm Address learning works for tree layout i.e. no closed loops (or cycles) But not for cyclic connected graph! Spanning Tree Algo. builds a network including all the nodes with selected links (i.e. edges) without closed loops Known as a spanning tree!

Spanning Tree for increased reliability, desirable to have redundant, alternative paths from source to dest but need to avoid cycles solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

Some bridge features Isolates collision domains resulting in higher total max throughput (i.e. amount of data transmitted within an interval) Transparent (“plug-and-play”): no configuration necessary

Routers vs. Bridges (1) both store-and-forward devices routers: network layer devices (examine network layer headers) bridges are link layer devices routers maintain routing tables, implement routing algorithms bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

Routers vs. Bridges (2) Bridges pros (+) and cons (-) + Bridge operation is simpler requiring less data unit processing + Bridge tables are self learning - All traffic confined to spanning tree, even when alternative bandwidth is available - Bridges do not offer protection from broadcast storms (i.e. forwarding of broadcast traffic)

Routers vs. Bridges (3) Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher packet processing bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Ethernet Switches Essentially a multi-interface bridge layer 2 (frame) forwarding, filtering using LAN addresses Incoming frame from particular station switched to appropriate output line Unused lines can switch other traffic More than one station can transmit at a time Multiplying capacity of LAN

Shared Hub and Switch 

Types of Ethernet Switches Store-and-forward switch Accepts frame on input line Buffers it briefly, then forwards it to appropriate output line Error checking, boosts integrity of network Cut-through switch Takes advantage of dest address appearing at beginning of frame Switch begins repeating frame onto output line as soon as it recognizes dest address Highest possible throughput Risk of propagating bad frames Switch unable to check CRC prior to retransmission Netgear GS108UK GB Switch Latency ~ 10 µs for 64-byte frames Throughput 32 Mfps MAC database (8000 entries)

Ethernet Switch Benefits No change to attached stations to convert bus LAN or hub LAN to switched LAN For Ethernet LAN, each station uses Ethernet MAC protocol Each station has dedicated capacity equal to original LAN Assuming switch has sufficient capacity to keep up with all devices Switch scales easily Con: still has broadcast storm problem!

Subnetwork with layer-3 device! Solution: break up network into subnetworks connected by routers or layer-3 switch (faster!) Packet forwarding done in the hardware MAC broadcast frame limited to stations and switches contained within a single subnetwork

Typical Large LAN Organization Thousands to tens of thousands of stations Desktop systems links 10 Mbps to 100 Mbps Into layer 2 switch Wireless LAN connectivity available for mobile users Layer 3 switches at local network's core Form local backbone Interconnected at 1 Gbps Connect to layer 2 switches at 100 Mbps to 1 Gbps Servers connect directly to layer 2 or layer 3 switches at 1 Gbps

Typical Large LAN Organization Diagram

Summary comparison

Summary LAN topologies IEEE 802 reference model Types of MAC protocols Interconnection Devices Hubs, bridges, switches, routers Read Stallings chapter 15 Next: Specific MAC protocols