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Overview
Last Lecture
» Routing in WAN
» Source: chapter 10
This Lecture
» X.25
» Source: chapter 10
Next Lecture
» Congestion control
» Source: chapter 12
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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Lecturer Dr Z. Huang
Virtual circuit service
Virtual Call
» Dynamically established
Permanent virtual circuit
» Fixed network assigned virtual circuit
TELE202 Lecture 7 X.25
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Lecturer Dr Z. Huang
Virtual call
Sequence of events
TELE202 Lecture 7 X.25
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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 DTEDCE link
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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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.
L2
h4
h5
R1
subnet
R2
L1
h6
R3
h1
h2
h3
L3
h7
TELE202 Lecture 7 X.25
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h8
h9
h10
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
DS NS
h1
h1
h2
h2
h3
h3
h4
R2
h5
R2
h6
R2
h7
R3
h8
R3
h9
R3
h10 R3
TELE202 Lecture 7 X.25
R2
NS
R1
R1
R1
h4
h5
h6
R3
R3
R3
R3
R3
NS
R1
R1
R1
R2
R2
R2
h7
h8
h9
h10
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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)
TELE202 Lecture 7 X.25
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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)
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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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
TELE202 Lecture 7 X.25
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Lecturer Dr Z. Huang