Overview Principles of Packet Switching Standards Structure of X.25

1. X.25 Packet Switching Overview • Principles of Packet Switching • Standards Structure of X.25 • Packet-Switch DTE and DCE Concepts • Theory of Operation • Packet Sequencing • X.25 Interface Protocol Structures • Network Layer Functions • User Connectivity • X.75 Internetworking Protocol

2. X.25 Packet Switching Early Reasons for Packet Switching • Create standard interfaces between computing devices • More efficient use of expensive transmission bandwidth • Interconnection of a large number of computing devices and • Ability to extend computer communication over noisy transmission facilities

3. X.25 Packet Switching Principles of Packet Switching • Packet switching divides user data into blocks called the payload information. Packet switching adds overhead to the user data blocks, resulting in a block of data called a packet • The functions that use the packet overhead are • Link layer: performs error detection, multiplexing of multiple logical connections, basic network management capability • Network layer: identify a uniquely addressed network station • Pack layer: allows multiple logical users to share a single physical network access circuit by providing buffers which are used during times of overflow

4. X.25 Packet Switching Principles of Packet Switching (continue…) • Packet switching extends the concept of statistical multiplexing to an entire network • Refer to Figure 9.1 (p. 325) • Packet switching employs queueing to control loss and resolve contention at the expense of added, variable delay • There are two types of flow and congestion control • Implicit: adaptively alter the rate of packets sent into the network by estimating loss and delay using a layer 4 protocol (ex. TCP) • Explicit: the sender gets notified on congestion events, if the sender or receiver reacts to the explicit indication of congestion quickly enough, the loss can be avoided entirely

5. X.25 Packet Switching X.25 Packet Switching Defined Origins of X.25 • The CCITT X.25 packet-switching standard was developed to provide a reliable system of data transport for computer communications over the noisy, unreliable analog-grade transmission medium • Many packet switches today can provide networking speeds up to and including DS1 and E1, with packet sizes up to 4096 bytes

6. X.25 Packet Switching Standards Structure of X.25 • The CCITT set of “X dot” standards for the physical, link, and packet layer protocols are known collectively as X.25 • Refer to Figure 9.2 (p. 327) • The physical layer is defined by the X.21 and X.21bis standards. X.21 specifies an interface between DTE and DCE • The data link layer standard is based upon the High-level Data Link Control (HDLC) ISO standard. The modified X.25 version of this called LAP-B • The packet layer standard is called the X.25 Packet Layer Protocol (PLP). The packet layer defines Permanent Virtual Circuit (PVC) and virtual call, or Switched Virtual Circuit (SVC) message formats and protocols

7. X.25 Packet Switching Standards Structure of X.25 (Continue…) • User connectivity to the packet-switched network takes on mnay forms. The suite of “X dot” standards is: X.121, X.21, X.25, X.28, X.29, X.3, and X.32. • X.121 defines the network layer numbering plan for X.25 • CCITT Recommendations X.3, X.28, and X.29 define the method for asynchronous DTEs to interface with X.25 networks via a Packet Assembler/Dissembler (PAD) function • A PAD delivers a stream of asynchronous characters to the DTE from a received X.25 packet

8. X.25 Packet Switching What does X.25 Packet Switching Provide? • X.25 packet switching provides logical user access and logical channel multiplexing onto a single line by interleaving low-speed user channels into a single high-speed data channel which then interfaces to the network • Refer to Figure 9.3 (p. 329) • Packet switches allocate user bandwidth in virtual circuits. This gives the user the perception of a dedicated circuit of full bandwidth • Packet-switch networks are designed based upon the statisticalchance that not all users will want to transmit at the same time, nor for the same duration

9. X.25 Packet Switching Packet-Switch DTE and DCE Concepts • Packet switching’s first practical commercial application was designed for host computers to communicate through multiple nodes over a packet-switched network. • Refer to Figure 9.4 (p. 329) • The X.25 protocol defines the protocols and procedures that manage the transfer of data between the DTE, typically a user terminal, and the DCE, typically the network node • The end-user DTE accesses the X.25 network through a communications controller, FEP, or host • A DCE packet assembler/dissembler (PAD) is often used within the network for DTE access

10. X.25 Packet Switching Theory of Operation Traffic Characteristics • The two primary requirements that best suit packet networks are • burstiness:data transmission having a duration of less than five seconds • delay insensitive traffic: packet switched networks that are not delay sensitive • Refer to Figure 9.5 (p. 331)

