1. TELE202 Lecture 7 X.25 1 Lecturer Dr Z. Huang Overview ¥Last Lecture »Routing in WAN »Source: chapter 10 ¥This Lecture »X.25 »Source: chapter 10 ¥Next Lecture »Congestion control »Source: chapter 12

2. TELE202 Lecture 7 X.25 2 Lecturer Dr Z. Huang X.25 ¥Approved in 1976 ¥Interface between host and packet switched network ¥Almost universal on packet switched networks and packet switching in ISDN ¥Defines three layers »Physical »Link »Packet ¥Physical layer »Interface between attached station and link to node: DTE (user equipment) and DCE (node) »Uses standard X.21, sometimes EIA-232 »Reliable transfer across physical link »Sequence of frames ¥Link layer »Link Access Protocol Balanced (LAPB) –Subset of HDLC (ABM) ¥Packet layer »External virtual circuits »Logical connections (virtual circuits) between subscribers

3. TELE202 Lecture 7 X.25 3 Lecturer Dr Z. Huang Virtual circuit service ¥Virtual Call »Dynamically established ¥Permanent virtual circuit »Fixed network assigned virtual circuit

4. TELE202 Lecture 7 X.25 4 Lecturer Dr Z. Huang Virtual call ¥Sequence of events

5. TELE202 Lecture 7 X.25 5 Lecturer Dr Z. Huang Packet format ¥Header sizes »24-bit, 32-bit, or 56-bit ¥Sequence numbers »3-bit, 7-bit, or 15-bit ¥Virtual circuit number: 12 bits ¥Multiplexing »DTE can establish 4095 simultaneous virtual circuits with other DTEs over a single DTE- DCE link

6. TELE202 Lecture 7 X.25 6 Lecturer Dr Z. Huang Packet layer ¥Virtual Circuit Numbering ¥Flow and Error Control »Same as HDLC »This is why X.25 is not efficient –Two layers of flow and error control

7. TELE202 Lecture 7 X.25 7 Lecturer Dr Z. Huang Packet layer ¥Packet Sequences »Complete packet sequences »Allows longer blocks of data across network with smaller packet size without loss of block integrity »A packets –M bit 1, D bit 0 »B packets –The rest »Zero or more A followed by B ¥Reset and Restart »Reset –Reinitialize virtual circuit –Sequence numbers set to zero –Packets in transit lost –Up to higher level protocol to recover lost packets –Triggered by loss of packet, sequence number error, congestion, loss of network internal virtual circuit »Restart –Equivalent to a clear request on all virtual circuits –E.g. temporary loss of network access

8. TELE202 Lecture 7 X.25 8 Lecturer Dr Z. Huang A case study of WAN ¥Assume we have the following WAN »L1 is a star LAN running IEEE802.4 »L2 is a ring LAN running IEEE802.5 »L3 is a bus LAN running IEEE802.3 »R1, R2, and R3 are routers which are connected by a subnet »R1 is also connected to L1, R2 to L2, and R3 to L3 ¥A host has two addresses »hi, network layer address »hi_dl, data link layer address ¥A router has »a network layer address Ri »but more than one data link addresses, according to how many LANs and routers it connects to. We use Ri_dl to represent its data link addresses in general. L1 L2 L3 h1 h2 h3 h4 h5 h6 h7 h8 h9h10 R1 R2 R3 subnet

9. TELE202 Lecture 7 X.25 9 Lecturer Dr Z. Huang A case study of WAN ¥Suppose stations in L1, L2, L3 and the subnet are connected at DL layer »The data link layer can provide an interface for the network layer »L1 provides –L1_DL_send(packet, dl_address) –L1_DL_recv(packet) »L2 provides –L2_DL_send(packet, dl_address) –L2_ DL_recv(packet) »L3 provides –L3 _DL_send(packet, dl_address) –L3 _ DL_recv(packet) »The subnet provides –SN_ DL_send(packet, dl_address) –SN_ DL_recv(packet) ¥Suppose routers have the following routing table » R1 R2 R3 »DS NS NS NS »h1 h1 R1 R1 »h2 h2 R1 R1 »h3 h3 R1 R1 »h4 R2 h4 R2 »h5 R2 h5 R2 »h6 R2 h6 R2 »h7 R3 R3 h7 »h8 R3 R3 h8 »h9 R3 R3 h9 »h10 R3 R3 h10

