Network topologiesL. Lefèvre

INRIA, Lyon, FranceLaurent.lefevre@inria.fr

Coaxial cable (1)• Widely installed for use in business and corporation

ethernet and other types of LANs.• Consists of inter copper insulator covered by cladding

material, and then covered by an outer jacket• Physical Descriptions:

Covered by sheath materialOuter conductor is braided shielded (ground)

Separated by insulating materialInner conductor is solid copper metal

Optical Fibers (2)

• multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. used for relatively short distances because the modes tend to disperse over longer lengths (this is called modal dispersion) .

• For longer distances, single mode fiber(sometimes called monomode) fiber is used. In single mode fiber a single ray or mode of light act as a carrier

Wireless LAN

• Wireless LAN (802.11b..) : 11 to 108Mbits• HiperLAN (European standard; allow

communication at up to 20 Mbps in 5 GHz range of the radio frquency (RF) spectrum.

• HiperLAN/2 operate at about 54 Mbps in the same RF band.

• WiMAX

Major Categories of Networks

• PAN• SAN / SAN• Local Area Networks (LAN)• Metropolitan Area Networks (MAN)• Wide Area Networks (WAN)

Issues of size and breadth.

Circuit Switching Technologies• Circuit switching is a dedicated communications path

established between two stations or multiple end points• Transmission path is a connected sequence of physical

link between nodes.• On each link, a logical channel is dedicated to the

connection. Data generated by the source station are transmitted along dedicated path as rapidly as possible.

• At each node, incoming data are routed or switched to the appropriate outgoing channel without excessive delay. However, if data processing is required, some delay is experienced.

• Example of circuit switching above is the telephone networks.

Packet Switching Technologies• It is not necessasry (as in circuit switching) to dedicate transmission

capacity along a path rather data are sent out in a sequence of small chucks, called packets.

• Each packet, consisting of several bits is passed through the network from node to node along some path leading from the source to the destination

• At each node along the path, the entire packet is received, stored briefly, and then transmitted to the next node.

• At destination all individual packets are assembled together to form the complete text and message from the source. Each packet is identified as to its place in the overall text for reassembly.

• Packet switching networks are commonly used for terminal-to-computer and computer-to-computer communications.

• If packet errors occur, the packet is retransmitted.

Network Interface Card (NIC)

• Every computer and most devices (e.g. a network printer) is connected to network through an NIC.

Network CharacteristicsNetwork Characteristics• Topology:

– Physical interconnection structure of the network graph:• Node Degree: Number of channels per node. • Network diameter: Longest minimum routing distance between any two nodes in

hops.• Average Distance between all pairs of nodes .• Bisection width: Minimum number of links whose removal disconnects the graph

and cuts it in half.• Symmetry: The property that the network looks the same from every node.• Homogeneity: Whether all the nodes and links are identical or not.

– Type of interconnection:• Static or Direct Interconnects: Nodes connected directly using static point-to-point

links.• Dynamic or Indirect Interconnects: Switches are usually used to realize dynamic

links between nodes: – Each node is connected to specific subset of switches. (e.g

Multistage Interconnection Networks, MINs).– Blocking or non-blocking, permutations realized.

• Shared-, broadcast-, or bus-based connections. (e.g. Ethernet-based).

Introduction

• Physical and Logical Topologies• Topologies

– Bus– Ring– Star– Extended Star– Mesh– Hybrid

Static PointStatic Point--toto--point Connection Network Topologiespoint Connection Network Topologies• Direct point-to-point links are used.• Suitable for predictable communication patterns matching topology.

Linear Array:

Ring:

Examples: Token-Ring, FDDI, SCI, FiberChannel Arbitrated Loop, KSR1

Route A -> B given by relative address R = B-A

N-1 Links -> O(N) complexityNode Degree: 1-2Diameter = N -1Average Distance = 2/3NBisection Width = 1

N Links -> O(N) complexityNode Degree: 2Diameter = N/2Average Distance = 1/3NBisection Width = 2

Fully Connected Network: Every node is connected to all other nodes using N- 1 direct links

N(N-1)/2 Links -> O(N2) complexityNode Degree: N -1 Diameter = 1Average Distance = 1Bisection Width = (N/2)2

Ring Topology

Ring Topology (2)