11. X.25 Packet Switching Basic Operation • DTE devices transmit data to the network via synchronous mode X.25 protocol through the use of packets. • As packets flow through the network, each mode checks for errors and then retransmits if necessary • All operations are transparent to the user • Refer to Figure 9.6 (p. 332) • An elaborate acknowledgment and retransmission scheme is provided in packet switching, primarily because of the large amount of errors which could be experienced when using voice grade lines

12. X.25 Packet Switching Acknowledgments • under normal conditions, all packets are acknowledged by each receiving node as they pass through the network • Refer to Figure 9.7 (p. 333) Packet Sequencing • Packet switching utilizes queuing and store-and-forward protocols with packet-sequencing information contained within each packet. • Sequencing errors can be caused by delay incurred by message division (different packet delays) over simultaneous long and short hops

13. X.25 Packet Switching Example of X.25 Store-and-Forward Operation • The LAP-B protocol uses a store-and-forward approach to ensure reliable delivery of packets across noisy, error-prone transmission links • Two types of packets are exchanged between th enodes: Link Data (LD) and ACKnowledgment (ACK) • Each message has a pair of numbers associated with it: • Receive Sequence number (RSN): indicates the next SSN that is expected by the receiver • Send Sequence Number (SSN): number of packet that is being sent • Refer to Figure 9.8 (p. 335)

14. X.25 Packet Switching Packet Error & Lost Packets • To alleviate these problems, each node only transmits acknowledgments to the previous node when it passes a packet on to the next destination node. • This scheme adds additional delay to the queuing process. If a certain level of packet loss can be reasonably tolerated, this scheme does not need to be implemented. • Refer to Figure 9.9 (p. 336) Packet duplication • It can be caused by a failure of a node to acknowledge a packet that has continued through the network

15. X.25 Packet Switching X.25 Interface Protocol Structure • The X.25 interface protocols define two levels of control procedures: • The subscriber link access procedure which defines the physical movement of data between two user locations • The packet layer DTE/DCE interface which defines the transfer of information across the physical medium at the network layer CCITT Recommendation X.25 • X.25 acts as the interface between DTE and DCE for terminals and computers operating in the packet switching mode • Refer to Figure 9.10 (p. 338)

16. X.25 Packet Switching X.25 and the OSI Reference Model • Refer to Figure 9.11 (p. 338) • Refer to Figure 9.12 (p. 339) Data link Frame Structure • X.25 network access can be accomplished over • a single-link procedure (SLP): uses standard ISO HDLC framing • a multilink procedure (MLP): uses LAP-B over multiple channels based on the ISO MLP standard • Refer to Figure 9.13 (p. 340)

17. X.25 Packet Switching The Packet Layer DTE/DCE Interface • Each packet to be transferred across the DTE/DCE interface is carried or packaged within the data link layer HDLC (LAPB) frame information field • Refer to Figure 9.14 (p. 341) X.25 Layer 3 Packet Format • The X.25 layer is composed of a header and a user data, or information, field • Refer to Figure 9.15 (p. 342)

18. X.25 Packet Switching Network Layer Functions • Each physical interface can establish one or multiple virtual circuits to remote physical interfaces • Each of these circuits is then assigned logical channel numbers • Virtual circuits assure sequence integrity in the delivery of user data, established either administratively as a PVC, or as an SVC through call control procedures. • Over these PVCs flow both normal data and call control packets. • An X.25 “call” is set-up using a global X.25 address, and a SVC channel number is negotiated and allocated between each switch

19. X.25 Packet Switching Network Layer Functions (continue…) • The X.25 global address is only present in call set-up and tear-down, and all data in between is identified by its SVC channel number. • Thus, the X.25 address is global significant, and the SVC channel number is locally significant • Datagrams are messages unto themselves that do not require control procedures. • Datagrams may be sent in a best-effort mode, or have an explicit receipt notification

20. X.25 Packet Switching Permanent Virtual Circuits and Virtual Calls • The network layer provides the user with up to 4095 logical channels designated either as Permanent Virtual Circuits (PVCs) or Virtual Calls (VCs) over one LAPB physical channel • PVCs are virtual circuits permanently established between a source and destination node • The user always sees the virtual circuit as a dedicated circuit for his or her use only, whereas the network provides the same circuit as a shared resource to multiple users upon demand. • Refer to Figure 9.16 (p. 343) • Switched Virtual Circuits are established and disabled on demand