10. TELE202 Lecture 7 X.25 10 Lecturer Dr Z. Huang A case study of WAN ¥The router R1 works as below »Check if there is any packet coming –Use L1_DL_recv(packet) to get the packets from L1 –Use SN_DL_recv(packet) to get the packets from the subnet »Check the destination of the packet and look up the routing table –If the next stop is R1 or R2, forward the packet using SN_DL_send(packet, R1_dl/R2_dl) –If the destination of the packet is one of the hosts in L1, send the packet using L1_DL_send(packet, hi_dl) (1<=i<=3) »Repeat the above steps until the router crashes or is shut down ¥Routers R2 and R3 work in a similar way ¥The network layer at each host »If a packet is destined for outside of the LAN, it sends the packet to the router of the LAN using Li_DL_send(packet, Ri_dl) »If a packet is destined for some host inside the LAN, it sends the packet to the host using Li_DL_send(packet, hi_dl) »It receives packets from the data link layer in the host using Li_DL_recv(packet)

11. TELE202 Lecture 7 X.25 11 Lecturer Dr Z. Huang A case study of WAN ¥Assume h1 want to send a packet to h9 »h1 first set the destination field of the packet as h9. »Because the packet is destined for outside of L1, h1 sends the packet to the router R1 using L1_DL_send(packet, R1_dl) »When R1 receives the packet using L1_DL_recv(packet), it checks the destination of the packet and finds it h9 »R1 checks its routing table and finds the next stop is R3. So R1 sends the packet to R3 using SN_DL_send(packet, R3_dl) »When R3 receives the packet using SN_DL_recv(packet), it checks the destination of the packet and finds it h9 »R3 checks its routing table and finds the next stop is h9. So R3 sends the packet to h9 using L3_DL_send(packet, h9_dl) »Finally h9 can receive the packet using L3_DL_recv(packet)

12. TELE202 Lecture 7 X.25 12 Lecturer Dr Z. Huang Network layer (WAN) functions ¥All the work done above is part of the functionality of network layer »Route packets »Maintain routing tables ¥The network layer is the lowest layer which can achieve end-to-end transmission (transmit a packet from an end host to another), but it may not guarantee reliable delivery of the packet ¥Can provide the following functions for the higher layer protocol »NL_send(higher_layer_packet, NL_address) »NL_recv(higher_layer_packet) ¥If the WAN is connection-oriented, a connection has to be set up before using NL_send and NL_recv. Then the connection should be torn down after finishing data transfer ¥Besides the above work, network layer needs to do congestion control and other bookkeeping work, e.g. billing

13. TELE202 Lecture 7 X.25 13 Lecturer Dr Z. Huang Deadlocks ¥Deadlock »The situation, in which nodes in a network are waiting for an event that won’t happen ¥Store-and-forward deadlock »Three nodes, A,B, and C have reached the point where their buffers are full and can’t accept any more packets »Though A can send packets, it can’t remove any packets from its buffer until B sends an acknowledge, which is not possible until B has more space to accommodate packets from A »B and C, C and A are in the same situation

14. TELE202 Lecture 7 X.25 14 Lecturer Dr Z. Huang Summary ¥X.25 »Physical layer »Link layer »Packet layer »Virtual call »Packet format »Block reassemble ¥Relationship between data link layer and network layer »How a packet is transmitted in a WAN? »Understand the case study of WAN ¥Network layer functions ¥Deadlocks in computer networks