• No beginning or end (a ring in fact !!)• All devices of equality of access to media• Single ring – data travels in one direction only, guess

what a double ring allows !?• Each device has to wait its turn to transmit• Most common type is Token Ring (IEEE 802.5)• A token contains the data, reaches the destination,

data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use

Star Topology

Star Topology (2)

• Like the spokes of a wheel (without the symmetry)

• Centre point is a Hub• Segments meet at the Hub• Each device needs its own cable to the Hub• Predominant type of topology• Easy to maintain and expand

Star Topology (3)• Advantages• Easy to add devices as the

network expands• One cable failure does not

bring down the entire network (resilience)

• Hub provides centralised management

• Easy to find device and cable problems

• Can be upgraded to faster speeds

• Lots of support as it is the most used

• Disadvantages• A star network requires more

cable than a ring or bus network

• Failure of the central hub can bring down the entire network

• Costs are higher (installation and equipment) than for most bus networks

Star Network– Bisection bandwidth

difficult to scale– Easy to set up– No path redundancy

Switch

Computer

Clos– Named for Charles Clos paper

in ‘53.– Full bisection bandwidth– Scalable to arbitrary size– Based on a xbar

technology(old-16, new-32)– Redundant and Reconfigurable– # of Input links = # of output

links– Vampire has 2 myrinet clos

networks with 0.5Tb/s and 1Tb/s

8 hosts/Xbar16

1 link pair/host

4 link pairsSpreader Network

Torus– Constant incremental cost

as you add nodes– Automatically scales

bisection bandwidth– Redundant paths– Wiring nightmare!– Best for applications that

can exploit the topology– Care must be taken when

assigning nodes to a job– Built-in switch!

Ring

2D Torus

Static Network Topologies Examples:Static Network Topologies Examples:Multidimensional Meshes and Tori

3D cube2D mesh 2D torusd-dimensional array or mesh:

– N = kd-1 X ...X kO nodes– described by d-vector of coordinates (id-1, ..., iO)

– Where 0 ≤ ij ≤ kj for 0 ≤ j ≤ d-1

d-dimensional k-ary mesh: N = kd

– k = d√N– described by d-vector of radix k coordinate. – Diameter = d(k-1)

d-dimensional k-ary torus (or k-ary d-cube): – Edges wrap around, every node has degree 2d and

connected to nodes that differ by one (mod k) in every dimension.

1D torus

1D mesh

Multidimensional Meshes and Tori PropertiesMultidimensional Meshes and Tori PropertiesRouting:

– Relative distance: R = (b d-1 - a d-1, ... , b0 - a0 )– Traverse ri = b i - a i hops in each dimension.– Dimension-order routing.

Average Distance:– d x 2k/3 for mesh.– dk/2 for cube.

Degree:– d to 2d for mesh.– 2d for cube.

Bisection bandwidth:– k d-1 bi-directional links when k is even.

• Physical layout?– 2D in O(N) space.

Static Connection Networks Examples: Static Connection Networks Examples: HypercubesHypercubes

• Also called binary n-cubes. • Dimension = n = log2N • Number of nodes = N = 2n

• Diameter: O(log2N) hops• Good bisection BW: N/2• Complexity:

– Number of links: N(log2N)/2– Node degree is n = log2N

0-D 1-D 2-D 3-D 4-D

5-D

Static Connection Networks Examples: Static Connection Networks Examples:

TreesTrees

• Diameter and average distance are logarithmic.– k-ary tree, height d = logk N– Address specified d-vector of radix k coordinates describing

path down from root.• Fixed degree k.• Route up to common ancestor and down:

– R = B XOR A– Let i be position of most significant 1 in R, route up i+1 levels– Down in direction given by low i+1 bits of B

• H-tree space is O(N) with O(√N) long wires.• Low Bisection BW = 1

Fat Tree– Aggregate Bandwidth as you

move “up” the tree– Implicit max scaling for Full

Bisection bandwidth– Limited path redundancy

4x link

16x link

Dynamic Network Building Blocks:

CrossbarCrossbar--Based SwitchesBased Switches

Cross-bar

InputBuffer

Control

OutputPorts

Input Receiver Transmiter

Ports

Routing, Scheduling

OutputBuffer

The end

• let‘s see some NSI challenges andexamples

Some references

• Documents from E. Lawley• Documents from Shaaban