21. X.25 Packet Switching Permanent Virtual Circuits and Virtual Calls (Continue…) • Virtual Calls (VCs) or Switched Virtual Calls (SVCs) act as circuit-switched calls, with the characteristic of being connected and disconnected after the data has been sent between the source and destination node • Therefore, one source can connect to many destinations at the different times. • Refer to Figure 9.18 (p. 345) • SVCs can add even more delay than when using a PVC

22. X.25 Packet Switching Virtual Calls and Logical Channel Numbers • Logical Channel Numbers (LCNs) are assigned to each of the incoming and outgoing virtual calls for each DCE and DTE, respectively, as well as to all PVCs • Out of the 4095 logical channel numbers available per physical circuit, PVCs are assigned the lowest numbers, followed by one-way incoming virtual calls, then two-way incoming and outgoing calls, and the highest numbers are reserved for one-way outgoing virtual calls • Refer to Figure 9.19 (p. 346)

23. X.25 Packet Switching X.25 Control Packet Formats • Control packets are used for virtual call setup and termination. • Refer to Figure 9.21 (p. 347) • Control packets perform many functions, including call request and incoming call packets, call accepted and call connected clear request and clear indication, etc. • Clear packets are used to clear the user-to-user session (DTE-to-DTE) • Interrupt packets are used when the user wants to bypass the normal flow control protocol • Reset packets will reset the entire PVC or SVC during data transfer • Restart packets will reset all of the PVCs and clear all SVCs on a specific physical interface port

24. X.25 Packet Switching X.25 Control Packet Formats (Continue…) • Diagnostic packets are used as the catch-all identifier of all error conditions not covered by the other control packets • Reject packets reject a specific packet and retransmission occurs from the last received packet Normal Data Packet Formats • Normal data packets are transferred after call setup and before call termination • PVCs only require normal packet formats, as the virtual circuit is permanent and does not need to be set up or terminated • Refer to Figure 9.22 (p. 349)

25. X.25 Packet Switching Flow Control and Windowing • The send and receive sequence numbers in the X.25 packet layer are also used to provide flow control between the packet layer source and sink. • The send sequence number is a sequential number for the current packet • Refer to Figure 9.23 (p. 349) • Numbers are incremented modulo the maximum window size. • The receive sequence number acts as an acknowledgement for all packets up to one less than the receive sequence number

26. X.25 Packet Switching Flow Control and Windowing (Continue…) • The transmitter can send no more packets than the modulo minus 1 without acknowledgment • This is called a sliding window flow control protocol. This process allows the receiver to control the maximum rate of transmission over a virtual circuit, and is therefore a form of traffic control Datagram Function • Datagrams are single-packet messages which contain a destination address and are independently routed through the packet-switched network

27. X.25 Packet Switching Datagram Function (Continue…) • All the information for a single transaction is contained within the datagram packet. • These packets each select the best route through the network, do not require acknowledgements, are unsequenced, and thus are a form of connectionless service. • Datagrams are ideal for when the user interface data transfer is for a very short period of time and where response time must be fast, or is not even needed, and where user control over the transmission is not required. • Datagrams were dropped from the X.25 standard

28. X.25 Packet Switching User Connectivity • The network user interface is accomplished through seven important standards: X.121, X.21, X.28, X.29, X.3 and X.32 User Network Interface • CCITT Recommendation X.121 - Defines the international numbering plan for packet-switched networks • CCITT Recommendation X.28 - Defines the operational control of these functions between character mode terminal DTE and the DCE PAD • CCITT Recommendation X.29 - Defines the same controls but for the host computer destination

29. X.25 Packet Switching User Network Interface (Continue…) • CCITT Recommendation X.3 - Defines a Packet Assembler/Disassembler (PAD) concentrator function for start-mode or character-mode DTE devices • CCITT Recommendation X.32 - Defines X.25 synchronous dial-up mode for DTE services • There are two types of “user-to-network” interfaces using these protocols: • packetized virtual circuits: assure sequencing of user data and on ordered flow of messages which require multiple packets • packetized datagram: are messages unto themselves and do not follow the ordered packet flow of virtual circuits

