Data Communications TDC 362 / TDC 460 Circuit Switching and Packet Switching

8.1 Circuit Switching Space-Division Switch Time-Division Switch TDM Bus Combinations

Figure 8.1 Circuit-switched network

Figure 8.2 A circuit switch

Blocking or Non-blocking A network is unable to connect stations because all paths are in use A blocking network allows this Used on voice systems Short duration calls Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections

Figure 8.4 Crossbar switch

Figure 8.5 Multistage switch

Figure 8.6 Switching path

Three Stage Switch

Figure 8. 7 Time-division multiplexing, without and with a Figure 8.7 Time-division multiplexing, without and with a Time-slot interchange

Figure 8.8 Time-slot interchange

Figure 8.9 TDM bus

Figure 8.10 TST (Time-space-time) switch

Circuit-Switched Routing Many connections will need paths through more than one switch Need to find a route Efficiency Resilience Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending on traffic Uses a peer structure for nodes

Alternate Routing Possible routes between end offices predefined Originating switch selects appropriate route Routes listed in preference order Different sets of routes may be used at different times

Alternate Routing Diagram

Control Signaling Functions Audible communication with subscriber Transmission of dialed number Call can not be completed indication Call ended indication Signal to ring phone Billing info Equipment and trunk status info Diagnostic info Control of specialist equipment

Control Signals

Location of Signaling Subscriber to network Within network Depends on subscriber device and switch Within network Management of subscriber calls and network ore complex

In Channel Signaling Use same channel for signaling and call Inband Requires no additional transmission facilities Inband Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path Out-of-band Voice signals do not use full 4kHz bandwidth Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present Need extra electronics Slower signal rate (narrow bandwidth)

Drawbacks of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling

Common Channel Signaling Control signals carried over paths independent of voice channel One control signal channel can carry signals for a number of subscriber channels Common control channel for these subscriber lines Associated Mode Common channel closely tracks interswitch trunks Disassociated Mode Additional nodes (signal transfer points) Effectively two separate networks

Common vs. In Channel Signaling

Signaling Modes

Signaling System Number 7 SS7 Most widely used common channel signaling scheme Internationally standardized and general purpose

SS7 SS7 network and protocol used for: Basic call setup, management, tear down Wireless services such as PCS, roaming, authentication Toll free and toll (900) wireline services Enhanced features such as call forwarding, caller ID, 3-way calling Efficient and secure worldwide telecommunications

SS7 SS7 messages are exchanged between central offices and specialized databases via signal transfer points (packet switches). Control plane Responsible for establishing and managing connections Information plane Once a connection is set up, info is transferred in the information plane

SS7 Signaling Network Elements Service switching point (SSP) SSPs enable central offices to communicate with SS7 databases (the user entry point into SS7) Signal transfer point (STP) A signaling point (packet switch) capable of routing control messages Service control point (SCP) SCPs contain databases with call routing instructions

SS7 SCP SCP STP SSP Central Office STP SSP Central Office SSP Central

SS7 Characteristics SSPs are telephone switches that send signaling messages to other SSPs to setup, manage, and release voice circuits An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number) Because the SS7 network is critical to call processing, SCPs and STPs are deployed in mated pair configurations in separate physical locations Links between signaling points are also in pairs

Packet Switching Principles Circuit switching designed for voice Resources dedicated to a particular call Much of the time a data connection is idle Data rate is fixed Both ends must operate at the same rate What if we don’t want a dedicated call, or the data rate is bursty? You want packet switching!

Basic Operation Data transmitted in small packets Typically 1000 bytes Longer messages split into series of packets Each packet contains a portion of user data plus some control info (such as addressing info or packet type) Packets are received, stored briefly (buffered) and passed on to the next node Store and forward (only ATM does not do this)

Advantages Line efficiency Data rate conversion Single node to node link can be shared by many packets over time Packets queued and transmitted as fast as possible Data rate conversion Each station connects to the local node at its own speed Nodes buffer data if required to equalize rates Packets are accepted even when network is busy Delivery may slow down Priorities can be used

Two Basic Forms of Packet Switching Packets handled in two ways Datagram Virtual circuit

Datagram Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may get lost or delayed Up to receiver to re-order packets and recover from missing packets

Virtual Circuit Preplanned route established before any packets sent Call request and call accept packets establish connection (handshake) Each packet contains a virtual circuit identifier instead of destination address No routing decisions required for each packet Clear request to drop circuit Not a dedicated path

Figure 18.2 Virtual Circuit Identifier (VCI) VCI is known only between two switches. (It is not a global address.)

Figure 18.4 Switch and table

Figure 18.5 Source-to-destination data transfer

S(witched)VC vs. P(ermanent)VC setup A virtual circuit can be either switched or permanent. If permanent, an outgoing VCI is given to the source, and an incoming VCI is given to the destination. The source always uses this VCI to send frames to this particular destination. The destination knows that the frame is coming from that particular source if the frame carries the corresponding incoming VCI. If a duplex connection is needed, two virtual circuits are established.

S(witched)VC vs. P(ermanent)VC setup A PVC has several drawbacks: 1. Always connected, so always paying 2. Connection is between two parties only. If you need a connection to another point, you need another PVC. Don’t like these disadvantages? Use an SVC.

