CPS 356: Introduction to Computer Networks
Lecture 7: Switching technologies
Ch 2.8.2, 3.1, 3.4
Xiaowei Yang
Review: link layer functions• Encoding
– NRZ, NRZI, Manchester, 4B/5B
• Framing: bit, byte, time-synchronization
• Error detection– Parity bits, Internet checksum, CRC
• Reliable transmission– Stop-and-wait
– Sliding window
– Concurrent logical channels
• Media Access Control– Ethernet, Token Ring, Wifi (today)
Ethernet: Physical propertiesSensing the line; if idle, sends signals
10Base5
CSMA/CD: Carrier Sense Multiple access with CollisionDetection
Collision Domain
• Any host hears any other host– A single segment
– Multiple segments connected by repeaters
– Multiple segments connected by a hub
Transmitter Algorithm
Begin: Wait until the line is idle and has data to send, the adaptor sends it, and listens to collision
– If no, go back to Begin– else exponentially backoff
• randomly selects a k between [0,2n-1], waits for k x 51.2 μs to try Begin again
• Gives up after n reaches 16
Collision detection
• An adaptor senses the signals on the line and compares it with its own– If same, no collision; otherwise, collision
– Sends 32-bit jamming sequence after collision
• In the worst case, a sender needs to send 512 bits (46+14+4 = 64B) to detection collision– Why?
• One way delay is d• A needs to send for 2d duration to detect collision• 2d = 512 μs. On a 10Mps Ethernet, corresponds to 512 bits
• The IEEE 802.3 Baseband 5-4-3 – Five physical segments between any two nodes
– Four repeaters between the nodes.
– Three of these physical segments can have connected node
• Each segment < 500m Total < 2500m
• Propagation delay for this maximum-extent Ethernet network is 25.6us
• 2*d = 512us (tolerating errors)
• Minimum Ethernet packet size is 512 bits (64B)– Header 14B, payload 46B, CRC 4B
Today• Types of switching
– Datagram
– Virtual circuit
– Source routing
• Switching hardware design
Packet switching• Problem: single link networks have limited scale
• Ethernet < 1024 hosts, 2500 meters• Wireless limited by radio ranges• Point-to-point links connect only two nodes
• Packet switches enable packets to travel from one host to another without being directly connected
• A packet switch is a device with several inputs and outputs leading to and from the nodes that the switch interconnects
A star topology• A switch has a limited number
of inputs and outputs ports
• Switches can be connected to each other to build larger networks
• Adding a new host may not reduce the performance for other hosts– Not true for shared media
networks– Why?
Switching• A switch connects links
• Each link can be of different types
• A switch runs an appropriate link layer protocol
• Switching (or forwarding): receive incoming packets and send to different outgoing links
Switching technologies• Problem: how does a switch decide on which
output port to place each packet?
• Solution: look at the packet header and makes a decision– Connectionless: datagram
– Connection oriented: virtual circuit
– Source routing
Challenges
• Contention– Input rate exceeds output rate
• Multiple input ports may send to the same output port
– Switches queue packets until contention disappears
• Congestion– When a switch runs out of buffer, it discards packets.
– Too frequent packet loss is said to be congested
Datagram
• Every packet contains the destination address– A global unique identifier
– Ethernet has 48-bit addresses
• A switch maintains a forwarding table that maps a packet to an output port
Switch 2’s forwarding table
A 3
B 0
C 3
D
E
F
G
HQ: how does a switch compute the table?