30. X.25 Packet Switching User Applications • The primary application used with packet switching is the terminal-to-host application access and vice versa. • Packet switching was designed for terminals and PC interactions, and compressed voice can also be passed through HDLC over the packet-switched network. Performance And Design Considerations • Performance in a packet-switched network is measured in packets per second throughput • Packet delay through a typical packet switch node is 50 to 200 ms due to packet processing

31. X.25 Packet Switching Performance And Design Considerations (Continue…) • The switch performance should be constant, irrespective of packet size. Some packet switches drastically degrade performance and packet processing as the packet size increases above 128 or 256 kbits. • Another consideration is the protocol conversion.The greater the amount of protocol conversion that the switch or PAD has to perform, the lower the throughput and performance.

32. X.25 Packet Switching X.75 Internetworking Protocol • X.75 can be used to connect the larger backbone packet switches to one another • X.75 functions reside in the network layer above X.25 functions • All functions of the X.75 protocol are similar to the X.25 protocol • X.75 also supports the multilink procedures to support multiple links between STE • STE is Signaling Terminal Exchange and acts as the internetwork interface point and performs both packet-transfer procedures and packet signaling • Refer to Figure 9.27 (p. 358)

33. Cisco Virtual LANs (VLANs)

34. Objectives • Describe Virtual LANs • Describe Frame Tagging • Describe Inter-Switch Link Routing • Describe Virtual Trunking Protocol

35. Virtual LANs (VLANs) • Definition:A logical grouping of network users and resources connected to administratively assigned ports on a switch. • Essentially…smaller broadcast domains • Organized by: • Location • Function • Department • Application or protocol

36. Virtual LANs

37. Broadcast Control • Broadcasts occur in every protocol • Bandwidth greedy & Broadcast applications • Flat network = one broadcast domain • VLANs & Broadcasts • all devices in a VLAN are members of the same broadcast domain and receive all broadcasts

38. Security Flat network problems addressed • By using VLANs and creating multiple broadcast groups, administrators now have control over each port and each user

39. Flexibility & Scalability • Layer-2 switches only read frames • Can cause a switch to forward all broadcasts • VLANs create broadcast domains • Reduces broadcast traffic • Add users to VLAN regardless of physical location • Additional VLANs can be created when network growth consumes more bandwidth • Switch fabric • Group of connected switches sharing same VLAN information

40. Traditional Collapsed Backbone

41. Switched & Routed Network

42. VLAN Memberships • Static VLANs • Typical method of creating VLANs • Most secure (switch port assigned to a VLAN always maintains that assignment until changed) • Dynamic VLANs • Node assignment to a VLAN is automatic • MAC addresses, protocols, network addresses, etc • VLAN Management Policy Server (VMPS) • MAC address database for dynamic assignments • MAC-address to VLAN mapping

43. Identifying VLANs • Access links • A link that is part of only one VLAN • Trunk links • Carries multiple VLANs

44. Frame Tagging Frame Tagging Definition: Keeping track of users/frames as they travel the switch fabric & VLANs • User-defined VID (VLAN ID) assigned to each frame • Removed before exiting trunked & access links

45. VLAN ID (VID) Methods • Inter-Switch Link (ISL) • externally tagging VLAN info onto Ethernet frame • Cisco proprietary • FastEthernet & Gibabit Ethernet only • Allows VLANs to be multiplexed over trunk line • IEEE 802.1q • Must use if trunking between Cisco & non-Cisco switch

46. Trunking & Routing • Trunking (layer 2) • Single port can be part of multiple VLANs • Cisco switches use Dynamic Trunking Protocol (DTP) for trunk management (using ISL or 802.1q) • Routing (layer 3) • A layer-3 device (router or switch) is necessary for any host or device to communicate between VLANs

47. VLAN Trunk Protocol (VTP) • VTP Purpose: • Manage all configured VLANs across a switch internetwork & maintain consistency • Allow an administrator to add, delete, & rename VLANs • Permits trunking over mixed networks • VTP server must be created to manage VLANs

48. VTP Modes

49. VTP Modes of Operation • Server • Default for all Catalyst switches • Minimum one server for a VTP domain • Client • Receives information + sends/receives updates • Cannot make any changes • Transparent • Does not participate in a VTP domain but forwards VTP advertisements • Can add/delete VLANs • Locally significant

50. Revision Number