Figure 18.6 SVC setup request 1 - Setup frame sent from A to Switch I. Note how the Outgoing VCI is not yet known.

Figure 18.7 SVC setup acknowledgment As the acknowledgment frame goes back, the VCI number is placed into the Outgoing VCI entry in each table.

Virtual Circuits vs Datagram Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node looses all circuits through that node Datagram No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network

Packet Size

Event Timing

Routing Complex, crucial aspect of packet switched networks Characteristics required Correctness Simplicity Robustness Stability Fairness Optimality Efficiency

Performance Criteria Used for selection of route Minimum hop Least cost Dijkstra’s algorithm most common Finds the least cost path from one starting node to all other nodes Algorithm can be repeated for each starting node

Dijkstra’s Least Cost Example

Dijkstra’s Least Cost Example

Decision Time and Place Packet or virtual circuit basis Place Distributed Made by each node Centralized - dead Source - dead

Basic Routing Strategies Adaptive versus Fixed (dead?) Distributed versus Centralized (dead?) Flooding

Centralized and Distributed Routing Tables

Flooding No network info required Packet sent by node to every neighbor Incoming packets retransmitted on every link except incoming link Eventually a number of copies will arrive at destination Each packet is uniquely numbered so duplicates can be discarded Nodes can remember packets already forwarded to keep network load in bounds Can include a hop count in packets

Flooding Example

Properties of Flooding All possible routes are tried Very robust At least one packet will have taken minimum hop count route Can be used to set up virtual circuit All nodes are visited Useful to distribute information (e.g. routing)

Adaptive Routing Used by almost all packet switching networks Routing decisions change as conditions on the network change Failure Congestion Requires info about network Decisions more complex Tradeoff between quality of network info and overhead Reacting too quickly can cause oscillation Reacts too slow to be relevant

Adaptive Routing - Advantages Improved performance Aid congestion control Complex system May not realize theoretical benefits

Where does routing info come from? Local (isolated) Route to outgoing link with shortest queue Can include bias for each destination Rarely used - do not make use of easily available info Adjacent (neighbor) nodes only All nodes in network

Unicast Routing Overview: Chapter 21 Unicast Routing Overview: Routing Protocols (Details in TDC 365/463)

Three basic unicast routing protocols: RIP, OSPF, BGP Figure 21.1 Unicasting In unicast routing, the router forwards the received packet through only one of its ports. Three basic unicast routing protocols: RIP, OSPF, BGP

R1, R2, R3 and R4 use an interior and exterior routing Figure 21.3 Autonomous systems R1, R2, R3 and R4 use an interior and exterior routing protocol. The other routers use only an interior protocol. RIP and OSPF are interior, BGP is exterior.

RIP (Routing Information Protocol) is an interior routing Protocol based on distance vector routing which uses the Bellman-Ford algorithm. Each router shares its routing knowledge with its neighbors, every 30 seconds. This shared information is used to update a router’s routing table. An entry in the routing table consists of the destination network address, the shortest distance to reach the destination in hop count, and the next router to which the packet should be delivered. (see next slide)

Table 21.1 A distance vector routing table Destination Hop Count Next Router Other information 163.5.0.0 7 172.6.23.4 197.5.13.0 5 176.3.6.17 189.45.0.0 4 200.5.1.6 115.0.0.0 6 131.4.7.19

RIP Updating Algorithm Receive: a response RIP message 1. Add one hop to the hop count for each advertised destination. 2. Repeat the following steps for each advertised destination: 1. If (destination not in the routing table) 1. Add the advertised information to the table. 2. Else 1. If (next-hop field is the same) 1. Replace entry in the table with the advertised one. 1. If (advertised hop count smaller than one in the table) 1. Replace entry in the routing table. 3. Return.

Figure 21.4 Example of updating a routing table

OSPF (Open Shortest Path First) protocol is another interior routing protocol for autonomous systems. Special routers called autonomous system boundary routers are responsible for dissipating information about other autonomous systems into the current system. To handle routing efficiently and in a timely manner, OSPF divides an autonomous system into areas.

Figure 21.7 Areas in an autonomous system

In OSPF, each router sends the state of its neighborhood to every other router in the area. It does this by flooding. The state of its neighborhood is only shared when there is new information. This generates much less traffic than does distance vector routing (RIP). OSPF keeps information on its links (the connection between two routers). There are 4 types of links: point-to-point, transient, stub, and virtual. To share information about their neighbors, each entity distributes link state advertisements (LSAs).

There are 5 different types of LSAs: router link, network link, OSPF There are 5 different types of LSAs: router link, network link, summary link to network, summary link to AS boundary router, and external link. Every router in an area receives the router link LSAs and network link LSAs from every other router and forms a link state database. Dijkstra’s least cost algorithm is applied to this link state database to create the routing table. The routing table shows the cost of reaching each network in the area.

RIP and OSPF have shortcomings. BGP RIP and OSPF have shortcomings. RIP (distance vector routing) is not always optimal because The smallest hop count is not always the optimal route. Plus, bad news moves slowly. OSPF (link state routing) has the shortcoming of a possibly huge routing table. To use link state routing for the whole internet would require each router to have a huge database. What about BGP (Border Gateway Protocol)? It is an inter- autonomous system routing protocol and is based on a routing method called path vector routing.