Features of datagram switching
• Connectionless: hosts can send anytime. No need to wait for connection to set up
• Unknown network state: not sure whether a packet can reach the destination
• Independent forwarding: packets may take different paths
• Robust to failures: a failure of a switch may not disrupt communications– Switches can re-compute forwarding tables
Virtual circuit switching
• Connection oriented– Set up a virtual circuit
– Data transfer
• Connection setup phase– Set up connection state
– A virtual circuit identifier, an incoming interface, an outgoing interface, and an outgoing virtual circuit identifier
Virtual circuit table (switch1)
Incoming
interface
Incoming
VCI
Outgoing
interface
Outgoing
VCI
2 5 1 11
5
11
Virtual circuit switching
• Algorithm:– If a packet arrives on the matching incoming port with the
matching incoming VCI, it will be sent to the corresponding outgoing port with the corresponding VCI
• VCIs are link-local
How to setup connection state
• Administrator configured– Permanent virtual circuit (PVC)
– Admin manually sets up VC tables
– Does not suit large networks
• Signaling– A host sends messages to dynamically setup or
tear down a VC
VC setup• A host A sends a setup message to first hop switch, including
the final destination address– Similar to a datagram packet
• The switch picks an unused VCI to identify the incoming connection, and fills part of the VC table– Why not let the host pick it?
• Every switch repeats the process until the packet reaches the destination B
• The destination B sends an ack to inform its upstream switch the VCI for the connection
Setup B
IF VCI OF VCI2 5 1
Setup B
IF VCI OF VCI3 11 2
Setup B
IF VCI OF VCI0 7 1
Setup B
IF VCI OF VCI0 7 1
VCI
4
IF VCI OF VCI2 5 1
IF VCI OF VCI3 11 2
IF VCI OF VCI0 7 1
VCI
4
ACK, 4
4
ACK,
Characteristics of VC switching
• Connection setup wait
• Data packets contain a small VCI, rather than the full destination addresses
• One switch failure tears down the entire connection
• Connection sets up require routing algorithms– Setup packet is forwarded using a datagram algorithm
VC allows resource reservation
• Buffers can be allocated during the setup phase to avoid congestion
• An example (X.25)– Buffers allocated during connection setup
– Sliding window is run between pairs of nodes (hop-by-hop flow control)
– Circuit is rejected if no more buffer
Quality of service (QoS)
• Connectionless network is difficult to allocate resources– Switches send packets independently
– How to associate one packet with other packets?
• Virtual circuit can be used to provide different QoS– Allocate a fraction of link bandwidth to each circuit
Switching technologies
• Connectionless: datagram
• Connection oriented: virtual circuit
• Source routing
Source routing
• Source host provides all the information for packets to travel across the network– Packets carry output port numbers– Packets carry switch addresses– Variable header length
Handling source routing headers
• Rotation
• Stripping– No return path!
• Pointer
Loose or strict source routing
• Strict– Must visit every node on the path
• Loose– Waypoints rather than the complete route
Overview
• Types of switching– Datagram
– Virtual circuit
– Source routing
• Switching hardware design– How to build switches?
Software switch
• Packets cross the bus twice– Half of the memory bus speed
• 133Mhz, 64-bit wide I/O bus 4Gpbs
• Short packets reduce throughput– 1Mpps, 64 bytes packet– Throughput = 512 Mbps– Shared by 10 ports: 51.2Mbps
Hardware switches
• Ports communicate with the outside world– Eg, maintains VC tables
• Switching fabric is simple and fast
Performance bottlenecks
• Input port– Line speed: 2.48 Gbps
• 2.48x109/(64x8) = 4.83 Mpps
• Buffering– Head of line blocking– May limit throughput to only 59%– Use output buffers or sophisticated buffer
management algorithms to improve performance
Fabrics
• Shared bus– The workstation switch
• Shared memory– Input ports read packets to shared memory
– Output ports read them out to links
Fabrics
• Cross bar– Each output ports need to accept from all input
ports
Fabrics
• Self routing– a self-routing header
added by the input port
– Most scalable
– Often built from 2x2 switching units
An example of self-routing
• 3-bit numbers are self-routing headers• Multiple 2x2 switching elements
– 0: upper output; 1: lower output
Summary• Types of switching
– Datagram• Forwarding table• Simple, difficult to have QoS
– Virtual circuit• Datagram for connection setup• Smaller VC identifiers
– Source routing• Packets carry forwarding information• Variable header length
• Switching hardware design– Shared bus, shared memory, cross-bar, self-routing
• Next lecture– How is forwarding table computed?– ATM network
